History of the Development of Low Dosage Hydrate Inhibitors - Energy

VOLUME 20, NUMBER 3

MAY/JUNE 2006

© Copyright 2006 American Chemical Society

ReViews History of the Development of Low Dosage Hydrate Inhibitors Malcolm A. Kelland* Department of Mathematics and Natural Sciences, Faculty of Science and Technology, UniVersity of StaVanger, 4036 StaVanger, Norway ReceiVed December 21, 2005. ReVised Manuscript ReceiVed March 2, 2006

Low dosage hydrate inhibitors (LDHIs) are a recent and alternative technology to thermodynamic inhibitors for preventing gas hydrates from plugging oil and gas production wells and pipelines. LDHIs are divided into two main categories, kinetic inhibitors (KHIs) and anti-agglomerants (AAs), both of which are successfully being used in field applications. This paper reviews the research and development of LDHIs with emphasis on the chemical structures that have been designed and tested. The mechanisms of both KHIs and AAs are also discussed.

Introduction Gas hydrates are ice-like clathrate solids that are formed from water and small hydrocarbons at elevated pressures and at lower temperatures (Figure 1).1 The temperature below which hydrates can form increases with increasing pressure and can sometimes be as high as 30 °C. Gas hydrates are a problem to the oil and gas industry as they can block flow lines, valves, wellheads, and pipelines, causing loss of production. Low dosage hydrate inhibitors (LDHIs) have been researched and developed over the past 15 years as an alternative method to control gas hydrates. They are now established tools for the prevention of gas hydrate plugging of oil and gas pipelines and gas wells. A wide range of OPEX savings, possible extended field lifetime and multi-million dollar CAPEX savings, are economic drivers for choosing LDHIs instead of other hydrate prevention methods.2 This paper documents the chemistry and history of the development of LDHIs. It is written from a somewhat Norwegian perspective, where the author is based, and includes * Telephone: +47 51 83 18 23. Fax: +47 51831750. E-mail: [email protected]. (1) Sloan E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. (2) Frostman, L. M.; Thieu, V.; Crosby, D. L.; Downs, H. H. In Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, February 5-8, 2003; SPE 80269.

Figure 1. Two commonest clathrate hydrates, structure I (McMullan and Jeffrey, 1965) (left) and structure II (Mak and McMullan, 1965) (right).

previously unpublished work from RF-Rogaland Research (RF, now International Research Institute of Stavanger, IRIS). Some qualitative theory regarding the mechanism of LDHIs is also provided. Due to the confidential nature of much of the research, very few research papers on the chemistry of LDHIs have been published in the public domain. Consequently, this paper has made extensive use of the patent literature. In particular, patent applications (and not just awarded patents) give a good indication of what each research group was working on at the time. To give a full flavor of the chemistries investigated, both successful and unsuccessful attempts to commercialize LDHIs are described.

10.1021/ef050427x CCC: $33.50 © 2006 American Chemical Society Published on Web 04/01/2006

826 Energy & Fuels, Vol. 20, No. 3, 2006

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crystal.3,4 THF forms structure II hydrate, the same structure that is usually formed by natural gas hydrates. A second apparatus is the ball-stop rig or rocker rig. These are small cells or test tubes that are placed in a cooling bath within which a metal ball rocks back and forth. When the balls stop moving, the cell has plugged with hydrates. The time to hydrate formation (induction time) is determined as the time when the liquid goes cloudy and/or as the time when gas consumption due to hydrate formation is observed. Both THF hydrate5,76 and natural gas hydrate6,76 can be made in such ball-stop cells. The rocker rig using natural gas hydrate is a simple but excellent test equipment for AAs. A third commonly used apparatus is

Figure 2. Pressure-temperature graph for a typical natural gas hydrate.

Before we delve into the history, it will help to explain a few terms and types of equipment that are used in the research stages. The term LDHI was coined in the mid 1990s to differentiate these inhibitors from the more well-known thermodynamic inhibitors such as methanol and glycols. LDHIs are usually dosed at a concentration of ca. 0.1-1.0 wt % (active component) based on the water phase, whereas thermodynamic inhibitors are dosed at much higher concentrations of ca. 2050 wt %. For commercial applications, LDHIs can be subdivided into two basic categories: •kinetic hydrate inhibitors (KHIs or KIs) •anti-agglomerants (AAs) KHIs act primarily as gas hydrate anti-nucleators, although most of them also delay the growth of gas hydrate crystals. KHIs are generally water-soluble polymers. There are a number of non-polymeric hydrate crystal growth inhibitors that are poor anti-nucleators (e.g., tetrapentylammonium bromide, butoxyethanol, certain polyetheramines). These often work as KHI synergists. KHIs allow you to transport hydrate-forming fluids for a certain period of time before hydrates start to form. The time to the formation of the first hydrate crystals is called the induction time. AAs allow hydrates to form, but they prevent them from agglomerating and subsequently accumulating into large masses. An AA enables the hydrates to form as a transportable nonsticky slurry of hydrate particles dispersed in the liquid hydrocarbon phase. As will become apparent later, there are two mechanisms by which the AA effect can be accomplished. In general, the water-cut for AAs should be below approximately 50%, otherwise the hydrate slurry gets too viscous to transport. The best AAs perform at higher subcoolings than the KHIs. The subcooling (∆T) is a measure of the driving force for hydrate formation in a system. Subcooling is the difference between the hydrate equilibrium temperature and the operating temperature at a given pressure (Figure 2). In this paper, the performance of an LDHI is given as the maximum subcooling for which a produced fluid can be safely transported without fear of hydrate deposition and plugging. The subcooling values quoted for a given LDHI are generic. In practice, the actual performance of a KHI, and to a lesser extent some AAs, depends on the detailed composition of the liquid hydrocarbon phase. Other factors such as pressure, salinity, other additives, and mixing also have an effect. Several types of equipment have been designed for studying LDHIs, and very brief descriptions are given here. The simplest, but a very effective, technique for studying growth inhibition involves measuring the growth rate of a single THF hydrate

(3) Makogon, T. Y.; Larsen, R.; Knight, C. A.; Sloan, E. D. J. Crystal Growth 1997, 179, 258-262. (4) Zeng, H.; Wi, L. D.; Walker, V. K.; Ripmeester, L. A. In Proceedings of the 4th International Conference on Natural Gas Hydrates, Yokohama, Japan, May 19-23, 2002. (5) Jussaume, L.; Canselier, J. P.; Montfort, J. P. In Proceedings of the AICHE Spring National Meeting, Houston, TX, March 14-18, 1999. (6) Deaton, W. M.; Frost, E. M. Oil Gas J. 1937, 36 (1), 75. (7) Arjmandi, M.; Ren, S.-R.; Yang, J.; Tohidi, B. In Proceedings of the 4th International Conference on Natural Gas Hydrates, Yokohama, Japan, May 19-23, 2002. (8) Oskarsson, H.; Lund, A.; Hjarbo, K.; Uneback, I.; Navarrete, R. C.; Hellsten, M. In Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, February 2-4, 2005; SPE 93075. (9) Lund, A.; Akporiaye, D. E.; Tayebi, D.; Wendelbo, R.; Hjarbo, K. W.; Karlsson A.; Dahl, I. M. U.S. Patent 6688180, 2004. (10) Urdahl, O.; Lund, A.; Mork, P.; Nilsen, T. Chem. Eng. Sci. 1995, 50 (5), 863-870. (11) Lund, A.; Urdahl, O.; Gjertsen, L. H.; Kirkhorn, S.; Fadnes, F. In Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, France, June 2-6, 1996; pp 407-414. (12) Lippmann, D.; Kessel, D.; Rahminian, I. In Proceedings of the 5th International Offshore and Polar Engineering Conference, The Hague, The Netherlands, June 11-16, 1995. (13) Reed, R. L.; Kelley, L. R.; Neumann, D. L.; Oelfke, R. H.; and Young, W. D. In Proceedings of the 1st International Conference on Natural Gas Hydrates, New York, 1993; p 430. (14) Kuliev, A. M. GazoV. Delo 1972, 10, 17-19. (15) Matthews, R. R.; Clark, C. R. European Patent Application 309210, 1989. (16) Sugier, A.; Bourgmayer, P.; Behar, E.; Freund, E. European Patent Application 323307, 1989. (17) Sugier, A.; Bourgmayer, P.; Behar, E.; Freund, E. European Patent Application 323774, 1989. (18) Sugier, A.; Bourgmayer, P.; Stern, R. European Patent Application 323775, 1989. (19) Bourgmayer, P.; Sugier, A.; Behar., E. In Proceedings of the 4th Multiphase Flow Conference, BHR, 1989. (20) Sloan, E. D. U.S. Patent 5420370, May 30, 1995. (21) Long, J.; Lederhos, J.; Sum, A.; Christiansen, R. L.; Sloan, E. D. In Proceedings of the 73rd Annual GPA ConVention, New Orleans, LA, March 7-9, 1994. (22) Sloan, E. D. U.S. Patent 5432292, July 11, 1995. (23) Sloan, E. D.; Christiansen, R. L.; Lederhos, J.; Panchalingam, V.; Du, Y.; Sum, A. K. W.; Ping, J. U.S. Patent 5639925, June 17, 1997. (24) Sloan, E. D. U.S. Patent 5880319, March 9, 1999. (25) Yeh, Y.; Feeney, R. Chem. ReV. 1996, 601, 96 (2) and references therein. (26) Edwards, A. R. In Proceedings of the 1st International Conference on Natural Gas Hydrates, New York, 1993; p 543. (27) Anselme, M. J.; Reijnhout, M. J.; Klomp, U. C. WO Patent Application 93/25798, 1993. (28) Klomp, U. C.; Kruka, V. C.; Reijnhart, R. In Proceedings of Controlling Hydrates, Waxes and Asphaltenes, IBC Conference, Aberdeen, October 1997. (29) Muijs, H. M.; Beers, N. C.; Van Om, N. M.; Kind, C. E.; Anselme, M. J. Canadian Patent Application 2036084, 1991. (30) Reijnhout, M. J.; Kind, C. E.; Klomp, U. C. European Patent Application 0 526 929 A1, 1993. (31) Klomp, U. C.; Kruka, V. C.; Reijnhart, R. WO Patent Application 95/17579, 1995. (32) Duncum, S.; Edwards, A. R.; Osborne, C. G. European Patent Application 0536950 A1, 1993. (33) Duncum, S.; Edwards, A. R.; Gordon, K. R.; Holt, C. B.; Osborne, C. G. WO Patent Application 94/25727, 1994. (34) Duncum, S.; Edwards, A. R.; Lucy, A. R.; Osborne, C. G. WO Patent Application 94/24413, 1994.

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the high-pressure stirred cell or autoclave.7 The cell is placed in a cooling bath, and the pressure, temperature, and sometimes torque exerted on the stirrer are measured. Some cells have

windows for visual observations or may be entirely made of sapphire. Recently, mini-autoclaves have been developed for rapid screening of LDHIs.8,9,221The next apparatus in terms of

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the level of complexity is the vertical placed pipe-wheel or loopwheel. The pipe is usually of 1-3 in. i.d. and can have a window for visual observation. It is pressurized and then rotated in a cooling chamber. Pressure, temperature, and torque exerted on the wheel can be measured.10-12 The last apparatus is the horizontal flow loop. A simple flow loop can be built for tests (117) Bakeev, K.; Chuang, J.-C.; Winkler, T.; Drzewinski, M. A.; Graham, D. E. WO Patent Application 01/27055, 2001. (118) Bakeev, K.; Chuang, J. C.; Winkler, T.; Drzewinski, M. A.; Graham, D. E. U.S. Patent 6281274, 2001. (119) Bakeev, K.; Myers, R.; Graham, D. E. U.S. Patent 6180699, 2001. (120) Cohen, J. M.; Young, W. D. U.S. Patent 6093863, 2000. (121) Cohen, J. M.; and Young, W. D. U.S. Patent 6096815, 2000. (122) Phillips, N. J.; Grainger, M. In Proceedings of the Annual Gas Technology Symposium, Calgary, Alberta, Canada, March 15-18, 1998; SPE 40030. (123) Palermo, T.; Argo, C. B.; Goodwin, S. P.; Henderson, A. Ann. N.Y. Acad. Sci. 2000, 912. (124) Leporcher, E. M.; Fourest, J. M.; Labes Carrier, C.; Lompre, M. In Proceedings of the 1998 SPE European Petroleum Conference, The Hague, The Netherlands, October 20-22, 1998; SPE 50683. (125) Klomp, U. C. WO Patent Application 99/13197, 1999. (126) Klomp, U. C. Shell Global Solutions, personal communication, 2005. (127) Knott, T. Holding hydrates at bay. Offshore Eng. 2001, February. (128) Shoup, G. BP, personal communication, 1999. (129) Glenat, P.; Palermo, T. In Proceedings of the 3rd Natural Gas Hydrate Conference, Salt Lake City, July 1999. (130) Velly, M.; Gateau, P.; Sinquin., A.; Durand, J. P. European Patent Application EP 0896123. (131) Toulhoat, H.; Sinquin, A. U.S. Patent 6028236, 2000. (132) Velly, M.; Delion, A.-S.; Durand, J. P. European Patent Application EP 812977. (133) Velly, M.; Hillion, M.; Sinquin, A.; Durand, J. P. European Patent Application EP 905350. (134) Kelland, M. A.; Namba, T.; Tomita, T. Norwegian Patent Application 2278, 1999. (135) Kelland, M. A.; Tomita, T. Norwegian Patent Application, 1999 (withdrawn before publication). (136) Klomp, U. C. WO Patent Application 01/77270, 2001. (137) Rivers, G. T.; Crosby, D. L. U.S. Patent Application 2004/0110998, 2004. (138) Rivers, G. T.; Crosby, D. L. WO Patent Application 2004/22909, 2004. (139) Klomp, U. C. Shell Global Solutions, personal communication, 2004. (140) Dzialowski, A.; Patel, A.; Nordbo, K. In Proceedings of the Offshore Mediterranean Conference, Ravenna, Italy, March 28-30, 2001. (141) Fu, B.; Neff, S.; Mathur, A.; Bakeev, K. SPE Production and Facilities, August 2002; SPE 78823. This is the revised version of Fu, B.; Neff, S.; Mathur, A.; Bakeev, K. In Proceedings of the SPE Annual Technical Conference and Exhibition, New Orleans, LA, September 30October 3, 2001; SPE 71472. (142) Rasch, A.; Mikalsen, A.; Austvik, T.; Gjertsen, L. H.; Li, X. In Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, May 19-23, 2002. (143) Rasch, A.; Mikalsen, A.; Gjertsen, L. H.; Fu, B. In Proceedings of the 10th International Multiphase Conference, Cannes, France, June 1315, 2001. (144) Neubecker, K. BASF, personal communication, 2005. (145) Angel, M.; Neubecker, K.; Stein, S. World Patent Application WO 2004/042190, 2004. (146) Dahlmann, U.; Feustel, M.; Kayser, C.; Morschchaeuser, R. U.S. Patent Application 2004/0030206, 2004. (147) Thieu, V.; Bakeev, K.; Shih, J. S. U.S. Patent 6451891, 2002. (148) Toyama, M.; Seye, M. World Patent Application WO 02/10318, 2002. (149) Colle, K.; Talley, L. D.; Longo, J. M. World Patent Application WO 2005/005567, 2005. (150) Talley, L. D. Nucleation Workshop; Ecole Nationale Superior de Mines: St. Etienne, France, June 2003. (151) Glenat, P.; Peytavy, J. L.; Jones, N. H.; Grainger, M. In Proceedings of SPE Middle East Conference, Abu Dhabi, U.A.E, 2004; SPE 88751 (152) Fu, S. F. Nalco, personal communication, 2005. (153) Frostman, L. M.; Crosby, D. In Proceedings of the Deep Offshore Technology Conference, Marseille, France, November 19-21, 2003. (154) Mehta, A. P. Shell Global Solutions, personal communication, 2005. (155) Petrie, M. Clariant, personal communication, 2005. (156) Swanson, T. A.; Petrie, M.; Sifferman, T. R. Flow Assurance Forum, Galveston, TX, 2004.

