Energy Fuels 2010, 24, 1234–1241 Published on Web 01/07/2010
: DOI:10.1021/ef9010687
Experimental Investigation of Wax Deposition in Kermanshah Crude Oil through a Monitored Flow Loop Apparatus M. Lashkarbolooki, A. Seyfaee, F. Esmaeilzadeh,* and D. Mowla School of Chemical and Petroleum Engineering Department, Shiraz University, Shiraz, Iran Received September 22, 2009. Revised Manuscript Received November 14, 2009
The main objective of this study was to determine the effect of temperature difference between the oil and the pipe wall on deposition thickness and wax and asphaltene þ resin contents on the pipe wall surface of a single-phase crude oil. All of the tests were performed under laminar flow conditions (Reynolds number is ∼450) with a wax appearance temperature (WAT) of 31 °C and a 13°API crude oil from the Kermanshah oil field (Paydar-East Reservoir). A new experimental apparatus was designed and constructed to simulate the deposition thickness in flow lines. The heat-transfer method was used to measure the deposition thickness during the tests, and the chemical analysis method was additionally used to determine wax and asphaltene þ resin contents at the end of the each test. It was found that the deposition thickness increased by increasing the temperature difference between the oil and pipe wall. In addition, the chemical analysis method showed that wax content on the deposit was increased by increasing the deposition thickness and temperature difference.
shear dispersion, Brownian movement, and gravity settling.7-13 Several investigators suggested that temperature gradient causing molecular diffusion to be the main reason of deposition.4,14,15 The temperature at the walls is less than the temperature at the center of the flow line. This leads to a temperature gradient and molecular diffusion of the paraffin crystals toward the wall. Several factors are responsible for the wax deposition in pipelines, including crude oil composition, temperature, flow rate, thermal history, and pressure. Moreover, the amount of deposited wax is affected by the concentration of paraffins, light ends, and nucleating or inhibiting materials in the crude oil.1,16-19 Pressure is not considered as an important factor for wax deposition, especially for dead or stock tank oils.16,17
1. Introduction Waxy or paraffinic crude oils are complex mixtures that contain aromatics, paraffins, naphthenes, asphaltenes, resins, etc. Of these compounds, long-chain paraffins (n-alkanes) and some naphthenes, which are wax components, cause serious problems, because of their tendency to deposit on the cold pipe wall. The temperature at which the first crystals of paraffin wax start to appear upon cooling of a crude oil is called the wax appearance temperature (WAT).1 Under reservoir conditions (a temperature range of 70-150 °C and a pressure range of 8000-15000 psi), wax remains dissolved in the crude oils. As the crude oil leaves the reservoir and flows through cold environment pipelines, the crude oil temperature in the region of the pipe wall decreases below the WAT; wax components precipitate out of the solution and crystallize, which leads to solid deposition on the pipe wall. Holder and Winkler2 observed wax deposits under a cross-polarized microscope and found that the wax crystallites have structures of platelets that overlap and interlock. Moreover, the previous experimental investigations have shown that crude oils with wax content of at least 2% could cause wax deposition problems.3-6 Four possible mechanisms have been identified and considered for paraffin deposition including molecular diffusion,
(9) Weingarten, J. S.; Euchner, J. A. SPE Prod. Eng. 1988, 3 (1), 121-126. (Paper No. SPE 15654-PA.) (10) Hamouda, A. A.; Ravnoy, J. M. Prediction of Wax Deposition in Pipelines and Field Experience on the Influence of Wax on Drag-Reducer Performance. Presented at the 24th Annual Offshore Technical Conference, OTC 7060, Houston, TX, May 4-7, 1992. (11) Majeed, A.; Bringedal, B.; Overa, S. Oil Gas J. 1990, 88 (25), 63– 69. (12) Azevedo, L. F. A.; Teixeira, A. M. A Critical Review of the Modeling of Wax Deposition Mechanisms. Presented at the AIChE 2002 Spring National Meeting, New Orleans, LA, May 10-14, 2002. (13) Rygg, O. B.; Rydahl, A. K.; Ronningsen, H. P. Wax deposition in Offshore Pipeline Systems. Presented at the 1st North American Conference, June 1998. (14) Hamouda, A. A.; Davidsen, S. In Proceedings of the SPE International Symposium on Oilfield Chemistry, San Antonio, TX, Feb. 14-17, 1995; Paper No. SPE 28966-MS. (15) Brown, T. S.; Niesen, V. G.; Erickson, D. D. In Proceedings of the SPE Annual Technical Conference and Exhibition, Houston, TX, Oct. 3-6, 1993; Paper No. SPE 26548-MS. (16) Brown, T. S.; Niesen, V. G.; Erickson, D. D. In Proceedings of the SPE Annual Technical Conference & Exhibition, New Orleans, LA, Sept. 25-28, 1994; Paper No. SPE 28505. (17) Ruffier-Meray, V.; Volle, J. L.; Schranz, C. J. P.; Le Marechal, P.; Behar, E. I. In Proceedings of the SPE Annual Technical Conference & Exhibition, Houston, TX, Oct. 3-6, 1993; Paper No. SPE 26540. (18) Hammami, A.; Raines, M. A. Soc. Pet. Eng. J. 1999, 4, 9. (19) Monger-McClure, T. G.; Tackett, J. E.; Merrill, L. S. SPE Prod. Facil. 1999, 14, 4.