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using THF hydrates at atmospheric pressure; however, this type of loop has limited scope of use.98 Most loops used today are (157) Swanson, T. A.; Petrie, M.; Sifferman, T. R. In Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, February 2-4, 2005; SPE 93158. (158) Collins, I. R. WO Patent Application 03/021078, 2003. (159) Frostman, L. M.; Downs, H. In Proceedings of the 2nd International Conference on Petroleum and Gas-Phase BehaViour and Fouling, Copenhagen, Denmark, August 2000. (160) Frostman, L. M. In Proceedings of the SPE Annual Technical Conference and Exhibition, Dallas, TX, October 1-4, 2000; SPE 63122. (161) Frostman, L. M.; Przybylinski, J. L. In Proceedings of the International Symposium on Oilfield Chemistry, Houston, TX, February 1316, 2001; SPE 65007. (162) Mehta, A. P.; Herbert, P. B.; Cadena, E. R.; Weatherman, J. P. In Proceedings of the Offshore Technology Conference, Houston TX, May 6-9, 2002; OTC 14057. (163) Frostman, L. M.; Crosby, D. In Proceedings of the Deep Offshore Technology Conference, Marseille, France, November 19-21, 2003. (164) Frostman, L. M.; Crosby, D. Poster presented at the SPE Forum on Gas Hydrates, St. Maxime, France, September 1999. (165) Thieu, V.; Frostman, L. M. In Proceedings of the International Symposium on Oilfield Chemistry, Houston, TX, February 2-4, 2005; SPE 93450. (166) Przybylinski, J. L.; Rivers, G. T. U.S. Patent 6596911 B2, 2003. (167) Klomp, U. C.; Le Clerq, M.; Van Kins, S. In Proceedings of the 2nd Petromin Deepwater Conference, Shangri-La, Kuala Lumpur, Malaysia, May 18-20, 2004. (168) Gateau, P.; Sinquin, A.; Beunat, V.; Vilagines, R. U.S. Patent Application 2004/0162456, 2004. (169) Palermo, T. IFP, personal communication, 2005. (170) Fadnes, F. H. Fluid Phase Equilib. 1996, 117, 186-192. (171) Le Porcher, E. M.; Peytavy, J. L.; Mollier, Y.; Sjoblom, J.; LabesCarrier, C. In Proceedings of the SPE Annual Technical Conference and Exhibition, New Orleans, LA, September 27-30, 1998 and references therein; SPE 49172. (172) Camargo, R.; Palermo, T.; Sinquin, A.; Glenat, P. Ann. N.Y. Acad. Sci. 2000, 912, 906. (173) Sinquin, A.; Bredzinsky, X.; Beunat, V. In Proceedings of the SPE Annual Technical Conference and Exhibition, New Orleans, LA, September 30- October 3, 2001; SPE 71543. (174) Sinquin, A.; Miao, M.; Beunat, V.; Jussaume, L. In Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, May 19-23, 2002. (175) Fu, S.; et al. Patent pending, filed October 2004. (176) Milburn, C. R.; Sitz, G. M. U.S. Patent 6444852 B1, 2002. (177) Dahlmann, U.; Feustel, M. U.S. Patent Application 2004/0163306, 2004. (178) Dahlmann, U.; Feustel, M. U.S. Patent Application 2004/0163307, 2004. (179) Dahlmann, U.; Feustel, M. U.S. Patent Application 2004/0164278, 2004. (180) Dahlmann, U.; Feustel, M. U.S. Patent Application 2004/0167040, 2004. (181) Dahlmann, U.; Feustel, M. U.S. Patent Application 2004/0159041, 2004. (182) Cowie, L.; Shero, W.; Singleton, N.; Byrne, N.; Kauffman, L. Deepwater Technology; Gulf Publishing Co.: 2003; pp 39-41. (183) Fu, S. F. In Proceedings of the 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 4040. (184) Burgazli, C. R. World Patent Application WO 2004/111161, 2004. (185) Burgazli, C. R.; Navarrete, R. C.; Mead, S. L. Presented at the Petroleum Society’s Canadian International Petroleum Conference, Calgary, Alberta, Canada, June 10-12, 2003; Paper 2003-070. (186) Storr, M. T.; Montfort, J.-P.; Taylor, P. C.; Rodger, P. M. In Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, May 19-23, 2002. (187) Storr, M. T.; Taylor, P. C.; Montfort, J.-P.; Rodger, P. M. J. Am. Chem. Soc. 2004, 126, 12569-1576. (188) Duffy, D. M.; Moon, C.; Irwin, J. L.; Di Salvo, A. F.; Taylor, P. C.; Arjmandi, M.; Danesh, A.; Ren, S. R.; Todd, A.; Tohidi, B.; Storr, M. T.; Jussaume, L.; Montfort, J.-P.; Rodger, P. M. Chemistry in the Oil Industry, Symposium VIII, Manchester, England, 2003. (189) Dahlmann, U.; Feustel, M.; Holtrup, F.; Jestel, M.; Fuss, R.-W.; Papenfuhs, B.; Steuer, M. WO Patent Application 02/084072, 2002. (190) Walker, V.; Ripmeester, J. A.; Zeng, H. WO Patent Application 03/087532, 2003. (191) Kelland, M. A.; Iversen, J. E. Unpublished results. (192) Makogon, T.; Sloan, E. D. In Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, 2002; pp 498-563. (193) Frostman, L. M. In Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, February 13-16, 2001; SPE 65006.

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high-pressure loops using natural gas, condensate, or oil and an aqueous phase. They can range from the mini-loop (e.g., 1/4 in. i.d.) to the full scale pilot loop of 4 in. i.d. or more.13,76 A disadvantage is that in some cases the pump can crush hydrates making AA experiments difficult to interpret. Early History of the Development of LDHIs The history of LDHIs actually begins in the early 1970s with a Russian engineer called Kuliev, who was well before his time.14 He was experiencing gas hydrate problems in his gas wells. He decided to try adding commercial surfactants to the top part of the wells and discovered that the hydrate problem went away. It is unclear from his paper how the surfactants (194) Crosby, D. Baker Petrolite, personal communication, 2005. (195) Boyne, K.; Horn, M.; Bertrane, D.; Fournie, F.; Cooper, T.; Quinn, P.; Coudeville, F.; Buchan, D.; Allan, K.; Arnott, S. In Proceedings of the Offshore Europe Conference, Aberdeen, UK, September 2-5 2003; SPE 83975. (196) Panchalingham, V.; Rudel, M. G.; Bodnar, S. H. U.S. Patent Application 20050081714, 2005. (197) Panchalingham, V.; Rudel, M. G.; Bodnar, S. H. U.S. Patent Application 20050081432, 2005. (198) Panchalingham, V.; Rudel, M. G.; Bodnar, S. H. U.S. Patent Application 20050085396, 2005. (199) Panchalingham, V.; Rudel, M. G.; Bodnar, S. H. U.S. Patent Application 20050085675, 2005. (200) Panchalingham, V.; Rudel, M. G.; Bodnar, S. H. U.S. Patent Application 20050085676, 2005. (201) Maximilian, A.; Neubecker, K.; Sanner, A. U.S. patent 6867262, 2005. (202) Dalhlmann, U.; Feustel, M. U.S. Patent Application 20050101495. (203) Moon, C.; Taylor, P. C.; Rodger, P. M. J. Am. Chem. Soc. 2003, 125, 4706. (204) Høiland, S.; Askvik, K. M.; Fotland, P.; Alagic, E.; Barth, T.; Fadnes, F. J. Colloid. Interface Sci. 2005, 287, 217. (205) Karaaslan, U.; Parlaktuna, M. Energy Fuels 2002, 16, 1387. (206) Zanota, M. L.; Dicharry, M.; Graciaa, A. Energy Fuels 2005, 19, 584. (207) Angel, M.; Stein, S.; Neubecker, K. U.S. Patent 6878788, 2005. (208) Ohtake, M.; Yamamoto, Y.; Kawamura, T.; Wakisaka, A.; de Souza, W. F.; de Freitas, A. M. V. J. Phys. Chem. 2005, 109, 16879. (209) Pakulski, M.; Qu. Q.; Pearcy, R. SPE International Symposium on Oilfield Chemistry, The Woodlands, TX, February 2-4, 2005; SPE 92971. (210) Szymczak, S.; Sanders, K.; Pakulski, M.; Higgins, T. SPE Annual Technical Conference and Exhibition, Dallas, October 9-12, 2005; SPE 96418. (211) Dong Lee, J.; Englezos, P. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 44. (212) Zeng, H.; Brown, A.; Wathen, B.; Ripmeester, J. A.; Walker, V. K. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 1. (213) Hawtin, R. W.; Moon, C.; Rodger, P. M. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 118. (214) Grainger, N.; Hawtin, R.; Moon, C.; Rodger, P. M.; Rogers, S. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 317. (215) Arla, D.; Dicharry, C, Graciaa, A.; Hurtevent, C.; Jussaume, L.; Peytavy, J. L.; Sinquin, A. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 322. (216) Vebenstad, A.; Larsen, R.; Straume, E.; Argo, C. B.; Fung, G. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 1193. (217) Camargo, R. M. T.; Concalves, M. A. L.; Montesami, J. R. T.; Cardoso, C.; Minami, K. Offshore Technology Conference, Houston, 2004; OTC 16687. (218) Palermo, T.; Mussomeci, A.; LePorcher, E. Offshore Technology Conference, Houston, 2004; OTC 16681. (219) Arjmandi, M.; Ren, S. R.; Tohidi, B. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 1178. (220) Clark, L. W.; Anderson, J. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 1249. (221) Oskarsson, H.; Uneback, I.; Navarrete, R. C.; Hellsten, M.; Lund, A.; Hjarbo, K. W. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 1283.

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worked, but it is the first recorded example of using a low dosage of a chemical to prevent hydrate plugging. We have to go forward to the mid 1980s before any further work on LDHIs took place. Conoco patented the use of scale inhibitors as gas hydrate inhibitors.15 They cited an example using a fairly high concentration of scale inhibitor to prevent gas hydrate in the gas well. The use of scale inhibitors at these concentrations is uneconomical for pipeline use, and no further work was reported. In 1987, IFP (The French Petroleum Institute) filed a series of patent applications on using surfactants as LDHIs.16-19 They listed a wide range of surfactants and basically claimed all surface-active chemicals as hydrate inhibitors. This last claim was to cause some concern to oil companies developing or using LDHIs and not wishing to be dependent on IFP’s patent. It was not clear from IFP’s patents which surfactants they were focusing on or the mechanism for their ability to prevent hydrate agglomeration. Later, it became apparent that the surfactants were producing a special kind of water-in-oil emulsion. (The surfactant dosage was ca. 0.8 wt % based on the water phase.) This emulsion confined hydrates to form within the water droplets, and the hydrates never agglomerated. The end product was a slurry of hydrate particles in a hydrocarbon phase. This we shall call the IFP AA mechanism. IFP calls their AAs dispersant additives. Examples of surfactants in IFP’s patents include diethanolamides, dioctylsulfosuccinates, sorbitans, ethoxylated polyols, ethoxylated fatty acids, and ethoxylated amines. They also gave examples of polymeric surfactants based on polyalkenyl succinic anhydride. The products in the examples in their patents showed only weak KHI effects. Also, in the late 1980s the Colorado School of Mines (CSM) was carrying out research on gas hydrates. CSM noticed that some materials catalyzed hydrate formation at low dosages and were encouraged by their consortium members to look for materials that would have the opposite effect. This initiated a long program on searching for kinetic hydrate inhibitors. CSM constructed a THF hydrate ball-stop rig and began screening a vast range of commercial products. One other initiative on LDHIs began in the late 1980s in Norway within the state-funded PROFF research program. The program included two projects on gas hydrates. The first, carried out at SINTEF, studied the consistency of hydrates under flowing conditions. They observed that at low water conversion the hydrates were sticky and liable to agglomerate and deposit on the pipe walls. However, at high water conversion the hydrates were a nonsticky powder that appeared to be transportable in the hydrocarbon phase. Thus if a chemical could be found to prevent the hydrates from agglomerating during the early stages of water conversion, it should be possible to produce a transportable slurry of hydrates. The second gas hydrate project in the PROFF program was carried out at RF. RF began screening additives in high-pressure cells to observe any inhibiting effects. The best product from RF’s studies was a quaternary corrosion inhibitor from Blacksmith, which showed some positive AA effect, but the work was not taken further. In 1991, at the end of the PROFF program, Esso Norge (now ExxonMobil Norge) began sponsoring RF on a 3-year program to develop LDHIs. The results were to be fed into Exxon’s own internal program on LDHIs at Exxon Production Research (EPR, now ExxonMobil Upstream Research Company) in Houston. Meanwhile, Shell and BP were also putting together their own programs for developing LDHIs. Thus, at the beginning of the 1990s, three institutes and three oil companies were involved in the search for LDHIs.

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Figure 3. Structure of poly(vinylpyrrolidone). Figure 6. Structures of polyethylacrylamide, polyvinyl-N-methyl acetamide, and polyethyloxazoline.

Figure 4. Monomer units in the terpolymer Gaffix VC-713.

Figure 5. Structure of polyvinylcaprolactam.

Early 1990s Discovery of PVP and PVCap at CSM. The first breakthrough in KHI technology came at CSM in 1991. In their ballstop rig tests they came across a polymer, poly(vinylpyrrolidone) (PVP), which delayed the formation and the agglomeration of THF hydrates (Figure 3).20,21 The apparent AA effect of PVP as noted by CSM does not transfer to real hydrocarbon systems, which plug rapidly once a certain amount of gas hydrate is formed, but the KHI effect does. They also found that hydroxyethylcellulose (HEC) performed moderately in the ball-stop rig, but PVP was better. PVP is the five-ring member of the series of polyvinyllactams, which have two main suppliers, ISP and BASF. Nippon Shokubai also manufactures PVP. CSM kept to this series and tested a hair-care product from ISP called Gaffix VC-713 a terpolymer, which contains a high proportion of the seven-ring monomer, vinylcaprolactam (VCap) as well as vinyl pyrrolidone (VP) and dimethylaminoethyl methacrylate (DMAEMA) (Figure 4). This polymer outperformed PVP in the ball-stop test. CSM then set up a collaboration agreement with ISP to develop better KHIs. Realizing that VCap was the key monomer in Gaffix VC-713, CSM tested the homopolymer polyvinylcaprolactan (PVCap) (Figure 5). PVCap gave a similar performance to Gaffix VC-713 and became a standard by which other KHIs would be compared. This early high molecular weight version of PVCap (or Gaffix VC-713) gave 24 h hydrate nucleation delays up to a subcooling of 8-9 °C at a dosage of 0.5 wt %. The results from CSM’s work were patented22,23 and communicated to their consortium members, who began to test the polyvinyllactams in their own laboratories. CSM also discovered that other polymers could act as synergists increasing the performance of the polyvinyllactams, particularly PVCap.24 In their patent applications in 1994, they claimed polyelectrolytes, polyether block copolymers, polyvinylamides, polyalkylacrylamides, and polyalkyloxazolines as synergists for polyvinyllactams. The examples included polyethylacrylamide, poly(N-methyl-N-vinyl acetamide), and polyethyloxazoline (Figure 6). Each of these polymers was shown to be a hydrate growth inhibition synergist for PVCap. In the patent application, the amide polymers were claimed to have C1-4 alkyl groups, although no examples were given with C3-4 alkyl groups.

Shell’s Research and the Discovery of Quaternary Surfactant AAs. In the early 1990s, Shell became aware of certain fish, such as the winter flounder, which contained anti-freeze proteins and glycoproteins (AFPs and AFGPs).25,26 These AFPs and AFGPs prevent ice crystals from forming in the fish by binding to the surface of ice nuclei. This allows the fish to survive in sub-zero temperatures. It occurred to Shell that artificial AFPs and AFGPs might bind to the surface of gas hydrate nuclei. The AFPs and AFGPs themselves were found to be expensive and fairly poor KHIs, but a screening of perceived “protein-like” water-soluble polymeric amides led Shell to independently (although somewhat later than CSM) discover PVP. Shell tested PVP and a polymer called Antaron P-904 or Agrimer P-904 (butylated PVP) in its own laboratories. They discovered that the butylated PVP performed better than PVP. They patented the results in 1992 but later reassigned the rights to CSM.27 After studying AFPs and PVPs, and based on the results of a field trial in Michigan and in the Groningen field in which PVP failed to prevent hydrate formation, Shell realized that kinetic inhibitors would not give them the high subcooling performance they desired for their oncoming deepwater projects. They calculated using standard nucleation theory that you could not stop hydrates nucleating indefinitely at subcoolings above 10 °C.28 Since Shell was mostly interested in deepwater applications with high subcoolings, they concentrated on developing AAs. The first useful class of AAs they came across were the alkylarylsulfonates.29 (Interestingly, these surfactants were chosen among others by Kuliev back in the early 1970s.) However, it was found that their performance was mediocre, so Shell moved on to other products. The next class of products that was patented by Shell were alkyl glucosides.30 The multi-hydroxyl headgroup was envisaged to bond to water molecules on the hydrate surface. However, after further testing, it appeared that these surfactants gave very limited AA performance, and Shell moved on to what was to be a major breakthrough. Shell reasoned that quaternary ammonium surfactants would be ideal candidates as AAs as they are known to be good particle dispersants. Shell was also aware of the work on quateranry ammonium and phosphonium salt clathrates as reported by Jeffrey.230 In particular, they noticed that tetrabutylammonium bromide (TBAB) and tetrapentylammonium bromide (TPAB) (222) Zeng, H.; Walker, V. K.; Ripmeester, J. A. In 5th International Conference on Gas Hydrates, Trondheim, Norway, June 13-16, 2005; p 1295. (223) Pakulski, M.; Hurd, D. In 5th International Conference on Gas Hydrates, Trondheim, Norway, Norway, June 13-16, 2005; p 1444. (224) Cowie, L.; Bollavaram, P.; Erdogmus, M.; Johnson, T.; Shero, W. Offshore Technology Conference, 2005; OTC 17328. (225) Pakulski, M.; Dawson, J. C. U.S. Patent Application, 2004/ 0231848, 2004. (226) Pakulski, M.; Dawson, J. C. U.S. Patent 6756345, 2004. (227) Kvamme, B.; Huseby, G.; Førrisdahl, O. K. Mol. Phys. 1997, 90 (6), 979. (228) Lee, J. D.; Englezos, P. Chem. Eng. Sci. 2005, 60, 5323. (229) Lee, J. D.; Englezos, P. Chem. Eng. Sci. 2006, 61, 1368. (230) Jeffrey, J. Inclusion Compounds, Vol. 1; Academic Press: New York, 1984; pp 159-190.

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Figure 7. Structure of good quaternary ammonium or phosphonium hydrate growth inhibitors, where M ) N or P and at least two of the R groups are n-butyl, n-pentyl, or isopentyl.

Figure 8. Structure of Shell’s quaternary AAs (R1 ) long chain hydrocarbon tail; R2 ) n-butyl, n-pentyl, or isopentyl; M ) N or P; X is an optional spacer group).

had clathrate hydrate structures with the 51264 cage, the same cage found in structure II gas hydrate. They tested a large range of quaternary salts and found that salts with two or more n-butyl, n-pentyl, and isopentyl groups were the best at delaying the growth of THF hydrate crystals (Figure 7). Shell’s next inspiration was to replace one or two of the small alkyl groups with a long hydrophobic tail (8-18 carbon atoms). These materials are not anti-nucleators, so they are poor KHIs. In fact, some of these small quaternary ammonium and also phosphonium salts can promote hydrate nucleation by being templates. Shell called these chemicals “hydrate growth inhibitors” (HGIs). These quaternary surfactants salts with two or three n-butyl, n-pentyl, and isopentyl groups performed extremely well as hydrate AAs (Figure 8). Shell patented their results at the end of 1993.31 The mechanism for Shell’s quaternary AAs is not the same as that for IFP’s water-in-oil emulsifying AAs. Shell’s AAs are designed with a hydrate-philic (hydrate seeking) headgroup and a hydrophobic tail. Being surfactants, the AAs will accumulate at the water-oil interface, where hydrate formation first occurs. The hydrate-philic headgroup, which is the quaternary center, will bind to hydrate particles. The butyl/pentyl groups penetrate open 51264 cavities on the hydrate surface and can even become embedded in the surface as the hydrate grows around the alkyl groups. The long hydrophobic tail prevents the hydrate from continuing to grow on that surface. The hydrophobic tail also makes the surface more attractive to the hydrocarbon phase. Once several AA molecules have attached to the surface, the particle is easily dispersed in the hydrocarbon phase. The quaternary AA will also be attracted to the pipe walls (also indicated by the fact that some of these AAs are very effective corrosion inhibitors). The hydrophobic tails will then help prevent hydrate from growing or adhering to the pipe walls. All in all, the Shell AAs are well-designed molecules for their task, and it seemed like Shell was on to a winner, perhaps even the silver bullet. Shell divided up the development work. Shell Houston was to develop the more water-soluble single-tailed quaternary AAs, while Shell Amsterdam was to optimize the more oil-soluble twin-tailed quaternary AAs. BP’s Research. BP’s own work on LDHIs began with the study of amino acids. Tyrosines and related chemicals gave some weak KHI effect, and the results were patented.32 BP also patented various blends containing vinyl polymers such as PVP and synergists.33-35 The synergists included corrosion inhibitors, amino alcohols, amino carbohydrates, lactones, amino acids, hydroxyacids, and glycol ethers. Some of the patent applications were later withdrawn in favor of better KHIs. BP was a member of the CSM consortium and therefore had access to the

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polyvinyllactam KHIs. They were also collaborating with Shell in the mid 1990s and had access to their quaternary AA technology. They combined a polyvinyllactam with small quaternaries such as tetrabutylammonium bromide (TBAB) and found they gave a synergistic KHI effect.36 (Actually, Shell had included blends of Gaffix VC-713 and tetrapentylammonium bromide (TPAB) in their AA patent but was not so interested in the KHI effect.) BP called their blends “threshold hydrate inhibitors”, but perhaps since the acronym THI could be confused with thermodynamic hydrate inhibitors, it did not catch on. BP patented the results along with blends with other production chemicals.37,38 BP had their sights set on using their synergistic KHI blends on their Southern North Sea gas fields where the subcooling was less than 10 °C and ideal for KHIs. BP was also thinking ahead to a planned field in the ETAP province where the subcooling was about 6-8 °C. BP licensed their KHI technology to TR Oil Services (TROS, now Clariant Oilfield Services), who were used to bring the technology to the field. TROS built their own pressure cells based on BP’s design and worked on finding the optimum blends for BP’s fields. BP also initiated molecular modeling studies of PVP on gas hydrate surfaces at the University of Reading, U.K.39 The results show that the pyrrolidone ring bonded to the structure I hydrate surface via hydrogen bonding to the amide and van der Waals interaction between the ring and the hydrate surface. At that time, no studies were carried out showing interactions with or penetration of structure II cavities. It is thought that TPAB (or TBAB) works synergistically with PVCap because of their different geometries. Thus, TPAB and PVCap should attach to different sites on the hydrate crystal surface. So what is happening at the molecular level with TPAB? Molecular modeling carried out at the University of Reading for RF in the mid 1990s shows that TPAB penetrates a 51264 cavity on the 1,1,1 structure II hydrate surface. Two of the other pentyl groups lay in channels on the hydrate surface where new 51264 cages would normally be formed. It therefore seems possible that these cages could partially form, trapping or imbedding the pentyl groups in the hydrate surface. Below the critical nuclear size, growth of the nuclei is energetically unfavorable (∆G is positive). So, TPAB will not be embedded in the surface of the nuclei but will more easily detach. Above the critical nuclear size, TPAB can become embedded in the hydrate surface as partial hydrate cages form around the pentyl groups, but further structure II growth is prevented by the remaining pentyl groups. The embedding mechanism for the quaternary salts explains why tetrapropylammonium bromide (TPrAB) and tetrahexylammonimum bromide (THAB) are poor hydrate growth inhibitors. TPrAB has an even weaker van der Waals interaction with cavities and channels on the hydrate surface than TBAB. TPrAB will more easily detach from the surface even if some water molecules start to build cavities at the ends of the propyl groups (i.e., it is less quickly embedded in the surface). On the other hand, THAB has alkyl groups that are too long to give strong van der Waals interactions with more than one cavity on the surface and will therefore detach easily. In fact, THAB cannot easily get embedded in the surface of structure I or II hydrate. RF-Rogaland Research (RF) Projects for Esso/Exxon (now ExxonMobil) and Elf (now Total). Back at the beginning of the 1990s, RF began their research program with Essso Norge and Exxon Production Research (now ExxonMobil Upstream Research Company, EMURC). RF carried on from where they left off in the PROFF project by looking for a better AA than

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Figure 9. Structure of poly-L-proline.

the Blacksmith corrosion inhibitor. Tests were carried out on a condensate with 20% water cut in stirred sapphire cells. The cell was cooled from 20 to 4 °C over 2 h at a pressure of ca. 90 bar at 700 rpm stirring rate. RF tested a range of surfactants with varying headgroups, ionicity, and tail-lengths. Some surfactants with multiple hydroxyl or carboxylic acid headgroups were prepared in-house hoping to find hydrate-philic surfactants. Positive results were obtained with some neutral and cationic surfactants, but only at low subcooling. (An n-butylpolyglycol from Witco gave a slower growth of hydrates but was not identified as a synergist for KHI polymers at the time). RF had the right idea for making AAs, but not the right hydrate-philic headgroups such as those found by Shell. In their work on looking for surfactant AAs, RF observed that products with a high degree of propoxylation gave beneficial effects. This initiated a study of over 30 polypropoxylated products. Polypropoxylates are interesting molecules. They are not as hydrophilic as polyethoxylates, which tend to surround themselves with water molecules, neither are they like alkylchains, which tend to partition in nonpolar solvents. Because of this amphiphilic behavior, polypropoxylates often end up at the interface between the aqueous and nonpolar phase. This makes them useful as defoamers and demulsifiers. The best AA was an amine polyalkoxylate having a molecular weight of approximately 6000. This product gave excellent dispersion of hydrates in the hydrocarbon phase at subcoolings below ca. 10-13 °C at 700 rpm stirring. It is interesting to observe that BJ Unichem patented related polyether/polyamines as LDHIs in the late 1990s and that Akzo Nobel patented polyalkoxylated amines in the new millennium. RF carried out two smaller projects on LDHIs for Elf (now Total) in the early 1990s.40-42 The first project looked at natural and biodegradable surfactants as potential AAs. The best product was Plantaren 600 CPUS, an alkyl glucoside from Henkel. This product worked well at moderate subcooling at 0.5 wt %. It is interesting that Shell patented similar surfactants as AAs called alkyl glycosides. The second project tested various water-inoil emulsions made with commercial monomeric surfactants. No good results were obtained, indicating that compounds that are effective in creating water-in-oil emulsions do not necessarily show good AA performance. Returning to the Exxon/Esso program, RF moved away from studying AA surfactants and looked more at water-soluble polymers as potential KHIs. The first class of polymers that gave results were polyamino acids, which are related to fishanti-freeze proteins. The best results were obtained with polyL-proline (Figure 9). This polymer gave a subcooling of only a few degrees Celcius, but its structural relationship to PVP was the most important feature. Both poly-L-proline and PVP contained a five-member ring and an amide group in the repeating unit. During RF’s work for Exxon/Esso, RF did indeed discover PVP as a KHI giving about 5 °C subcooling and shortly thereafter PVCap, which gave about 8-9 °C subcooling.43 These discoveries were made independently of, but later than those of CSM and Shell (PVP only). RF showed that PVCap gave a lower subcooling performance with structure I methane hydrates as compared to structure II natural gas hydrates.80 A reduced

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Figure 10. Structure of N-methyl-N-vinylacetamide:vinyl caprolactam 1:1 copolymer (VIMA:VCap) where a ) b.

Figure 11. Structures of polyacryloylpyrrolidine, polydiethylacrylamide, and polyisopropylacrylamide.

Figure 12. Structures of polyethylmaleimide, ring-opened polyethyloxazoline, and ring-closed polyethyloxazoline.

performance with structure I hydrates relates to the higher symmetry of structure I hydrate crystals. On a suggestion from Exxon to test VCap copolymers, RF synthesized and tested N-methyl-N-vinylacetamide:VCap 1:1 copolymer (VIMA:VCap 1:1) (Figure 10). This copolymer performed better than PVCap by 2-3 °C subcooling in a sapphire cell test. Exxon, who had commissioned the work at RF and owned the results, synthesized similar copolymers in their own laboratories. Tests in Exxon’s mini-loop confirmed RF’s findings, subsequently Exxon and a researcher at RF jointly patented the copolymer as a KHI in 1995.44 RF synthesized and tested other VCap copolymers such as VCap:vinyl imidazole. This also gave good performance, but the work was not taken further. The last polymer of note that RF tested was a ring-opened polyethyloxazoline, which contains ethylamide groups in the repeating unit. Although the performance in the THF hydrate ball-stop rig was only moderate, interestingly it belonged to a different class of KHIs than the vinyl lactam polymers. LDHIs from Exxon Production Research (EPR). Esso Norge and Exxon Production Research (EPR, now EMURC) discontinued their support of RF’s work at the end of 1994 and decided to continue the work on their own. Exxon had deduced that a key element in many KHI polymers was the presence of an amide group attached to a hydrophobic group in the repeating unit. Exxon had their own theory for KHIs which involved maximizing the hydration volume of the polymer in water. The hydrophobic group would form hydrate cavities around it while the carbonyl oxygen atom of the amide group would form hydrogen bonds with water molecules. Several of these interactions would be needed; hence, a polymer or oligomer was required. The inhibitor could thus interrupt nucleation of hydrates in the water phase. Exxon used molecular modeling to deduce the best inhibitor structures. Their theory is somewhat related to that of other modelers.45-47,213,214,227 Exxon filed patents on a number of classes of polymercontaining amide groups. These included polyalkylacrylamides48 (Figure 11), polydialkylacrylamides48 (Figure 11), polyvinylamides,49 polyallylamides,50 polymaleimides51 (Figure 12), polymers of cycliciminoethers52 such as polyacylalkyleneimines and polyalkyloxazolines (ring-opened and ring-closed) (Figure 12), and amides or esters of N-acyldehydroalanine.53 The best

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Figure 13. Polyisobutylacrylamide.

Figure 14. Polyisopropylmethacrylamide.

Figure 15. Structure of polyisopropylmethacrylamide:N-vinyl-Nmethyl acetamide copolymer (VIMA:iPMA).

KHI performance was obtained with the acrylamide polymers, probably because the optimum size alkyl group was not used for the other classes of polymer. (For example, it is difficult to synthesize N-vinyl amides with the wide variety of alkyl groups that is possible for acrylamides.) The best acrylamide homopolymer was polyacryloylpyrrolidene (polyAP) followed by polydiethylacrylamide and then polyisopropylacrylamide (polyiPAm). Polyisobutylacrylamide was not tested because it is insoluble in water (Figure 13). (In 1996, RF tested some copolymers of isobutylacrylamide with VIMA and VP with KHI performance results similar to polyisopropylacrylamide.) Exxon found that the addition of a methyl group to the backbone of the acrylamide polymers increased their performance. For example, the subcooling performance of polyisopropylmethacrylamide (Figure 14) was found to be approximately 2 °C greater than that of polyisopropylacrylamide. Exxon patented this idea of adding a small alkyl group to the backbone of a KHI polymer to increase its performance.54 The only polyvinylamide to be tested by EPR was polyVIMA, which was found to be a poor KHI. However, copolymers of VIMA with other alkylamide polymers gave surprisingly high subcoolings, very much like what RF had seen with the VIMA: VCap copolymer. For example, compared to polyiPMAM,55 a 1:1 VIMA:iPMA copolymer (Figure 15) gave a 4.2 °C higher subcooling. For this copolymer, Exxon reported an actual subcooling of 17.5 °C. However, it should be noted that a subcooling of 4 °C was obtained by a test with no additives, and Exxon’s test method is different from that of others. Other interesting results were obtained with ring-closed oxazoline polymers. These represent the first class of polymers that do not contain amide groups and still show substantial KHI (anti-nucleation) performance. Exxon also found that a VIMA:vinyl butyrate copolymer gave a surprisingly high performance both as an anti-nucleator in the high-pressure mini-loop and as a hydrate crystal growth inhibitor with THF hydrates. The mini-loop performance was better than PVCap, polyiPMAM, and polyAP. (At that time none of the polymers were optimized for molecular weight.) It is clear

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that the butyrate groups in the VIMA:vinyl butyrate copolymer contribute to the inhibition performance since it is known that polyVIMA is a fairly poor KHI. As a hydrate crystal growth inhibitor, the propyl part of the butyrate group may be interacting with open large cavities on the structure II hydrate surface. It remains unclear whether the ester groups as well as the amide group in the VIMA units form hydrogen bonds with the hydrate surface. It is probable that both do, but hydrogen bonding with the ester group is probably weaker than with the amide. We can deduce this since polyvinyl esters are not water-soluble (low hydrophilicity), but polyvinylamides are water-soluble. (For example, polyvinyl acetate is not water-soluble at room temperature, but polyvinylacetamide is. Again, polyethylacrylate is not water-soluble but polyethylacrylamide is.) In fact, the reduced hydrophilicity of VIMA:vinylbutyrate polymer probably contributes to the increased KHI performance relative to polyVIMA. This is because, compared to polyVIMA, the copolymer will favor interactions with the hydrate surface more than with free water. Several research groups believe that the hydrophilicity of a polymer (measured as its cloud point) is related to its performance as a KHI, but no systematic study has been published. In addition, no group has studied watersoluble vinyl ester copolymers as KHIs. Exxon did carry out a study that concluded that cloud point is not a sufficient condition for increasing KHI performance.111 Cloud point can be adjusted by total carbon in the monomer or by salinity of the solution. Exxon found there was no change in rank order of subcooling for various structures when salinity greater than seawater was scanned. Increasing carbon in the monomer only resulted in an insoluble polymer that cannot inhibit hydrates. Exxon concluded that the exact positioning of carbon in a monomer or polymer is more important than the cloud point. This is the point of their methylated backbone patent where a carbon on the backbone was more effective than another carbon in the pendant amide group. Exxon obtained one of the polyalkyloxazolines from Nippon Shokubai in Japan (NS). After discussions with Exxon, NS became interested in LDHIs and built their own THF hydrate ball-stop rig. NS synthesized polyAP and found that it gave a very good performance in their rig. They patented the results in Japan in 1994,56 not knowing that Exxon had already done that a few weeks before. NS and RF remained unaware of this until 1996. NS also filed a patent application on alkyl acrylate: VP copolymers.57 They found, for example, that butyl acrylate: VP gave increased KHI performance as compared to PVP. The argument for its increased performance is identical to that given above for VIMA:vinyl butyrate copolymer tested by Exxon (i.e., a higher “hydrate-philicity” of the copolymer as compared to PVP and interaction of the butyl ester group with the hydrate surface). Exxon applied for two further patents in 1994 and 1995. The first patent covered surfactants with a hydrophobic tail of 12 carbons or less as AAs,59 while the second patent covered their use as synergists with KHI polymers.60 Examples given included butyl sulfate, sodium valerate, zwitterionics such as butyldimethylammonium butylene sulfonate, and N-dodecylpyrrolidone. On their own, these products gave reasonable performance in the THF hydrate ball-stop rig. When combined with alkylamide polymer KHIs, they gave good synergistic results. This can be explained by their function as hydrate growth inhibitors. The butyl group or pyrrolidone group can interact with open 51264 cavities on the hydrate surface. RF confirmed that butyl sulfate is indeed a hydrate growth inhibitor, although weaker than TBAB, in the single-crystal THF hydrate growth test.