*Author to whom correspondence should be addressed. Tel.: þ987112343833. Fax: þ987116287294. E-mail:
[email protected]. (1) Misra, S.; Baruah, S.; Singh, K. Soc. Pet. Eng. 1994, 28181, 50–54. (2) Holder, G. A.; Winkler, J. J. Inst. Pet. 1965, 51, 228. (3) Holder, G. A.; Winkler, J. J. Inst. Pet. 1965, 51, 235. (4) Singh, P.; Fogler, H. S.; Venkatsean, R.; Nagarajan, N. AIChE J. 2000, 46 (5), 1059–1074. (5) Tuttle, R. N. JPT, J. Pet. Technol. 1983, 35, 1192–1197. (6) Ajienka, J. A.; Ikoku, C. U. Energy Sources 1990, 12, 463–478. (7) Bern, P. A.; Withers, V. R.; Cairns, R. J. R. In Proceedings of the 1980 European Offshore Petroleum Conference Exhibition, Vol. 206; pp 571-578. (8) Burger, E. D.; Perkins, T. K.; Striegler, J. H. JPT, J. Pet. Technol. 1981, 33, 1075–1086. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 1234–1241
: DOI:10.1021/ef9010687
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Experimental investigations have shown that wax deposits obtained at higher flow rates are considerably harder than those formed at lower flow rates.4,14,18-24 In addition, several investigators have reported that an increase in flow rate resulted in a decrease in the amount of wax deposition.4,9,15,23-27 In 1994, Hsu et al.28 revealed that flow turbulence or shear dispersion effects depress wax deposition significantly, because of sloughing. In 1990, Agarwal et al.29 deduced that turbulence is the main factor for the decrease in deposition with increasing flow rate, while diffusion is responsible for the laminar region. Matzain30 (in 1996) and Lund31 (in 1998) confirmed that paraffin thickness was greater in laminar flow than in turbulent flow, and experienced a decrease in Reynolds number in turbulent flow. The composition of deposited solids has been determined to change somewhat with time under certain operating conditions.4,24 The hardening and compositional changes that occur within the deposit with time lead to its aging. This aging process was dependent on operating conditions and the fact that it was a stronger function of the temperature difference across the deposit than the compressive force due to flow.20 Wax deposition in production tubing and pipelines is a usual problem. Wax deposition along the inner walls of the pipeline may cause excessive pressure drop, as well as a reduction in flow rate, and it causes operational problems. To prevent blockage of pipelines, wax deposits should be removed periodically. Different mechanical, thermal, strong magnetic paraffininhibition, microbe paraffin-removing method, and chemical techniques can be used for wax removal.32-39 To achieve efficient remediation and use of various methods for its removal, it is necessary to understand the deposition rate, wax thickness, and wax and asphaltene þ resin contents of crude oil. Experimental results by Creek et al.24
have shown that the greater the temperature difference between the oil and the wall, the greater the deposition rate. However, they do not consider the important parameters, including the content of wax, asphaltene, and resin in deposition. For example, knowledge of the extent of deposition and deposit characteristics will be helpful in choosing the appropriate deposit removal technique and the suitable way to administer the chosen technique. To obtain insight into the wax deposition phenomenon, a model organic deposition-oil system has been studied using a laboratory flow loop. A series of tests were performed to investigate the effect of different temperatures on the deposition rate and wax and asphaltene þ resin contents on the pipe wall surface. The deposition thickness in the flow loop was determined using the heat-transfer method. The samples of the flowing crude oil were analyzed at the end of each test for wax and asphaltene þ resin contents. 