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Exxon conducted two mini-loop AA tests on butyl sulfate. At 0.25 wt % no effect was observed, but at 0.5 wt % the blocking temperature of the loop was over 13 °C below the equilibrium temperature. RF tested butyl sulfate as an AA in their sapphire cells but obtained very poor results. It seems that these small surfactants are best used as synergists for KHI polymers. IFP and Emulsion AAs. The last institute to be mentioned in the early 1990s is IFP. IFP continued to patent classes of AAs in the early 1990s.61-67 It was difficult to tell what chemistry they were focusing on, but IFP had begun to narrow their choice of dispersants to just a few products by 1994. It seemed from their patent application that they were concentrating on polymeric emulsifiers such as polyglycol derivatives of polyalkenylsuccinic anhydride. Early conference talks in Stavanger, Norway, in 1992 described the use of 0.5-2.0 wt % dispersant additives.68 IFP later gave a talk at a seminar in Trondheim, Norway, in 1994 where they showed successful results of an AA at 0.8 wt %.69 In addition, Norsk Hydro tested IFP’s products in autoclaves and got promising results using a dosage of 2.0 wt %. Mid 1990s At the beginning of the 1990s, LDHI technology was just a good research idea with a lot of potential. By the mid 1990s, many of the classes of LDHIs used today had been discovered, although there was plenty more chemistry yet to come. The service companies and several chemical companies began to get actively involved in designing and formulating LDHIs hoping to get a slice of the market that was opening up. By 1995 Shell, BP, and Exxon had decided on their strategies for developing their LDHIs. Shell had their quaternary surfactant AAs (“quat AAs”), which gave high subcoolings, BP had settled for KHI blends that gave moderate subcooling, and Exxon had a range of KHIs that could give a fairly high subcooling in their mini-loop. First KHI Field Trials. The polyvinyllactams discovered by CSM were beginning to reach the market via the chemical suppliers, ISP and BASF, and the service companies. One service company even advertised PVP under a new brand name as an effective KHI at 50 ppm at the exhibition hall of the SPE Technical Conference in 1994. It remained to be seen who would be first to try out these new LDHIs in the field. Who would dare risk using these new products with no track record? The first pioneers to report their field trials were Arco,70 Texaco,71 and BP.72 Arco was the first to go offshore in 1995 with a KHI when they tested Gaffix VC-713 on a Southern North Sea gas field. The trials showed that 0.5 wt % of polymer could handle 8-9 °C of subcooling. Texaco tested PVP in Wyoming and Texas showing it is useful at limited subcooling only. BP carried out six field trials on another Southern North Sea gas field (Ravensburn-Cleeton) in 1995-1996 using their KHI blends formulated by TR Oil Services (now Clariant). The KHI blends were based on mixtures of TBAB and VCap polymers. (Besides being a synergist, TBAB has the additional beneficial effect of increasing the cloud point of the VCap polymer). In addition, TBAB is roughly half the price of PVCap. The field trials were successful up to the maximum subcooling of 10 °C. This success led BP to switch from glycol to KHI on their West Sole/Hyde, 69 km wet gas pipeline in 1996 where the subcooling was a maximum of 8 °C.73,74 This became the first offshore field application of a KHI and, in fact, any LDHI. CSM’s Consortium. CSM continued to screen potential LDHIs up until the mid 1990s, but they did not make any further KHI discoveries besides that of the polyvinyl lactams and their

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synergy with some other polymer classes. CSM carried out a range of studies on PVCap, PVP, and VP:VCap copolymers supplied by ISP and BASF.75 They showed for example that laboratory autoclave experiments could be correlated with experiments in a pilot loop, in this case EPR’s loop.76,239 They also showed that the performance of PVCap was negatively impacted by methanol and a low concentration of salt.77,238 Higher salt concentrations (> 5.5 wt %) were actually beneficial to the performance of PVCap. In addition, CSM carried out studies on the dependence of the performance of PVCap on molecular weight.78 CSM found that the highest subcooling performance was obtained with a PVCap having a molecular weight of 900. Samples of PVCap were made at the University of Akron in Ohio. The University only measured the number average molecular weight, not the weight average molecular weight, which is the more common figure to quote. It is therefore difficult to say how many monomer units are in CSM’s low molecular weight PVCap polymers. Even so, at these low molecular weights, it is probably best to use the term oligomer rather than polymer. CSM also carried out extensive studies on THF hydrates giving structure II and ethylene oxide hydrate giving structure I.79 They confirmed RF’s findings80 with gas hydrates that PVCap gave a lower subcooling performance on structure I hydrate as compared to structure II hydrate at the same dosage. In addition, work carried out at CSM on the growth of single THF hydrate crystals showed that stirred solutions needed a lower dosage of KHI to totally inhibit crystal growth as compared to static solutions. This has implications in pipelines during shut-in situations. The dosage necessary to inhibit hydrates will be higher in a shut-in than under flowing conditions. This is because polymer diffusion to the hydrate surface is slower under static conditions. This also implies that a low molecular weight polymer with high mobility will perform better than a larger polymer. ISP and BASF. CSM’s agreement with ISP was not satisfactory for ISP, who wanted better control of the market for their KHI polymers. Consequently, ISP ended their collaboration with CSM and built their own autoclave equipment. In this way, ISP could test any new polymers synthesized in their laboratories and patent any new LDHI inventions. The first patent that ISP filed concerned the use of small alcohols and glycol ethers with a tail of 3-5 carbon atoms as synergists for vinyl caprolactam polymers.81 The main synergist example was butyl glycol ether (BGE). BGE was not only a good synergist for polymers such a PVCap, it could also be used as the solvent. This patent application was a problem to ISP’s competitor BASF. At the time BASF had been using small alcohols and glycol ethers in the manufacture of VCap polymers. Now BASF was forced to find other solvents. In the end, BASF ended up (231) Crosby, D. L.; Rivers, G. T.; Frostman, L. M. U.S. Patent Application 2005/0261529, 2005. (232) Kelland, M. A.; Svartaas, T. M.; Øvsthus, J.; Tomita, T.; Mizuta, K. Chem. Eng. Sci. Submitted for publication. (233) Kelland, M. A.; Svartaas, T. M.; Øvsthus, J.; Tomita, T.; Chosa, J. Chem. Eng. Sci. Submitted for publication. (234) Arjmandi, M.; Tohidi, B.; Danesh, A.; Todd, A. C. Chem. Eng. Sci. 2005, 60, 1313. (235) Koh, C. A.; Westacott, R. E.; Zhang, W.; Hirachand, K.; Creek, J. L.; Soper A. K. Fluid Phase Equilib. 2002, 192, 4, 143. (236) Koh, C. A.; Wisbey, R. P.; Wu, X. P.; Westacott, R. E.; Soper A. K. J. Chem. Phys. 2000, 113, 6390. (237) Kvamme, B.; Kuznetsova, T.; Aasoldsen, K. J. Mol. Graphics Modell. 2005, 23, 524. (238) Sloan, E. D.; Subramanian, S.; Matthews, P. N.; Lederhos, J. P.; Khokhar, A. A. Ind. Eng. Chem. Res. 1998, 37, 3124. (239) Lederhos, J. P.; Sloan, E. D. In SPE Annual Technical Conference and Exhibition, October 5-9, 1996, Denver; SPE 36588.

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using ethylene glycol and water as solvents for their products. ISP applied for several further patent applications in the late 1990s, which will be discussed later. In contrast to ISP, their competitor in vinyl lactam polymers, BASF, took a less active stance in trying to capture the market for their KHI polymers. During the mid 1990s they did not test their own polymers as LDHIs, nor did they file patent applications for any of their own products. (BASF did patent vinyl formamide copolymers with more hydrophobic monomers, but the performance of them appears to be poor.82) Instead, BASF relied on synthesising polymers at the request of any customer using their extensive experience in polymer chemistry, which went beyond the polyvinyllactams. Later BASF began to be more active in testing and patenting their own discoveries. ExxonMobil Research. Exxon (now ExxonMobil) approached BASF because they wanted to commercialize the VIMA:VCap copolymer that had been patented in 1995. BASF made samples of VIMA:VCap, which ExxonMobil tested in their loops. When ExxonMobil was convinced of the improved performance of VIMA:VCap copolymer, they signed a license agreement with Nalco Exxon Energy Chemicals (now Nalco) to commercialize the technology. ExxonMobil began using KHIs in the field from 1996.83,84 ExxonMobil and Nalco reported several field applications in 1999-2001 including the first deepwater application of an LDHI.85,86 Royalties from the use of VIMA:VCap copolymer were paid by Nalco to ExxonMobil and CSM because the copolymer contained VCap monomer. Sales of the copolymer were low until a North Sea application greatly increased the volume. Today however, VIMA:VCap is no longer commercially available due to the difficulty in obtaining the VIMA monomer and its high price. In the mid 1990s Clariant was the only manufacturer of VIMA monomer, which was used for making cheese wrap. Nalco bought a quantity of this monomer from Clariant to make VIMA:VCap copolymer, but they have not bought anymore since then. BASF and ISP have both been approached by Nalco to make further quantities of VIMA:VCap copolymer, but it seems that neither company has delivered this product. The other class of polymer that ExxonMobil wanted to commercialize was based on alkylacrylamides. They had used a proprietary theory to deduce that isopropylmethacrylamide (IPMA) polymer was the most potent KHI of the alkylacrylamides. Since it performed better than isopropylacrylamide polymer, ExxonMobil deduced that adding a methyl group to the backbone was beneficial. The same benefit was found for other polymer classes such as polyvinyloxazolines. ExxonMobil patented the idea of adding a methyl group to the backbone to increase polymer performance.54 ExxonMobil’s theory, that led them to the conclusion that the IPMA polymer is the most effective alkylacrylamide polymer, is proprietary. The theory has to do with maximizing the hydration volume of the polymer in order to perturb the nucleation of hydrates. Clearly IPMA and VCap polymers must be perturbing water molecules around the side chain functional groups at some distance so that they are unavailable for forming hydrate clusters, thereby preventing hydrate nucleation. They exploited this idea in a new patent application on modified AMPS (acrylamidopropylsulfonic acid) polymers. (Figure 16).87 PolyAMPS is a poor KHI, but ExxonMobil found that the addition of a hydrophobic tail between the amide and the sulfonic acid group increased the performance of the polymer. The optimum size of the tail was 5 carbon atoms long. A homopolymer of this monomer gave a performance in ExxonMobil’s mini-loop, similar to PVCap. Moreover, copolymers

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Figure 16. Structure of modified AMPS polymers where R1 is an alkyl tail of 1-6 carbon atoms and R2 is H or Me.

of this modified AMPS monomer with VCap gave even better performance. However, it does not appear that ExxonMobil has sought to commercialize this technology. Shell’s Quaternary AAs. While the first KHI field trials were taking place, Shell was preparing the first generation of the quaternary AAs. The Shell U.S. team was responsible for the development of the single-tail quaternary AA. The preferred molecules in their patents contained a hydrophobic tail of 1014 carbon atoms, a tributylammonium or tripentylammonium headgroup, and a counterion. (A tripentylammonium headgroup would give a better hydrate growth inhibitor, but the precursor tripentylamine is roughly twice the price of tributylamine.) This quaternary AA performed extremely well coping with subcoolings of 20+ °C in systems containing saline (> 1.5 wt % salt) water. However, the AA did not perform so well in freshwater. This may be due to the strength of the ion pairing in an aqueous phase of low ionicity. Second, the concentration needed for optimum performance was fairly high at 0.6-1.0 wt % actives. Baker Petrolite was given a worldwide license to commercialize the technology. They spent the mid and late 1990s building their own LDHI test equipment and getting up to speed on testing procedures. Shell and Baker Petrolite’s biggest hurdle in getting the singletail water-soluble quaternary AA to the field was its environmental impact. Quaternary surfactants are known to be toxic, and the quaternary AA was no exception. Added to that, the majority of the quaternary AA partitioned into the aqueous phase and would therefore be discharged into the sea (although the single-tail quaternary AA is seen to phase separate in a warm oil-water separator containing high salinity produced water. Therefore in such cases very little of it may be discharged in the overboard water). Second, being a tetraalkyl quaternary AA, it had low biodegradability. This meant that the toxic molecule would survive in the sea for a considerable time and could possibly bioaccumulate. The environmental impact of the quaternary AA made Baker Petrolite initially unsure about its commercial viability. Being a new molecule, it had to go through pre-market notification, and Baker Petrolite was unsure whether this was worth the trouble. But eventually the green light was given to go ahead with its manufacture and commercialization. However, no field trials were reported in the 1990s, indicating that field implementation was a fairly long process. The development of the twin-tailed quaternary AA was the responsibility of the Dutch Shell team. They enlisted the help of a neighboring chemical company, Akzo Nobel, to make their AAs. The first prototype molecules were based on tetraalkylammonium salts with two tails such as dicocoyldibutylammonium bromide.88 This quaternary AA performed well up to a subcooling of ca. 14 °C in Shell’s flow loop in several hydrocarbon fluids at 0.25 wt %. In contrast to the single-tail quaternary AAs, it also worked in freshwater systems. The biggest drawback with the tetraalkylammonium salts was their environmental impact. These quaternary AAs gave almost zero biodegradation in standard OECD (Organization for Economic Cooperation and Development) tests. Added to that they were

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toxic and even more bioaccumulative than the single-tail quaternary AAs. Akzo Nobel went back to the laboratory and synthesized diester quaternary AAs instead. These were diesters of dibutyldiethanolammonium halides and 2 mol of a long-chain alkyl carboxylic acid. These products biodegraded ca. 50% in 28 days. The ester groups degrade within 5 days, leaving a small, less surface active, and less toxic quaternary ammonium salt. The optimum chain length for the carboxylic acid appeared to be coconut fatty acids with 12-14 carbon atoms. This cocoyl diester quaternary AA performed well up to subcooling of 15 °C in the tests described by Shell in their patent. The product performed equally well in freshwater as in saline water. The concentration necessary was only 0.25 wt % based on the aqueous phase. In addition, this diester quaternary AA partitioned ca. 95% into the liquid hydrocarbon phase. This meant that the active concentration in the bulk water phase was only approximately 125 ppm. This seems surprisingly low for such a powerful product. However, the concentration of the AA at the liquid hydrocarbon/water interface, where hydrate formation first takes place, is undoubtedly a lot higher. Akzo Nobel formulated the cocoyl diester quaternary AA as a 33% solution in a hydrocarbon solvent. This was made available to the service companies for commercialization. One oil company interested in Shell’s quaternary AAs was Statoil. Statoil had been collaborating with the Dutch Shell team in the mid 1990s testing Shell’s/Akzo Nobel’s twin-tail quaternary AAs in their wheel-loops. Statoil was impressed by their performance and went ahead with a field trial on their Tommeliten field in 1995. Only a few details have been published on the field trial, the results of which do not appear to have dampened Statoil’s enthusiasm for using the quaternary AAs.89 In fact, Statoil wanted to use the twin-tail quaternary AA on a new field development called Kvitebjørn. The most economical solution was a tie-back using multiphase transportation. The subcooling for hydrate formation was calculated to be under 15 °C and ideal for the twin-tail quaternary AA. Statoil asked TROS (now Clariant Oilfield Services) to make an application to the Norwegian environmental authorities (SFT) to use the diester quaternary AA on the Kvitebjørn field.90 Unfortunately, the SFT turned down TROS and Statoil’s application. It seems that the reason given was that the diester quaternary AA was insufficiently biodegradable. The SFT did not like the fact that the degraded product, dibutyldiethanolammonium bromide, did not degrade any further in a 28 day OECD test even though this degraded product was far less toxic. TROS submitted further data to show that the degraded quaternary does degrade eventually in a 70 day test, but it was to no avail, the SFT stood by their decision. This meant that Statoil had to cancel their plans for developing the Kvitebjørn field since no alternative economical solution could be found. The field still remains undeveloped to this day. But the failure to get the twin-tail quaternary AA accepted for use on the Kvitebjørn field had wider repercussion. It meant that no quaternary AAs, singleor twin-tail would ever be allowed in Norwegian waters. Statoil then began to look elsewhere for alternative LDHI technology. They joined the CSM consortium as a late participant hoping to find a high-performing KHI. They also joined a new industry project (JIP), which started at RF in April 1995. They also carried out a field trial on BP’s PVCap/TBAB KHI technology commercialized by TROS (now Clariant).89 The field trial lasted only 3 h as the subcooling at 9 °C was just outside the performance limit of the KHI at 0.5 wt %. Meanwhile, the Dutch Shell team had to look elsewhere for field trials of their diester quaternary AA.

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Figure 17. Example of a polyetherdiamine.

IFP and Project EUCHARIS. Another JIP that started in the mid 1990s centered on the technology and facilities at IFP. The project was called EUCHARIS and was supported by six oil companies and the HSE Department of the U.K. The main aim of the project was to be a springboard for IFPs emulsion AA technology. After almost a decade of laboratory research, IFP had narrowed down the choice of AA candidates to just a few products, largely polymeric surfactants. One of them called Emulfip 102b made by FINA (now Total) was the preferred product. Emulfip 102b is a 50% solution of polymerized fatty acids and amides in rapeseed oil. Although no environmental data was published, this product was expected to be considerably greener than Shells quaternary AAs. To test Emulfip 102b further, IFP used their newly completed pilot-scale flow-loop. The product was tested at two concentrations and a range of conditions.91 At 0.4 wt % the results were poor, but at 0.8 wt % the tests were much more successful. Fine slurries of transportable hydrates were formed up to the maximum subcooling of 13 °C. There were, however, a couple of drawbacks to the emulsion AA technology. First, the water phase had to be thoroughly emulsified before entering hydrate-forming conditions, otherwise hydrate agglomeration and deposition is likely. Could this be guaranteed in the field? Second, hydrates formed from condensed water on the upper walls of the pipe during laminar flow or during shut down. Nevertheless the JIP committee recommended IFP to go ahead with field trials, which took place in the late 1990s. IFP also carried out studies on KHIs for the first time. They tested a BP KHI blend commercialized by TROS (now Clariant Oilfield Services) in their pilot loop. They also filed two patent applications on new KHIs.92,94 The first patent application claimed polymers with at least one sulfonate group. The best examples were acrylamide:AMPS copolymers.93 But these gave only moderate performance, no better than PVP. The second patent application claimed polymers with at least one nitrogen containing monomer such as the dimethylaminomethylacrylate monomer. These gave a similar fairly weak performance as the sulfonated copolymers. BJ Unichem and Gas Well AAs. The only service company to be actively involved in inventing LDHIs in the 1990s was BJ Unichem Chemical Services. A one-man effort has seen new products taken through from laboratory research all the way to several field applications. The laboratory test rig used was a 1 mm i.d., 64 m long flowline. The liquid phase was THF in 3.6% brine, and the flow rate was slow at less than 1 mm/s. BJ Unichem found that polyetherpolyamines and polyetherdiamines (particularly of the oxypropylene type) lowered the blocking temperature or gave a longer time to blocking than a test with no additives.95,96 Examples were taken from the Jeffamine range by Huntsman Corporation (Figure 17) and Ethoduomeen by Akzo Nobel. BJ Unichem also patented quaternized polyetherpolyamines by reaction of polyamines with a long-chain alkyl bromide.97 These products performed better in the THF hydrate flowline than PVCap, TBAB, and several Shell-type quaternary AAs. At least four field applications have been reported in gas wells both offshore and one onshore.98-101 One report included details of an autoclave test in which the polyetherpolyamine was a good synergist for PVCap. The same product dispersed gas hydrates in the remaining unconverted water. Use of the polyetheramines in completion and fracturing fluids has also been patented.226

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It is difficult to know how to categorize BJ’s products or indeed understand the mechanism for their action. (See also results in the early 1990s by RF on propylene glycols.) They are clearly not KHIs, as BJ’s autoclave tests on polyetherpolyamines gave zero induction time. (However, a later paper does show significant anti-nucleation effect for some polyetheramines, as well as good synergy with a polymeric KHI223.) Neither are they true AAs because they have not been proven to work at full water conversion with a liquid hydrocarbon phase. In both the laboratory studies and the field applications, gas hydrates that form are being dispersed in unconVerted water (or water plus a minor % of hydrocarbon phase). Therefore, when used alone it is probably best to call these products gas well AAs (GWAAs) in order to distinguish them from true AAs, which work in pipelines with a hydrocarbon phase at full water conversion. They are also synergists for KHI polymers such as PVCap.225 Regarding their mechanism as synergists, the polyetherpolyamine may be interacting with cavities on the hydrate surface via the amine headgroups. The polyether chain may interact weakly with the hydrate surface or just act as a barrier to prevent the hydrate nuclei from growing. Many small amines are known to form clathrate hydrates. This includes ethylamine, which is the same size as the ethyleneamine groups on the ends of the polyetherpolyamines. RF JIP: KI Studies. When Exxon dropped out of supporting RF’s studies at the end of 1994, this left RF briefly without any LDHI work. However, by April 1995, RF had launched its own JIP. The JIP was supported by seven oil companies, TR Oilfield Services (now Clariant Oilfield Services), Hoechst (now Clariant), and the Norwegian Research Council (NFR). The aim of the project was to design better LDHIs (both KHI and AA) than were previously available. RF’s approach was to use molecular modeling and a good deal of intuition to design improved products. The modeling work was carried out at the University of Reading. The goal was to find functional groups that would interact strongly with structure II hydrate surfaces.102 These functional groups could then be placed into water-soluble polymers to make KHIs and into surfactants to make AAs. Functional groups investigated by modeling included alkylamides, lactams and many other heterocyclic groups, amine oxides, quaternary ammonium salts, and betaines. The alkyl amides with the strongest structure II hydrate interactions were found to contain alkyl groups with 3-4 carbon atoms. Moreover, the interaction was strongest when the alkyl group was branched (i.e., isopropyl and isobutyl). To confirm the modeling results, RF sought to make KHI polymers containing optimum alkylamide groups. They employed the University of Oslo to make vinyl alkyl amide polymers. The University succeeded in making N-vinyl-N-propyl propanamide, but this monomer refused to polymerize, even with comonomers such as VP. Clariant also tried to make polyvinylalkylamides. They synthesized N-vinyl-N-isobutyl acetamide, but this also refused to polymerize. Stumped by the lack of access to polyvinylamides, RF looked for other polymers containing alkylamide groups. Polyallylamides with pendant alkyl groups of 1-5 carbon atoms were made available to RF by Nitto Boseki of Japan. The most effective KHI in this class was polyallyl isopentanamide, with a pendant isobutyl on the amide (Figure 18). This result was in agreement with the modeling studies. The performance was significantly less than Gaffix VC-713 for two reasons. First, the degree of amidation of the original polyallylamine was only 40%. Second, the alkylamide groups are separated from the polyvinyl backbone by methylene spacer groups. This gives the

Energy & Fuels, Vol. 20, No. 3, 2006 837

Figure 18. Structure of polyallyl isopentanamide.

Figure 19. Structures of VP:isobutylacrylamide and VIMA:isobutylacrylamide copolymers.

Figure 20. Structure of VP:butyl acrylate copolymer.

Figure 21. Structures of polypyrrolidinyl aspartate (polyAS) and polyglyoxylpyrrolidine (polyGP).

alkylamide a greater degree of rotational freedom as compared to polyvinyl amides. This means polyallylamides will lose this rotational freedom (entropy) on binding two hydrate surfaces making the process less favorable than with polyvinylamides. The next class of polymers with alkylamide groups that RF investigated were polyalkylacrylamides. Polyisopropylacrylamides (polyiPA), synthesized in house, performed better than polyethylacrylamides but a little worse than Gaffix VC-713. Polyisobutylacrylamide (polyiBA) was also synthesized but found to be water-insoluble. Therefore, 1:1 copolymers of iBA with VIMA and VP were prepared with cloud points of 30 °C (Figure 19). Both these copolymers performed somewhat better than polyiPA and similar to Gaffix VC-713. No further polyalkylacrylamides were synthesized in house as RF began to be supplied with this class of polymer by Nippon Shokubai Company Ltd. of Japan (NS). NS had independently discovered polyacryloylpyrrolidine (polyAP) as a KHI and had patented the result in 1995.56 RF later became a co-owner of the patent along with a second patent on VP:alkyl acrylates, which performed better than PVP, (e.g., VP:butyl acrylate copolymer) (Figure 20).57 Besides polyAP, the first patent also included other polymers with carbonyl pyrrolidine groups in the side chains (e.g.. polypyrrolidinyl aspartate (polyAS) and polyglyoxyl pyrrolidine (polyGP)) (Figure 21). Bayer later patented polyaspartate and related polymers with succinyl units.58 RF tested NS’s polyAP and found it to be an excellent KHI, superior to Gaffix VC-713. (PolyGP performed equally as well as polyAP and polyAS a little less so.) Extensive work was carried out optimizing polyAP. The optimum molecular weight (Mw) for best performance was found to be 1000-3000. This represents only eight monomers in the chain of the shortest polymers (oligomers). Only one copolymer, AP:VCap copolymer, was found that performed better than polyAP. The best of

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Figure 23. Structure of one of Clariant’s synergists for maleic anhydride copolymers. Figure 22. Structure of the active monomer unit in amidated maleic anhydride copolymers. R+ is H+, a metal ion or a quaternary ammonium ion. The isobutyl group can be exchanged with isopropyl.

the other AP copolymers was 1:1 AP:acrylamide copolymer. AP:VIMA copolymers were not investigated, although ExxonMobil’s results with methacryloylpyrrolidine:VIMA (MAP: VIMA) copolymers suggest they could outperform polyAP. Molecular modeling of AP monomer indicated that it interacted strongly with the deep holes on the (1,1,1) structure II hydrate surface. VCap did not interact well with the surface, and VP interacted in a different way to AP. This suggested to RF that PVCap and PVP might be synergists for polyAP. This was confirmed through laboratory experiments. The best synergistic blend that was formulated was a 4:1:1 triple blend of polyAP (Mw ) 5000):PVCap (Mw ) 19000):Antaron P904 (Mw ) 16000). A 6000 ppm solution of this triple blend prevented hydrates from forming for at least 17 h at 16 °C subcooling in autoclave and wheel tests. The conditions were 90 bar and 2 °C while the fluids were 3.6% synthetic seawater, Exxol D60, and a natural gas blend (C1-C4, CO2, and N2). By this time, RF and NS were aware that ExxonMobil had beaten them to the patent office for polyAP and other polyalkylacrylamides. Therefore, RF and NS took out a joint patent on their synergistic blend technology hoping that ExxonMobil would give them a license to commercialize them.103 Two hurdles got in the way of commercializing polyAP and synergistic blends. First, ExxonMobil would not give RF and NS a license before having tested the technology themselves. Second, it was discovered that pyrrolidine, the starting material for making polyAP, was prohibitively expensive (ca. 12 euro/ kg). Therefore, NS and RF decided to switch strategies and concentrate on polyiPA, which was expected to be cheaper to manufacture than PVCap. PVCap was found to be a synergist for polyiPA but not to the same degree as for polyAP. The best synergistic blend was found to be a 2:1 blend of 90/10 iPA/ AMPS:PVCap. A 6000 ppm solution of this blend delayed hydrates for over 1 day, including a 6 h shut-in at 12 °C subcooling in autoclave and wheel tests.104 A sample of this blend was sent to ExxonMobil for testing. A similar subcooling was obtained in loop tests as RF had found in the autoclave and wheel tests. ExxonMobil’s prior licensing agreements precluded a license to NS and RF. Not being able to commercialize AP or iPA polymers, NS decided to synthesize new classes of surfactant AAs that RF would test. This chapter in the history of LDHIs is discussed later in the section entitled “The Late 1990s”. In mid-1996, RF discovered a new class of KHIs based on amidation of maleic anhydride copolymers. Maleic anhydride does not easily polymerize by itself but does so with a variety of vinylic comonomers giving regular ABAB copolymers. Reaction of these copolymers with alkylamines (cyclic imines) opens the anhydride ring to give two different neighboring side chains: first, an alkylamide group and, second, a carboxylic acid or carboxylate group (Figure 22). RF made a series of amidated copolymers using various amines. They found that isobutylamine gave a copolymer with marginally better performance than a copolymer made using isopropylamine. Copolymers made from n-propylamine, nbutylamine, and tetrahydrofurfurylamine gave poorer perfor-

mance. RF patented the results in 1997. Unknown to RF at the time, Clariant, who had heard RFs initial results at a JIP meeting, had patented a modification of RF’s technology. Clariant had noted that RF’s maleic copolymers had low cloud points, a feature, which at the time was thought to be a disadvantage. Clariant’s idea was to substitute some of the alkylamine for a diamine in the amidation process. For example, reaction of a 80:20 blend of isobutylamine:dimethylaminopropylamine with a maleic anhydride:vinyl acetate copolymer gave a useful KHI with a high cloud point. The pendant dimethylamino groups help solubilize the copolymer in water to higher temperatures. The performance of the copolymer is a little less than a polymer amidated with isobutylamine alone but is still useful to a subcooling of approximately 8 °C at 5000 ppm (this is without added synergists). RF and Clariant decided to combine their two patents at a later stage.105,106 Clariant continued to carry out studies on amidated maleic anhydride copolymers with the German Petroleum Institute at Clausthal. Clariant also developed synergists for these polymers, which have been patented separately.107 These consist of small, surfactant-like molecules with alkylamide headgroups (Figure 23). The technology has been successfully field trialled and is now commercially available through Clariant Oilfield Services. Clariant. Clariant filed three further patent applications during their time in the RF JIP. The first application claimed the use of amphiphilic polymers as KHIs.108 The polymers contained equal amounts of cationic and ionic monomers. Initial results were obtained through the JIP and used in the patent application. RF later tested Clariants polymers as KHIs but obtained poor performance. Sometime later Clariant withdrew its patent application. The second patent application claimed the use of isobutyl glycol ether (iBGE) as a synergist solvent for KHI polymers.109 Clariant noted that ISP had already patented small alcohols and glycol ethers as synergists for their VCap polymers. ISP’s preferred synergist was n-butyl glycol ether (BGE), a commercial product. Clariant found that iBGE was a better synergist for the amidated maleic copolymers than BGE. Presumably, the isobutyl group interacts better with hydrate cavities than the n-butyl, which fits with RF’s observations on alkylamides. However, iBGE did not become a commercial product, and the patent application lapsed. It seems that the marginal performance improvement of iBGE over BGE was outweighed by the cost of commercializing a new solvent in limited quantities. The third patent application from Clariant appeared to hold a lot of promise.110 At that time, 1996, Clariant was selling PVCap/TBAB blends to BP through their daughter company TROS (now also called Clariant Oilfield Services). Neither PVCap or TBAB were manufactured by Clariant, who were keen to sell their own chemicals. In talks between RF and Clariant the idea arose of using amine oxides as a substitute for TBAB. TBAB, TPAB, and tributyl phosphine oxide were all known to form clathrate hydrates, so it was assumed that tributylamine oxide (TBAO) and tripentylamine oxides (TPAO) would do the same. Clariant synthesized these amine oxides by simple oxidation of the corresponding trialkylamines. RF found that TBAO performed at least as well as TBAB in inhibiting the growth of single crystals of THF hydrates. TPAO performed even better but was not studied further since the cost