2. Experimental Section 2.1. Experimental Apparatus. Most laboratory investigations on wax deposition have been performed using a coldfinger setup20,40 and flow loops setup.4,15,41,42 Because of advantages of flow loop to coldfinger apparatus,15 factors that influence wax deposition were investigated in a new flow loop setup; an apparatus was designed to simulate wax deposition in pipelines. The deposition flow loop consists of two test sections, a test fluid circulation system, a cooling system, a hot bath system, and a data acquisition system. During tests, the data acquisition system gathers data from eight input channels. The lookout program is used to monitor the system. A schematic diagram of the flow loop system in the lookout program is shown in Figure 1. In this work, Kermanshah crude oil with a 13°API crude oil was used for testing. The crude oil reservoir as the feed crude oil cycle is inserted in the hot water. To obtain a uniform temperature in the hot bath, a pump was used to circulate water. Also, a gear pump was used to circulate the crude oil at a constant flow rate. The flow loop consists of two test sections. Both test sections have a length of 60 cm. The inner pipe is made of stainless steel and has an internal diameter of 15.75 mm; the outer pipe has an inner diameter of 62.74 mm. While the test fluid is circulated in the inner pipe of the test sections by the gear pump, a coolant mixture (50 vol % glycol and 50 vol % water) is flowing countercurrently in the annulus and back to the refrigerant unit reservoir. This allows wall surface of the test sections to remain constant at the desired temperature throughout the experiment. The oil tank, flow lines, and the test sections are insulated by glass wood on the outside to minimize heat transfer to the surroundings. Thermocouples to monitor the temperatures in the inlet and outlet of the test sections, in the inlet and outlet coolant system of the test sections, in the hot water tank, and in the oil tank were installed. The inlet temperatures of the coolant mixture and crude oil were maintained at a constant temperature. The adjusted temperatures for all the tests were 0, -4, -8, and -12 °C for the inlet coolant mixture and 40, 44, and 48 °C for the inlet oil temperature. To adjust the inlet oil temperatures, a heater with 2 kW of power was used. The temperatures of the system were monitored in intervals of 30 s through a lookout program for a period of 24 h. First, for each experiment, the pipeline and oil reservoir are loaded with crude oil and the flow rate is adjusted using a bypass valve with the volumetric flow rate of 3 L/min. For deposition to occur, the pipe wall must be maintained at a lower temperature than the bulk fluid and below the fluid WAT. To achieve this, a coolant mixture is circulated
(20) Singh, P.; Youyen, A.; Fogler, H. S. AIChE J. 2001, 47, 2111– 2124. (21) Jessen, F. W.; Howell, J. N. Pet. Trans. AIME 1958, 213, 80. (22) Wu, C.; Creek, J. L.; Wang, K.; Carlson, R. M.; Cheung, S.; Shuler, P. J.; Tang, Y. Measurement of Wax Deposition in Paraffin Solutions. Presented at the AIChE 2002 Spring National Meeting, New Orleans, LA, March 10-14, 2002. (23) Bott, T. R.; Gudmunsson, J. S. Technical Report No. IP 77-007; Institute of Petroleum, London, U.K., 1977. (24) Creek, J. L.; Lund, H. J.; Brill, J. P.; Volk, M. Fluid Phase Equilib. 1999, 158-160, 801. (25) Patton, C. C.; Casad, B. M. Soc. Pet. Eng. J. 1970, 10 (1), 17. (26) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. AIChE J. 2001, 47, 6. (27) Creek, J. L.; Matzain, B.; Apte, M.; Volk, M.; Delle Case, E.; Lund, H. Mechanisms for Wax Deposition. Presented at the AIChE Spring National Meeting, Houston, TX, March 1999. (28) Hsu, J. J.; Santamaria, M. M.; Brubaker, J. P. In Proceedings of the SPE 69th Annual Technical Conference and Exhibition, New Orleans, LA, Sept. 25-28, 1994; Paper No. SPE 28480. (29) Agarwal, K. W.; Khan, H. U.; Surianarayanan, M.; Joshi, G. C. Fuel 1990, 69 (6), 794–796. (30) Matzain, A. M.S. Thesis, University of Tulsa, Tulsa, OK, 1996. (31) Lund, H. J. M.S. Thesis, University of Tulsa, Tulsa, OK, 1998. (32) Nguyen, D. A.; Fogler, H. S.; Chavadej, S. Ind. Eng. Chem. Res. 2001, 40, 5058–5065. (33) Andre, L. C. M.; Elizabete, F. L.; Gonzalez, G. J. Pet. Sci. Eng. 2001, 32, 159–165. (34) Shock, D.; Sudburg, J. D.; Crockett, J. J. J. Pet. Technol. 1955, 7 (9), 23–30. (35) Jorda, R. M. J. Pet. Technol. 1966, 237, 1605–1612. (36) Narvaez, C.; Ferrer, A. A.; Corpoven, S. A. Prevention of paraffin well plugging by Plunger-Lift use, SPE 21640. Presented at The SPE Production Operation Symposium, Oklahoma City, OK, April 7-9, 1991. (37) Biao, W.; Lijian, D. Presented at the International Meeting on Petroleum Engineering, Beijing, PRC, Nov. 14-17, 1995; Paper No. SPE 29954. (38) Svetgoff, J. Oil Gas J. 1984, 82 (9), 79–82. (39) Eastund, B. J.; Schmitt, K. J.; Meek, D. L.; Anderson, D. C.; Grisham, G. Pet. Eng. Int. 1989, 61 (1), 46–51.