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Energy & Fuels, Vol. 20, No. 3, 2006 839

of tripentylamine was rather high and would make the price of TPAO uneconomic. TBAO proved to be an excellent synergist for VCap polymers. Optimized blends produced some of the most powerful KHIs tested at RF in the 1990s, superseded only by the best polyAP triple blends. However, TBAO did not become a commercial product, and the patent application on amine oxides was withdrawn. The problem was its poor biodegradability. At less than 20% in a 28 day OECD test, TBAO was no better than TBAB and therefore offered no advantage to the best available technology at the time. Due to the high performance of TBAO, RF was convinced that there was more mileage in amine oxide products. In particular, they thought surfactants amine oxides would make good AAs. These are discussed in the next section under “RF JIP: AA Studies”. Late 1990s In the late 1990s a few research groups were still actively pursuing KHI technology, but it was becoming increasingly clear that this was not going to be an economic solution for high subcooling conditions. Improvements in KHI technology were few and incremental, and no big breakthroughs occurred. Thus, as far as LDHIs are concerned, deepwater applications were the domain of AAs only. Perhaps the only company that might justifiably argue with this conclusion is ExxonMobil. They had produced experimental data from their mini-loop to suggest that KHIs could be used at 15+ °C subcooling. At 1 wt % their best inhibitor VIMA:IPMA 1:1 low molecular weight copolymer (oligomer) failed at a subcooling of 22-23 °C.111 The experimental method is to slowly cool the mini-loop 1 °C at a time until the line is blocked. With no additives, the line was blocked at a subcooling of 4 °C. It should also be added that ExxonMobil has not used any synergists or synergist solvents in these experiments. It is difficult to say how long the induction time is at any one subcooling at this high concentration as this information is not available; but the data obtained is still impressive. ExxonMobil spent 3 years from 1998 to 2001 looking for manufacturers of their alkylacrylamide polymers. IPMA monomer was not commercial then and still is not today. ExxonMobil first looked for a manufacturer of VIMA:IPMA copolymer rather than VIMA:IPMA oligomer and then finally IPMA homopolymer. Their sources told them that IPMA monomer would cost 2-3 times the price of VCap monomer. Even at this high price ExxonMobil still believes they have an application for an IPMA polymer, and they wish to explore this avenue further. ExxonMobil finished their research program in 1998, and one or two patent applications are still to be released. CSM also carried out studies on KHIs into the late 1990s. In particular, they showed that PVCap oligomer performed better than higher molecular weight PVCap.3 Samples were obtained from BASF and the University of Akron, Ohio. BASF’s PVCap oligomer became a commercial product through the service company Nalco. Nalco now uses polymers supplied by ISP. CSM Studies on AAs. CSM’s consortium members could see that PVCap oligomer was not going to be good enough for deepwater applications at higher subcooling. They therefore encouraged CSM to study AAs as the only LDHI alternative. CSM responded by setting up equipment and test procedures for studying AAs. They then tested a range of commercial surfactants with different headgroups and HLB (hydrophilic lipophilic balance) values as well as about 70 new surfactant AAs synthesized at the University of Akron, Ohio.112 None of the commercial surfactants performed particularly well, the best

Figure 24. Structure of dodecyl-2-(2-caprolactamyl) ethanamide.

of them being a Span sorbitan surfactant that prevented agglomeration at 2 wt % concentration at up to 11.5 °C subcooling. The synthetic surfactants were designed to contain headgroups found in KHIs. The caprolactam ring was an obvious choice, as it was known from CMS’s studies to attach to hydrate crystal surfaces. The best AAs were mono-tail surfactants containing the caprolactam headgroup as well as polymeric surfactants based on vinyl caprolactam oligomer or N,Ndialkylacrylamides with alkyl thioether end groups. The single best product was dodecyl-2-(2-caprolactamyl) ethanamide (Figure 24) At 0.75 wt % this AA prevented hydrate agglomeration at 11.5 °C subcooling including a 3-day shut-in/start-up test. ISP KHI Patents. ISP filed a number of patent applications on KHIs in the late 1990s. They were all based on VCap polymer technology. Three of the patent applications claimed very low molecular weight PVCap (i.e., PVCap oligomer).113-115 The lowest molecular weight PVCap oligomer that gave the best performance had a molecular weight (Mw) of 1500. This represents about 11 monomer units in the oligomer. Some of the experimental work was carried out in collaboration with BP. The fourth and fifth patent applications claimed copolymers of VCap with small amounts of dimethylaminoethylacrylate or N-(3-dimethylaminopropyl)methacrylamide.116 The function of this comonomer in aiding KHI performance is not given, but it may be related to the comonomers’ ability to open out polymer chains that might otherwise curl up on themselves in aqueous solution. The sixth and seventh patent applications claimed the use of VCap:vinyl pyridine copolymers.117,118 The function of the second monomer in one patent application was to give the copolymer KHI and corrosion inhibition properties. An eighth patent application claimed the use of polyoxyalkylenediamines as synergists for VCap polymers.119 This synergy was originally discovered by BJ Unichem a couple of years earlier. Two further patents claimed a KHI synergist effect between VCap polymers and an alcohol containing 3-5 carbon atoms and one hydroxy group.120,121 KHIs for the Eastern Trough Area Project (ETAP). The last report on KHIs in the late 1990s concerns their use on the BP-operated Eastern Trough Area Project (ETAP) in the British sector of the North Sea.122,123 ETAP consists of several fields tied back to a central processing unit. Plans for ETAP began in the early 1990s, and production began in 1998. Fluids from two of the fields were expected to be operating 6-8 °C into the hydrate region. Both fields were therefore ideal for using BP’s KHI technology, which was being field-trialed in the Southern North Sea Gas Basin. (Shell, who is a partner in ETAP, hoped to use their AA technology in one of the fields, but this never materialized.) The use of KHI gave BP and their partners a CAPEX saving of 40 million U.S. dollars as compared to using methanol. The main cost saving was due to eliminating a methanol regeneration plant and the platform space it required. TROS (now Clariant) who had the license for BP’s KHI were asked to formulate KHI products for ETAP. They had to overcome compatibility issues such as the combined use of corrosion and scale inhibitors. Injection of KHI began in 1998 and the field has been operating smoothly since. The originally KHIs were based on BP’s PVCap/TBAB synergists blend technology (A similar product was used by Elf, now Total, to

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Figure 25. Early version of the twin-tail diester quaternary AA (A), improved version of the AA where R ) Me or Et (B), and a worse version of the AA with propylene spacer groups (C).

replace methanol in an onshore multiphase transport line124). The PVCap was supplied by BASF and amounted to approximately 100-120 ton per year. This made ETAP the biggest KHI application anywhere in the world. (The later RasGas KHI application in Qatar in the Middle East is of similar magnitude.) In 2001 Clariant lost the contract to Nalco for topside chemical injection on ETAP. Nalco also used PVCap technology for their KHIs, but they chose to source the polymer from BASF’s competitor ISP. This cut BASF’s sales of PVCap in half. (Clariant’s current biggest sale of KHI is for seasonal use of ca. 70 ton/year for a Middle East application.155) Thus we can estimate that at the beginning of the new millennium sales of PVCap-based polymers for KHI applications were ca. 300500 ton per year. Oil-Soluble Twin-Tail Quaternary AA. Compared to KIs, progress in getting AAs into full field applications was slow. One of the main reasons was that all the AAs were new chemicals and had to go through manufacturing regulations beginning with Pre-Market Notification (PMN). The twin-tail quaternary AA also had to undergo a small but important modification. The first version of the AA was made from N-butyl diethanolamine and had ethylene spacer groups between the ester groups and the quaternary nitrogen atom (Figure 25, molecule A). Shell found that addition of a small alkyl branch (methyl or ethyl) to the spacer group improved the performance of the AA (Figure 25, molecule B).125 Specifically, the performance of shut-in/start-up loop test with preformed hydrates slurries were improved. A longer spacer group such as propylene (molecule C) gave a worse performance. The twintail quaternary AA also contained a certain amount of unquaternized amine in the finished product from Akzo-Nobel. The amine actually improves the performance of the AA as well as helping break emulsions. Field trials on the oil-soluble twin-tail quaternary AA were carried out in Holland, Great Britain, and New Zealand from the mid to late 1990s.126 A first field application was planned on a British North Sea gas/condensate field in the Sole Pit area.127 However, the produced water became so saline that a hydrate inhibitor was not required anymore. Shell had to wait a few more years for the first field application of the oil-soluble twin-tail quaternary AA. Water-Soluble Single-Tail Quaternary AA. No data were published in the open literature in the late 1990s by either Shell or Baker Petrolite concerning progress with the water-soluble single-tail quaternary AA. The supplier Baker Petrolite was undoubtedly going through the motions of getting the product ready for field trials, which began in 1999. It seemed that Baker

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Petrolite had overcome any environmental hurdles for the Gulf of Mexico (GoM) at least. Here the environmental law is not so restrictive as it is for example in the North Sea. In the GoM, it is the toxicity of the discharged water and its dilution that are the critical factors. In the North Sea it is the toxicity, biodegradability, and bioaccumulation of the injected chemicals that are evaluated. On top of this, the British sector demands 20% biodegradability for new chemicals, something the singletail quaternary AA does not have but the twin-tail quaternary AA does. The Norwegian sector normally demands 60+% biodegradability on new chemicals, which rules out all LDHIs commercialized to date. Some news about the single-tail quaternary AA did filter through on the grapevine. For example, Amoco (now BP) had been testing it in their high-pressure autoclave and found it gave excellent results, especially in saline water.128 The single-tail quaternary AA was also tested as part of the Deep Star program alongside other LDHIs. Deep Star was a series of deepwater technology projects funded by most of the major oil companies and managed by Texaco (now Chevron Texaco). Westport Technology (now Intertek Westport Technology Centre) undertook the test program using a high-pressure autoclave. The single-tail quaternary AA outperformed all the other LDHIs giving transportable hydrate slurries at 16+ °C subcooling. It was clearly a product the oil companies wanted to see used in the field. DeepStar also began funding the reconstruction of an onland pipeline in Wyoming for LDHI testing. Unfortunately, the budget got out of control and the project was never finished. IFP Patents and Field Trials on Emulfip 102b. As part of the EUCHARIS project, IFP carried out two field trials on their emulsion AA, Emulfip 102b, in 1998-1999.129 The site was a 3 in., 2.5 km onshore pipeline as part of Total’s Canadon field in southern Argentina. The pipe pressure was 40 bar, water cut 20%, and the salinity 10 g/L. The subcooling in the first field trial was a maximum of 10-12 °C. The AA was pumped in pulses rather than using continuous injection. IFP deemed the first field trials as a success. A downstream emulsion was formed in the separator caused by Emulfip 102b, but this problem was not addressed by IFP. A second field trial was carried out in June-July 1999 at Total’s Canadon field. The subcooling varied from 10 to 20 °C depending on the time of the day. The product worked for a short time but then failed, resulting in the plugging of the line. Two plugging mechanisms were proposed. First, accumulation of hydrate particles in the line, not necessarily at low points. Second, deposition of hydrate at the upper pipe walls from condensed water. NB: The flow is stratified flow and Emulfip 102b is not in the gas phase. This effect was seen in IFP’s loop test, but being a loop the deposits did not build up. Thus, it appears that Emulfip 102b was able to disperse the hydrates formed. The condensed water problem and slurry transportation difficulties may be overcome when the flow is not stratified (higher liquid volumes and flow rates). IFP filed several patent applications in the late 1990s on both KHIs and AAs.130-133 The first patent application claimed watersoluble polyoxyalkylene macromers and/or polymacromers alone or in combination with known KHIs such as Gaffix VC713 as KHIs. A second patent application claimed water-soluble KHI polymers characterized by a specific number of rotational degrees of freedom and degree of polymerization. Examples are polymers of propylene glycol alginate, poly(ethylene glycol) monomethacrylate, and AMPs. A third patent application claimed AAs based on mixtures of copolymers of polyisobu-

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Energy & Fuels, Vol. 20, No. 3, 2006 841

Figure 26. Structure of amine oxide surfactant AAs.

tylene succinate/poly(ethylene glycol) and alkyl (meth)acrylate: VP copolymers. A fourth patent application claimed amphiphilic nonionic polymeric surfactants as AAs formed by reacting a polymerized unsaturated oil and an amino alcohol (e.g., flax oil and diethanolamine). RF JIP: AA Studies. The RF JIP began searching for new AAs as early as 1995. However, it was not until the late 1990s that they concentrated their efforts on this class of LDHI. They set a goal of finding an AA that at 0.5 wt % in 3.6% brine would perform as well as Shell/Akzo Nobel’s oil-soluble quaternary AA but was more environmentally friendly. This meant lower toxicity or higher biodegradability or both. Almost all the potential AAs were new surfactants synthesized by various chemical companies under collaboration agreements. A range of polymeric surfactants with pendant alkyl- and dialkylamide groups were tested, but they gave poor results. Better performance was obtained with monomeric surfactants. The first noteworthy AA was a triester quaternary ammonium salt with one long tail made by Stepan Europe. It performed well up to 10 °C subcooling and had >60% biodegradability. However, the performance of this class could not be improved so RF looked elsewhere. The next class of surfactants that gave positive AA results were the cationic caprolactams. They contained a caprolactam head and a quaternary nitrogen spacer group. They worked well up to 10-11 °C subcooling but were toxic and were not investigated further. Tributylamine oxide had been shown by RF to be an excellent THF hydrate growth inhibitor on a par with TBAB. It was therefore of great interest to investigate amine oxide surfactants as AAs. Commerical surfactants from Clariant with dimethylamine oxide headgroups gave poor results. Better results were obtained with two surfactants synthesized by Clariant although neither had an optimum structure. The first surfactant was dodecylbutylmethylamine oxide (DDBMAO) (Figure 26). By having only one butyl group rather than two at the head, it was not ideal. As expected it gave moderate results in the cell and wheel. The second surfactant is also shown in Figure 26 and has a diethylamine oxide headgroup. However, it also gave only moderate results in the cell and wheel. The surfactant also turned out to be very toxic to skeletonema. For these reasons Clariant decided not to synthesize further amine oxide surfactants, and the patent application on amine oxide was dropped. More recently, Stavanger University has synthesized the more desirable surfactants with dibutylamine oxide headgroups. In 1998-1999 RF and Nippon Shokubai prepared and tested monomeric surfactants with alkylamide or dialkylamide groups in the head.232 This led to a patent application in 1999.134 The groups were mostly carbonylpyrrolidine and isopropylamide groups that had been shown to perform well in KHI polymers. Several classes of these surfactants performed fairly well in cell tests, but two classes stood out for also having low environmental impact. These are shown in Figure 27. Both surfactant classes have a branching carboxylic acid group which gives them high biodegradability. Further, the higher the HLB (the shorter the tail) the lower the toxicity of the surfactants. Both classes performed very well in cell and wheel tests at 13 °C subcooling. Statoil also tested the isopropylamide surfactant in their wheel and obtained similar results. However, both classes failed at 15 °C subcooling. As there seemed no performance advantage over

Figure 27. Structures of carbonylpyrrolidine and isopropylamide carboxylic acid surfactants tested by RF. R ) C8-14.

Figure 28. Structure of betaines tested by RF. R is a long alkyl chain with various spacer groups.

the Shell quaternary AAs, further work on the alkylamide surfactants was dropped and the patent application withdrawn. The last AA idea RF investigated in their JIP was to take the single-tail quaternary AA and place the anion in the molecule thereby making it zwitteronic.135,233 Nippon Shokubai made a range of these betaine surfactants, which RF tested (Figure 28). The results were fairly promising in cell and wheel tests, but the subcooling limit at 0.5 wt % was approximately 13 °C. Clearly, the anion was best separated from the quaternary nitrogen atom. RF concluded that Shell’s original idea of using quaternary surfactant AAs with two or three butyl or pentyl groups in the head was still the best class of AAs discovered yet. Following this conclusion, RF did get a chemical company to make a new class of quaternary AA. They were only briefly tested with good results in RF cells before RF turned to other studies in the new millenium. Stavanger University is now testing this promising class. Into the New Millenium Polyesteramide LDHIs. By the end of the millennium, all three LDHI JIPs were coming to a close. CSM and RF had seen new KHIs brought to commercialization whereas IFP had still to see their AAs and KHIs reach full application. Exxon had shut down its R&D program and were no longer investigating new chemistry. So one may be excused for thinking that after all this effort the best chemistry had been discovered. However, Shell was still open for new LDHI ideas when they came across a new class of polymer marketed by the Dutch company, DSM, under the tradename Hybranes. Hybranes are hyper-branched polyesteramides. Shell found that certain polyesteramides performed well as KHIs while others showed AA behavior. They patented the results in 2002.136 The basic polyesteramides are fairly cheap to manufacture, cheaper than PVCap according to Shell. They are made by condensing a cyclic acid anhydride with a dialkanolamine in a ratio of n:n + 1 where n is an integer. (By varying n one can vary the molecular weight of the polymer.) This gives a polymer with hydroxyl groups at the tips. The hyper-branching is caused by the dialkanolamine, which has three reactive groups. By adding a third molecule to the reaction mixture, such as a secondary amine, the tips of the polymer can be modified to become more or less hydrophilic. In their patent application, Shell used polymers made from di-2-propanolamine as the dialkanolamine. The best choice for the cyclic acid anhydride appears to be hexahydrophthalic anhydride. This combination gives polymers with the structural units shown in Figure 29. The resemblance of the polyesteramide with earlier KHIs such as PVCap and polyIPMA is now apparent. All three KHI

842 Energy & Fuels, Vol. 20, No. 3, 2006

Figure 29. Repeat unit in a polyesteramide made from di-2propanolamine and hexahydrophthalic anhydride.

polymers have a hydrophobic group attached to an amide group. The hydrophobic group forms a van der Waals interaction with hydrate surfaces (and/or perturbation of free water), while the amide hydrogen bonds to water molecules in the hydrate surface. This disrupts further growth of the hydrate particles. In the case of the polyesteramide shown in Figure 29, the hydrophobic group is the cyclohexyl ring, although the methyl group of the di-2-propanolamine may make a contribution. The ester groups may also take part in hydrogen bonding. The feature that singles out the polyesteramides from all other KHI polymers is their hyper-branching. All other KHI polymers are linear snake-like molecules with a polyvinyl backbone. To use an analogy for KHI bonding to hydrate surfaces, the polyesteramides are like a hand clutching a hydrate ball, whereas other KHIs are like a single long finger attached to the ball. The molecular weight of the polymer does not need to be very high either. Shell found that polymers with molecular weights of 1500 gave good KHI performance. This is roughly the same figure found for polyvinyl-based KHIs such as PVCap. In Shell’s patents on polyesteramides, they found that some hydrophobically modified polymers gave moderate AA performance. Shell has spent considerable effort exploring polyesteramides as AAs, but in May 2004 they discontinued research concluding that the performance was not good enough to make a commercial AA product.139 Baker Petrolite and Polyesteramide KHIs. Polyesteramide KHIs are commercially available through Baker Petrolite. They will give PVCap some competition on the KHI market. Baker Petrolite has also patented new KHIs137 and synergists138 for polymers including polyesteramide dendrimers. In one example given in the synergist patent application, polyethyleneimine is reacted with formaldehyde and caprolactam. This gives polymers with pendant caprolactam rings just as one finds in PVCap. Although PVCap is also a synergist for the polyesteramides, it seems that Baker Petrolite is avoiding the use of PVCap in their KHI products. This gives them PVCap-free KHI technology, which is only rivalled by Clariant’s amidated maleic anhydride copolymers with ether carboxamide surfactant synergists. Another KHI synergist from Baker Petrolite is made by reacting N-methyl butylamine with formaldehyde and polyacrylamide. Other synergists mentioned are nonpolymeric surfactants with caprolactam or alkylamide headgroups. A small amount of a quaternary AA can also be formulated with the KHI blend to improve the performance. In their patent application, Baker Petrolite also mentions a typical polyesteramide as being made from di-2-propanolamine, hexahydrophthalic anhydride and bis(dimethylaminopropyl)imine. The latter molecule gives the polymer pendant dimethylamino groups, which help to make the polymer more water-soluble. Baker Petrolite has carried out two field trials on the polyesteramides in 2004, and several field applications have already begun. In addition, Baker Petrolite has found that their polyesteramide-based KHIs have better performance on structure I hydrates than other KHI chemistries such as PVCap.194 Some fields have such a high percentage of methane that structure I hydrate is formed rather than structure II. Baker Petrolite is already looking at structure I field applications for their polyesteramides.

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KHIs for Drilling and Completion Fluids. Gas hydrate formation can be a hazard in deep water drilling if gas breakthrough occurs. To avoid hydrates from forming the drilling fluid can be formulated with salts and glycols, which act as thermodynamic inhibitors. However, there are some situations in which the drilling fluid will be under-inhibited. For example, the density of the fluid required, so that the fracture gradient is not exceeded, might limit the amount of salt that can be added. Second, in ultra-deep water drilling, it may be impossible to avoid hydrate formation with some types of waterbased drilling fluids. Westport Technology Center International, RF, MI, and Norsk Hydro140 have all carried out projects to investigate the use of KHIs in under-inhibited drilling fluids. PVCap and other KHI polymers with low cloud points cannot be used, as they will not be compatible with highly saline drilling muds at the maximum circulation temperature of the mud (often 40-50 °C). Hence it has been necessary to design the KHI with a higher cloud point in saline solutions. Westport carried out a matrix of tests on KHIs for drilling fluids for a consortium of oil companies and mud companies. They determined that the KHIs lost performance probably by adsorption onto clay (mud) particles in the drilling fluid. RF carried out several projects for oil companies and MI Swaco. They also noted the effect of clay particles on some KHI polymers. However, they did identify two polymers that were useful KHIs for drilling fluids giving an extra 8-10 °C subcooling protection with a 24 h nucleation delay at 0.6 wt %. MI Swaco found a KHI polymer that gave 8 °C added subcooling protection with a 10 h nucleation delay at a concentration of 1 wt %.140 However, no clay was used in these tests. BJ Chemical Services has used a proprietary blend of thermodynamic inhibitor, KHI and GW AA in a packer fluid for a Gulf of Mexico application.209 KHIs Based on PVCap. Low molecular weight PVCapbased products with added synergists are probably the best KHIs for structure II hydrate inhibition on the market today. ISP and Nalco reported that such a KHI blend gave at least 48 h hydrate inhibition at a subcooling of 13 °C.141 This represents an improvement of about 5-6 °C subcooling since the VCap-based terpolymer Gaffix VC-713 was first discovered. ISP and Nalco had their low molecular weight PVCap-based KHI tested in the wheel loop by Statoil. The results were some of the best recorded for any KHI.142,143 The KHI was tested using two shutin/start-ups. For one condensate the KHI passed the test at 1719 °C subcooling, whereas for another condensate the maximum subcooling was only 8-12 °C. The maximum subcooling in a crude was 13-16 °C whereas in a gas/water system the maximum subcooling was 11-12 °C. This underlines the dependency of the KHI on the liquid hydrocarbon phase and the need to test a KHI in the actual fluids from the field. It would be interesting to determine what a condensate or crude contains that has such a beneficial effect on a KHI and then build this into improved KHIs. Nalco also showed that KHI polymer cloud point is not such a critical issue for field implementation.141 Earlier it was thought that the polymer had to be soluble up to the maximum temperature in the well stream, which is usually at the injection site. Otherwise it might precipitate out in the pipeline. This made it difficult to use PVCap, which has a cloud point of approximately 30-42 °C in freshwater. Nalco ran tests to show that PVCap polymer will remain within solvent droplets in the hydrocarbon phase until the temperature of the aqueous phase drops below the cloud point of the polymer. At this point the polymer will dissolve in the aqueous phase.

ReViews

Even with the good KHI results mentioned above Statoil could not use PVCap technology in their Norwegian fields. The reason is that the Norwegian Pollution Authority (SFT) demands that all new chemicals offshore have 60+ % biodegradability. Nalco’s PVCap-based KHI has very poor biodegradability, although it is low in toxicity like all water-soluble polymers. Since quaternary AAs were already banned from use in Norway, and now KHIs could not be used either, this led Statoil to shut down their LDHI research program in 2001. ExxonMobil Norge has a potential KHI application on their Ringhorn field where the subcooling was fairly low. However, the SFT would not allow the use of poorly biodegradable polymers, so the plan was dropped. The British environmental authorities demand only 20+ % biodegradability for new offshore chemicals. However, even this is higher than the biodegradation of normal PVCap. This has led BASF, Clariant and ISP to search for more biodegradable polymers. So far, BASF has reported that they have managed to synthesize a low molecular weight lactam-based polymer with approximately 20% biodegradability, which performs as well as earlier KHI VCap-based products.144 Recently, BASF took out a patent application on water-soluble vinyl lactam copolymers.145 A preferred polymer was 80:20 VP: butyl acrylate which performed better than a PVCap sample in the THF hydrate ball-stop rig. The copolymer is very similar to copolymers patented earlier by Nippon Shokubai and RF. A second patent application by BASF concerns the use of ethylene glycol as a high flash point solvent for VCap polymers.207 Although not a manufacturer of the VCap monomer, Clariant has also filed a patent application on copolymers where the bulk of the examples concern VCap:alkyl(meth)acrylate ester copolymers.146 With short-chain alkyl methacrylates, such as methyl methacrylate, the VCap copolymer gave improved KHI effect over PVCap samples. With long chain alkyl methacrylates, such as oleyl methacrylate the VCap copolymer gave some AA effect. A new synergist for PVCap has been identified as high molecular weight poly(ethylene oxide) (PEO).211,228 However, it does not perform as well as butoxyethanol.51 A small amount of PEO added to a PVCap solution also dramatically reduced the hydrate memory effect.229 KHIs Based on Isopropylmethacrylamide (IPMA). The high performance of polyIPMA, discovered by ExxonMobil, and its possible competition with PVCap has been recognized by Mitsubishi and ISP as they have both filed patent applications on polyIPMA. ISP claimed that polyIPMA synthesized in butyl glycol gave a far better performance than polyIPMA synthesized in 2-propanol.147 Mitsubishi patented polyIPMA with hydroxyl end groups, claiming it to be a better KHI polymer than normal polyIPMA.148 In 2003, ExxonMobil (EMURC) filed a patent application claiming KHIs that are polymers with a bimodal molecular weight distribution.149 The bimodal distribution can be made from a single polymerization or by mixing two polymers with unequal molecular weight distributions. The preferred polymer is polyIPMA although PVCap and other polymers are also claimed. The increase in performance over a polymer with a monomodal molecular weight distribution is most striking for polyIPMA. For example, a hold time of 20 h was possible at 43.5 °F (24.1 °C) subcooling using 0.5 wt % of a poly IPMA with a bimodal molecular weight distribution. The bulk of the polymer has a molecular weight average of ca. 1000-3000. This result is the highest subcooling recorded to date for a KHI product. The mechanism for the increased performance is not

Energy & Fuels, Vol. 20, No. 3, 2006 843

discussed in the patent application. RF had the same idea in the late 1990s carrying out studies on mixtures of high and low molecular weight polyAP. They observed some increase in performance relative to a low molecular weight sample only. RF postulated that the increase in KHI performance is related to the size and mobility of the different molecular weight samples. The low molecular weight sample, like a motorbike, moves quickly in solution to the site of hydrate nuclei growth. It will adsorb onto the surface of the hydrate but only weakly. The high molecular weight sample, like a lorry, moves slowly but can then replace the low molecular weight sample on the surface adsorbing more strongly and preventing hydrate nuclei growth for longer periods. The motorbike-lorry analogy can also be used for the synergistic blend of TBAB (motorbike) and PVCap (lorry) discussed earlier. In fact, RF has showed that TBAB is a better KHI synergist with high molecular weight PVCap than with low molecular weight PVCap. ExxonMobil also presented a paper in 2003 at a nucleation workshop on the results of mini-loop experiments of KHI hold time versus subcooling at fixed KHI concentrations in a gas condensate system.150 The hold time, which is repeatable, is the time from when the loop fluids reach the constant cooldown temperature until hydrates are first detected. The data fitted an exponential function in subcooling at all KHI concentrations of the form y ) mx + b where y ) ln(1/hold time), x ) subcooling, m was a constant at all KHI concentrations making the curves of hold time versus subcooling have the same shape, differing only in intercept, and b was dependent in a nonlinear fashion on KHI concentration. The function b versus KHI concentration was similar to the function maximum subcooling versus KHI concentration that approaches an asymptote above 0.5 wt % active polymer. KHI Applications Continue to Increase. By 2005, it is estimated that there are about 40-50 KHI applications worldwidespread between a handful of service companies. Recently, the first application in the Middle East was reported by Clariant and Total for a field called West Pars. Injection of KHI was successful throughout a 10-month period.151 Nalco has also reported that they have a KHI application for RasGas in the Middle East.152 A new field called Otter in the British sector of the North Sea and operated by Total will become only the second field, following ETAP, where a KHI application is planned during the field development. Otter is an oil field tied back in a 10 in. line to existing infrastructure. Production began in 2003.153,195 KHI is injected during planned shutdowns or short-term low flow. Another hurdle the service companies have had to overcome is combining KHIs with other production chemicals in a single injection line. On ETAP and a southern North Sea Gas field, KHIs have been successfully combined with corrosion and scale inhibitors by Clariant.155 Clariant also recently reported the successful use of KHI combined with paraffin inhibitor,156,157 while Baker Petrolite has reported studies on KHI/corrosion inhibitor blends.220 Another application area for KHIs is their combination with thermodynamic inhibitors. For example, late in field life the water cut may be too high (> 50%) to use AAs. The subcooling may also be too high for commercial KHIs alone. Boosting the subcooling performance by combining the KHI with methanol or glycol is a possibility. A West African field that is due to start up soon, and for which Shell is the operator, has been planned in this way. Baker Petrolite plans the early life of the field with application of a quaternary AA.154 BJ Chemical

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Services has published a paper on the use of KHI combined with methanol for a subsea production line in the Gulf of Mexico.210 Novel LDHIs. In 2003 BP filed a novel patent application for a method of delaying hydrate formation.158 The method comprises mixing a polymeric emulsifier and optionally a nonionic nonpolymeric co-emulsifier with the hydrocarbon and aqueous fluids so as to generate a water-in-oil emulsion. The emulsifiers form a substantially gas impermeable interfacial layer, which prevents hydrate from forming in the encapsulated, aqueous droplets. Nucleation delays of over 16 h were obtained at 12 °C subcooling. The concentration of the emulsifiers used was 2 wt %. BP recommends recovering and reusing the emulsifiers in order to make the process economically viable. However, no suggestions are made as to how this should be done. RF investigated two novel LDHI ideas in 2000. The first idea was to use super water-adsorbent polymers to take out all the water before hydrate formation occurred. The experiments failed because a suspension of water-adsorbed polymer particles could not be maintained. Coagulation may have been avoided with polymer particles with membranes but this is too expensive. However, the second idea gave some success. The idea was to use an ion pair blend of ionic KHI polymer and surfactant with the opposite charge to give an AA effect. The KHI would control hydrate growth and the surfactant would drag the hydrate KHIcoated particles to the hydrocarbon phase by electrostatic forces. An anionic IPA:AMPS copolymer with a cationic surfactant worked well at 14 °C and 90 bar but failed at higher subcoolings. No further work was carried out after the project finished at the end of 2000. First Field Trials and Field Applications of the Quaternary AAs. In the year 2000, Baker Petrolite presented a paper discussing the first field trials with the water-soluble quaternary AA.159 The first trial actually took place in the Gulf of Mexico (GoM) in 1999 and was a success. Later AAs have been used in the GoM to mitigate against hydrate plugging during shutin/start-up situations in black oil systems. The first field application with continuous injection of AA was in 2002, again in the GoM on a Shell field called Popeye.160 Since then the number of AA applications particularly in the GoM has increased rapidly.161-163 For example, ExxonMobil ran the Diana field oil flowlines with a Baker Petrolite AA from 2002 to 2003; oil production ended on high watercut.111 A field application in West Africa is also planned.153 Baker Petrolite claimed at an SPE Forum that 1 wt % actives of the water-soluble quaternary could treat any subcooling.164 It seems like this product is the proverbial silver bullet. So far it has been lab tested successfully up to 7000 psi (500 bar) and 22 °C (40 °F) subcooling, and used successfully in the field at ca. 14 °C (25 °F) subcooling. Along with KHIs they have also been found to work in highly sour systems where the H2S has been thought to affect the hydrate equilibrium properties.165 Finally, compatibility with materials193 and other production chemicals such as corrosion and wax inhibitors appears to have been overcome for both KHIs and AAs. Baker Petrolite has also patented a method to increase the performance of the water-soluble quaternary AA.166 The watersoluble quaternary AA (onium compound) is formulated with an amine salt and optionally a solvent. The amine salt will increase the ionic strength of the water phase, which may make the AA perform better in freshwater. The amine salt contained preferably alkyl or hydroxyalkyl groups with 1-3 carbon atoms or an ammonium salt could be used.