(40) dos Santos, J. S. T.; Fernandes, A. C.; Giulietti, M. J. Pet. Sci. Eng. 2004, 45, 47–60. (41) Knox, J.; Waters, A. B.; Arnold, B. B. Presented at the SPE Annual Fall Meeting, Los Angeles, Oct. 7-10, 1962; Paper No. 442. (42) Towler, B. F.; Rebbapragada, S. J. Pet. Sci. Eng. 2004, 45, 11–19.
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: DOI:10.1021/ef9010687
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Figure 1. Schematic diagram of the constructed flow loop system.
through the annulus of the test section with a flow rate of ∼12 L/ min. All of the tests were started after melting the deposit off the pipe wall from the previous test. To achieve this, the test fluid temperature is increased a sufficient amount more above the fluid WAT (∼40 °C more than the WAT). The tank oil temperature is increased by heating the hot bath. Oil is then flowed through the test section to aid the deposit removal. In addition, the deposit was washed with methyl ethyl ketone (MEK) between the two measurements to wash off any excess oil remaining on the pipe wall. The air is then flowed through the test section at a relatively high flow rate to aid the MEK removal in the flow loop.
In this study, all the tests were performed under laminar flow conditions. Therefore, the heat-transfer method was used to measure the deposition thickness, providing an indirect deposition thickness measurement continuously for the duration of the test. 3.1. Heat-Transfer Method. Wax deposition has been described as a nonisothermal phenomenon that seems to be driven by the heat flux between the flowing fluids and the surroundings. This method can give acceptable data for wax thickness if the film heat-transfer coefficients on the inside and outside pipe walls are predicted accurately. Before the occurrence of a wax layer on the pipe wall, the total resistance to heat transfer from the flowing fluid to the environment is comprised of the resistances that are due to convective heat transfer from the flowing fluid to the pipe wall, heat conduction through the pipe wall and any insulation or other coatings, and an appropriate heat-transfer process to the environment (e.g., convective heat transfer if the pipe is exposed to water, air, or another cooling fluid). After a layer of wax deposit is formed on the inside pipe wall, convective heat transfer with paraffin solidification will occur on the interface between the flowing fluid and the deposited wax layer. A thermal resistance term due to heat conduction through the wax layer is added to the total resistance to heat transfer from the flowing fluid to the environment. This added thermal resistance is approximately in direct proportion to the thickness of the wax layer on the pipe wall. Hence, the wax thickness can be determined from measurements of the relevant thermal parameters by solving the heat transfer. Heat transfer from the internal flowing fluid to the outside environment is described by43 1 1 ro ro ri ro ro 1 ¼ ln þ þ ln þ ð1Þ Uc ho ri -δw hc kw ri -δw kp ri
3. Deposit Estimation The wax thickness is difficult to be determined, even with the number of techniques used.24 Three online wax measurements; based on the pressure drop, the heat transfer, and the liquid displacement-level detection (LD-LD) method;were used in the literature to measure the deposition thickness in pipeline.43 The pressure drop method is based on the concept that wax deposition in a pipe section reduces the hydraulic diameter of the pipe, resulting in an increase in the frictional pressure drop over it. A waxy crude oil often behaves as a non-Newtonian fluid when the temperature becomes lower than the WAT. However, when the wax content is low (e.g.,