ReViews

Two methods to reduce the environmental impact of quaternary surfactant AAs have been patented. Shell has patented a method to phase separate a quaternary AA by adding sufficient inorganic salt to the produced water to render the AA insoluble. Baker Petrolite has patented methods to detoxify quaternary AA surfactants by the addition of anionic polymers or anionic surfactants.240-242 Very recently a new patent application from Baker Petrolite (Baker Hughes Incorporated) was laid open.231 They found that the effective concentration of a quaternary surfactant AA could be reduced by the addition of a minor amount of an anionic, nonionic, or amphoteric compound. The mixture, called an ion pair, allows lower concentrations of the quaternary surfactant to be used in practice, which reduces the treatment cost as well as the environmental impact. For example, the effective concentration of quaternary ammonium surfactant AA was reduced from 0.75 wt % to 0.15 wt % by the addition of 0.12 wt % alcohol ether sulfate (AES) for AA tests with a GoM condensate. In another example with a GoM condensate, the effective concentration of the quaternary AA could be reduced from 0.59 wt % to 0.30 wt % by the addition of 0.04 wt % sodium dodecyl sulfate (SDS). Both AES and SDS are known to have little or no AA hydrate inhibiting ability. In another version of the invention, Baker Petrolite found that mixing dodecylbenzenesulfonic acid (DDBSA) with a small quaternary ammonium compound with appendages containing less than 6 carbon atoms, to give an ion pair, also gave a product that performed as an AA. Neither the ammonium compound or DDBSA showed any appreciable AA activity individually. An example of a small quaternary compound could be TPAB, known to be a hydrate crystal growth inihibitor. Baker Petrolite’s idea is similar to RF’s idea of using an ion pair AA made from an anionic KHI with a cationic surfactant discussed earlier. Baker Petrolite has reversed the charges using a cationic hydrate growth inhibitor with an anionic surfactant. The first application for which the field was specifically designed to use an AA was Shell’s K7-FB field in the Dutch sector of the North Sea in 2003.167 The oil-soluble quaternary AA from Akzo Nobel is presently supplied through Clariant Oilfield Services. The field produces gas condensate and freshwater and the subcooling is a maximum of 8-9 °C. One of the drawbacks of the oil-soluble quaternary AA is that it has a shelf life at ca. 20 °C of about 1 year. One of the tails of the surfactant gradually degrades off leaving a mono-tail quaternary surfactant, which has much poorer AA performance. Consequently, the operator has to make sure that the turnover of the quaternary AA, especially in a hot climate, is kept to under a year. LDHIs from IFP. By 2005, IFP had still not managed to find a field application for their emulsion AA, Emulfip 102b, following mixed results from the field trials in Argentina. The tendency for the hydrates to drop out during shut-in was an issue that they addressed in a new patent application.168 This claimed the use of hydrophobic oil-soluble block copolymers that when used together with an emulsion AA would prevent the settling out of hydrates during shut-in. A typical block copolymer mentioned is made from styrene/ethylene/propylene. A typical emulsion AA is made by reacting a polyalkenylsuccinic anhydride with a monoether of poly(ethylene glycol). (240) Blytas, G. C.; Kruka, V. R. International Patent Application WO 01/38695. (241) Rivers, G. T.; Downs, H. H. U.S. Patent Application 20040144732, 2004. (242) Rivers, G. T.; Frostman, L. M.; Pryzbyliski, J. L.; McMahon, J. A. U.S. Patent Application 20030146173, 2003.

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Energy & Fuels, Vol. 20, No. 3, 2006 845

Figure 30. Structure of alkyl ether tributylammonium bromide AAs. R has 12-14 carbon atoms.

IFP has since put Emulfip 102b back on the shelf and instead are trying to promote another polymeric surfactant emulsion AA called IPE 202b, which they hope to commercialize through the service company Shrieve in the GoM.169 IFP’s KHIs such as AMPS:acrylamide are also in a stand-by situation and which they hope to be commercialized through a new partner. Natural LDHIs. It has been known since the mid 1990s that natural surfactants in crude oils affect hydrate plugging tendencies. Attempts to isolate these natural AAs have been carried out by chromatography but have led to very complicated mixtures.170 For crudes it seems the maltene fraction (which are surfactants) and possibly asphaltenes are responsible for the AA effect. Later work seems to show that there are two types of crude oil, only one of which has hydrate-plugging tendencies.171-173,215-218 New Quaternary AAs. By the new millennium it was becoming clear to the chemical manufacturers and service companies that the only surfactants that gave high AA performance were quaternary surfactants with 2 or 3 butyl or pentyl groups, first discovered by Shell. The research work of CSM, RF, and others had not provided any better AAs. Since the turn of the millenium Nalco,175 Goldschmidt,176 Clariant,177-181,202 and Champion Technologies196-200 have all filed patent applications on new quaternary surfactant AAs. Both Nalco and Clariant already have commercially available quaternary AAs that will compete with the Shell quaternary AAs available through Baker Petrolite and Akzo Nobel. Nalco appears to have found a way around the original Shell patents allowing them to use the desired surfactants with a quaternary center with butyl or pentyl groups. After 2 years of testing at Nalco and Westport Technology Centre, the new AA is already being used in a field application on BP’s Horn Mountain in the GoM.182 Injection is continuous and began in 2003. Other applications have recently been reported.183 Goldschmidt’s patent application176 claims the use of quaternary ammonium salt surfactants as AAs with an ether spacer group between the quaternary nitrogen atom and the long alkyl tail (Figure 30). The AAs are claimed to have improved water dispersibility over tetraalkylammonium salts such as hexadecyltributylammonium bromide. There appears to be no public information as to whether Goldschmidt’s AAs have been commercialized. Clariant now has its own quaternary AA, which has been successfully field-trialled and is commercially available. Clariant has filed several patent applications on surfactant AAs, some products of which can also be used as synergists for KHI polymers such as PVCap. For example, alkylaminoalkyl monoand diesters are claimed in one patent application and shown to work as AAs as well as synergists for KHI polymers such as PVCap. The products also exhibited good corrosion inhibition properties. In a second patent application N,N′-dialkylaminoalkyl ether carboxylates are claimed as corrosion and gas hydrate inhibitors with improved water solubility and biodegradability. Examples are shown to work as AAs as well as synergists for KHI polymers such as PVCap. A third patent application with

Figure 31. Structure of a preferred polyalkoxylated amine where a + b + c ) 14.9.

Figure 32. Structure of tributylammoniumpropylsulfonate (TBAPS).

similar claims covers alkylaminoalkyl/alkoxy monoesters. A fourth patent application with similar claims covers alkoxylated amine ether carboxylic acids and their quaternized products. These surfactants could in theory contain dibutylammonium headgroups, as originally claimed by Shell for AAs, although no examples are given. Their synergy with PVCap is also claimed. A fifth patent application covers alkenylsuccinimidoalkylamines as KHI synergists and corrosion inhibitors with improved water solubility and biodegradability. Besides various quaternary ammonium surfactants Champion Technologies’ patent applications196-200 also cover amine oxides and betaine surfactants, a subject first researched by RF in the late 1990s. There was a reluctance by operators to use the early quaternary AAs since their unit price per barrel of water treated was about the same as methanol. However, the later quaternary AAs are now much more competitive and can be dosed at lower concentrations.224,231 New KHIs. In 2003, Akzo Nobel filed a patent application on a new class of LDHI called polyalkoxylated amines.184,185 The alkoxylation is preferably done with propylene oxide (PO). A preferred amine for making KHIs is triethanolamine but ammonia and other alkanolamines can be used. The amine can also be quaternized. The best example given in the patent application is triethanol with a total of 14.9 PO units (Figure 31). At 1.0 wt % this reduced the hydrate onset temperature from ca. 42 °F (5.6 °C) in a blank test to 32.9 °F (0.5 °C). AAs are covered in the patent application by adding a long hydrophobic tail to the polyalkoxylated amine, but no examples are given. Molecular modeling work carried out at the Universities of Warwick and Reading, England, and hydrate testing at ENSIACET & LGC, Toulouse, France, and Heriot-Watt University, Edinburgh, Scotland, has led to a class of zwitterionic KHI synergists.186,187(They also have potential as AAs.219) The only chemical mentioned in detail is tributylammoniumpropylsulfonate (TBAPS), which performed only slightly better than PVP in the THF hydrate ball-stop rig (Figure 32). Instead of binding inside potholes on the hydrate surface, TBAPS and related compounds lie along the surface and actually cap the pothole. The modeling was carried out in a vacuum and does not take into account free water-LDHI interactions. A later paper mentions a product called J3 designed using molecular modeling, which performed significantly better than a commercial PVCap blend, giving an induction time of over 4000 min with structure II hydrates.188 It will be interesting to see how this class of KHI develops.

846 Energy & Fuels, Vol. 20, No. 3, 2006

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LDHI testing. The effect of pressure, and not just the subcooling, on the performance of LDHIs also needs further investigation.80,234 Conclusion Figure 33. Structure of poly(vinyl alcohol) based KHIs where R1 has 1-6 carbon atoms, R2 or R3 is H, COOH, C1-10 alkyl or C6-C12 aryl, and R4 is H or CH3.

Kuraray Specialties Europe filed a patent application in 2001 on a new class of polymer KHI.189 The polymers are based on derivatives of poly(vinyl alcohol) by reaction with aldehydes. The structure of a typical product is shown in Figure 33. It contains vinyl ester acetal functionalities besides some unreacted vinyl alcohol monomer units. A preferred aldehyde is butyraldehyde, giving pendant propyl groups. Some of the polymers are also claimed to work as AAs. Since the early 1990s, it has been known that anti-freeze proteins not only inhibit ice growth but also inhibit hydrate formation.26,28 Queen’s University, Kingston, Canada, has sought to commercialize the idea by claiming anti-freeze proteins, active fragments of these anti-freeze proteins, and mimetics thereof, as KHIs. The anti-freeze proteins or active fragments are derived from insects, plants, fungi, protests, or bacteria presumably in a process that allows for large-scale manufacture since isolation of large quantities of anti-freeze proteins from these sources is prohibitively expensive.190,212 In 2001 BASF patented the use of grafted polymers as KHIs.201 The backbone is preferably a polyalkylene glycol, a polyalkyleneimine, a polyether, or a polyurethane, and the active functional side groups are made by grafting VP or VCap to the backbone using radical initiators. The heteroatoms in the backbone should help to increase the biodegradability of the polymer. Although quaternary ammonium salts with 2 or 3 butyl or pentyl groups had been known for over a decade from Shell’s work to work as hydrate growth inhibitors and as AAs, no one it seems had tried to make polymer KHIs with quaternary ammonium groups. In 2004 RF and Stavanger University rectified this situation by testing polyquaternaries with pendant tributylammonium groups.191 Perhaps not unexpectedly they also turned out to be excellent synergists for VCap polymers such as Gaffix VC-713 performing better than TBAB as synergist. So far the work, which is still ongoing, has not been taken beyond the laboratory stage. Future LDHI Research LDHI R&D appears to have come near to the top of the S-curve. As of late 2005, there is relatively little activity by oil companies, institutes, and the academic community in designing higher performing products. It now seems to be primarily the job of the service companies and chemical manufacturers in fine-tuning existing chemistries. However, a number of new academic research groups have taken up the batton and are beginning to explore LDHIs, some in collaboration with service companies. This may also lead to new or greener versions of known LDHI classes. Further, fundamental studies on the mechanisms of nucleation and agglomeration of hydrates are also being carried out, either as molecular modeling203,213,214,237 or as experimental work.204-208,222,235,236 These studies will surely help our understanding of the mechanisms of LDHIs but are unlikely to result in fundamentally new commercial LDHIs. Better experimental procedures that match the true field conditions (not just a single subcooling limit) are also needed for

Low dosage inhibitors for mitigating other oilfield problems are well-known and include wax, scale, corrosion, and asphaltene inhibitors. Low dosage inhibitors for the prevention of plugging by gas hydrates are the latest tools to have been developed. This paper has reviewed the development of LDHIs with focus on the various chemistries that have been investigated. Both commercial and unsuccessful attempts have been included for completeness. Since the start of LDHI research in the late 1980s over 240 documents have been published. The fact that nearly 100 of these are patent applications gives some idea of the interest in this subject. However, there have been far fewer patent applications in the last 2-3 years. The use of LDHIs is now rapidly becoming an accepted method for preventing hydrate plugging in gas wells and oil and gas pipelines. Already LDHIs are a million dollar business for some of the oilfield service companies. By the end of 2005, there are and have been ca. 50-70 field applications of LDHIs, the majority of them related to KHIs although the number of AA applications are now increasing almost as rapidly. Most LDHI applications to date have been retrofits (i.e., methanol or glycol use has been exchanged with the more economical use of an LDHI). However, as the technology becomes more wellknown, more fields will be planned with the use of LDHIs since the CAPEX savings can be very significant.224 KHIs will continue to dominate the market for applications where the hydrate subcooling is below 10 °C. Some commercial KHIs when dosed at >5000 ppm have been used in the field at subcoolings up to approximately 15 °C.155 Other KHIs have been shown to work at much higher subcoolings, but they are not commercially available. The best example is ExxonMobil’s VIMA:IPMA oligomer. Blends of thermodynamic inhibitors and KHIs are also being used in the field.210 AAs can of course also be used at low subcoolings where the water cut is under 50%, and in some cases they may be more economical than KHIs. The mechanism for kinetic hydrate inhibition (nucleation inhibition) by water-soluble polymers is still not fully understood, at least in the public domain, although models have been presented. Usually these relate to adsorption of the polymer onto hydrate embryos or crystal structures,192 although some work now suggests that water pertubation is also critical.188,213,214 More work is needed on molecular modeling of hydratewater-polymer interactions in an aqueous environment not just hydrate-polymer interactions in vacuo. In addition, ExxonMobil’s proprietary theory related to maximizing the hydration volume of the KHI polymer in water needs investigating further. After all, their polymers (e.g., VIMA:IPMA oligomer) appear to have the highest performance of KHIs reported to date. Finally, the mechanism of the effect of the hydrocarbon phase on the performance of a KHI needs investigating. For high subcooling applications, AAs seem the only commercial LDHI alternative. Applications of AAs are expected to move to higher subcoolings as operators and service companies become experienced with their use. Single-tailed quaternary AAs appear to be the only LDHIs that can consistently perform at 15+ °C subcooling. So far they have only been field proven in flow lines to 14 °C subcooling, although laboratory experiments suggest that 20+ °C subcoolings can be obtained (BP’s Holstein field did use an AA at start-up where the subcooling could have

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been as high as 25 °C224). Emulsion-based AAs have yet to reach field applications. There is still a need for greener AAs for use in environmentally sensitive waters such as offshore Norway. GWAAs are different from pipeline AAs and have so far only been shown to work at low water-to-hydrate conversion in gas wells. They are however good synergists for PVCapbased KHI polymers. Acknowledgment. I would like to thank my fellow LDHI researchers at RF-Rogaland Research: Thor Martin Svartaas, Knut Lekvam, Svante Nilsson, Lindy Dybvik, Jorunn Øvsthus, and Jan Erik Iversen during the years 1991-2001. I would also like to thank the participants in LDHI projects carried out at RF-Rogaland Research. They include Larry Talley (ExxonMobil Upstream Research Company), Nick Wolf and Stan Swearingen (ConocoPhillips), George Shoup (BP), Emil LePorcher and Jean-Louis Peytavy (Total), Keijo Kinnari and Anita Rasch (Statoil), Finn Fadnes (Norsk Hydro), Agostino Mazzoni and Carla Consoloni (ENI), Nick Phillips (Clariant Oilfield Services, now Baker Petrolite), and Peter Klug (Clariant). Yuji Sugiura, Takashi Tomita, Takashi Namba, Yoshihiro Arita, Jun-ichi Chosa, and Keiichiro Mizuta at Nippon Shokubai Company Ltd, are also acknowledged for their collaboration and excellent synthetic skills. I thank Larry Talley and Ulfert Klomp (Shell Global Solutions) for reading the manuscript and making helpful comments. I am also grateful for the help of Piers Crocker who read to me many LDHI articles and reports while I was unable to do so. Finally, but not least, I would

Energy & Fuels, Vol. 20, No. 3, 2006 847 like to thank my wife, Evy, who patiently typed a large part of the manuscript while I was recovering from a long-term illness.

Nomenclature AA ) anti-agglomerant AFP ) anti-freeze protein AFGP ) anti-freeze glycoprotein AMPS ) acrylamidopropylsulfonic acid CSM ) Colorado School of Mines EMURC ) ExxonMobil Upstream Research Company IFP ) Institut Francais du Petrole IPA ) isopropylacrylamide IPMA ) isopropylmethacrylamide KHI ) kinetic hydrate inhibitor LDHI ) low dosage hydrate inhibitor NS ) Nippon Shokubai Company Ltd. OECD ) Organization for Economic Cooperation and Development PolyAP ) polyacryloylpyrrolidine PVP ) poly(vinylpyrrolidone) PVCap ) polyvinylcaprolactam RF ) RF-Rogaland Research THF ) tetrahydrofuran TROS ) TR Oil Services (now Clariant) TPAB ) tetrapentylammonium bromide TBAB ) tetrabutylammonium bromide VIMA ) N-vinyl-N-methyl acetamide EF050427X