The Return of Kinetic Hydrate Inhibitors - American Chemical Society

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The Return of Kinetic Hydrate Inhibitors Bahman Tohidi,*,†,‡ Ross Anderson,*,†,‡,§ Houra Mozaffar,*,‡,§ and Foroogh Tohidi*,† †

Centre for Gas Hydrate Research, Institute of Petroleum Engineering, Heriot−Watt University, Edinburgh, EH14 4AS, United Kingdom ‡ Hydrafact, Ltd., Heriot−Watt University, Research Park, Edinburgh, EH14 4AP, United Kingdom ABSTRACT: Kinetic hydrate inhibitors (KHIs) have been serving the petroleum industry for many years. They are normally used at dose rates of 50% by mass) may be required, and this can have a significant effect on capital expenditure (CAPEX) and operational expenditure (OPEX). Larger pumps, storage/ regeneration facilities, pipes, and associated topside footprints all act to increase CAPEX, while the cost of purchase, transportation, and regeneration of thermodynamic inhibitors can increase OPEX considerably. There are also environmental implications: residual inhibitor quantities in disposed produced water must be tightly controlled, increasing regeneration and waste handling costs. All these factors have driven the industry to seek alternative, lower-cost strategies for hydrate control. Over the past decades, “low-dosage hydrate inhibitors” (LDHIs) have seen increasing use as alternatives to thermodynamic inhibition. LDHIs include the “kinetic hydrate inhibitors” (KHIs), which are generally believed to delay the nucleation/growth of hydrates, and the “anti-agglomerants” © 2015 American Chemical Society

(AAs), which do not prevent hydrate formation, but instead restrict hydrate particle agglomeration and thus plugging potential. The major benefit of LDHIs, which has given them their generic name, is that the quantities required for hydrate problem prevention are very low, typically only a few mass percent in the aqueous phase (or liquid hydrocarbon for some AAs). This is at least an order of magnitude less than the volumes of thermodynamic inhibitors normally required for safe operation under similar subcooling conditions. Clearly, such a large reduction in inhibitor volume requirements offers the possibility of major CAPEX and OPEX savings, with the added benefit of potentially reduced environmental impact.1−3 While our understanding of LDHIsboth KHIs and AAs has grown considerably in recent years, there are still many unknowns. LDHI development, to a large extent, has been based around the screening of many chemicals/combinations in the search for a new/superior active component and/or synergist combination and/or green KHIs and/or high cloud point. However, often less time is devoted to establishing the precise mechanisms by which inhibitors actually work and how different system parameters might affect these possible mechanisms (such as polymer adsorption on the surface of the hydrate). Thus, when it comes to selecting an LDHI for use in a particular pipeline/field development, the same screening process must be used again to ensure system suitability/ compatibility (LDHI compatibility with other chemicals Received: August 6, 2015 Revised: November 9, 2015 Published: November 9, 2015 8254

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and aqueous phase compositions, pressure, polymer type, presence of other pipeline chemicals). It is known that hydrate formation consists of two stages; (1) nucleation and (2) growth. Our findings show that KHIs could affect both the nucleation and the growth of gas hydrates. However, nucleation is commonly stochastic and unpredictable (i.e., it is believed to be a function of many other parameters, e.g., the presence of solid particles, the rate of mixing, the degree of subcooling, the composition of hydrate forming gases, the competition between structure I and structure II hydrates, etc.). Therefore, in almost all of our tests, we eliminated the effect of nucleation by keeping some hydrate particles in the system. Although, in a large majority of real fluid systems, there is no history of hydrate formation and/or hydrate crystals, the results obtained in our investigations could be regarded as conservative, i.e., the time required for nucleation could be added to the time measured for crystal growth in our studies. Interestingly, our results have shown that, unlike nucleation, the effect of KHIs on growth is completely repeatable, i.e., data for a wide variety of different gas−water systems have consistently shown that aqueous KHI polymers induce a number of fixed, repeatable (and transferable between different set-ups) crystal growth inhibition (CGI) pressure−temperature (P/T) zones delineated by quite well-defined “phase boundaries” at specific subcoolings. These include the following: • A slow dissociation rate region (SDR) (illustrated later in this work in Figures 2 and 3), which can extend quite significantly (e.g., up to 7 K) beyond the hydrate phase boundary, where hydrate-polymer complexes (current understanding) may survive for weeks in a metastable state (orders of magnitude reduction in dissociation rates). • A complete crystal growth inhibition region (CIR) (again, illustrated later in this work in Figures 2 and 3) within the hydrate stability zone to quite high subcoolings (>15 K for some systems) where hydrate nucleation/growth is prevented indefinitely (hence, a very long/indefinite induction/hold time) and even dissociation (typically in the case of hydrates initially formed at high subcoolings) can occur. • A slow growth rate region (SGR) (see Figures 2 and 3) where hydrate growth can occur, but is varyingly inhibited in terms of growth rate; from almost fully inhibited (orders of magnitude reduction in growth rates) to steady but still polymer-moderated growth. Our investigations show that the conventional hold time (induction time) is applicable to this region and beyond. • A rapid growth region (RGR) (see Figures 2 and 3) where rapid/catastrophic/uncontrolled hydrate growth occurs upon nucleation with growth rates being largely unaffected by the polymer, i.e., where the latter ceases to inhibit to any measurable extent and therefore there is risk of hydrates to form and create a plug in a pipeline. The observed phase behavior is, as noted, entirely repeatable and transferable across different setups, while the extent of regions is dependent on a range of factors, including polymer properties, concentration, and the presence of other components (e.g., thermodynamic inhibitors). The consistency and repeatability of results is in direct contrast to the scattered, stochastic nature of data often yielded by induction time studies, although it has been determined that induction time (ti) patterns ultimately do relate closely to CGI regions. This CGI evaluation method has opened up significant opportunities for better understanding the mechanisms of KHI inhibition and has already been used successfully for this purpose over the past

injected into the pipeline such as corrosion inhibitors, scale inhibitors, thermodynamic inhibitors, etc.). Such an approach is very time-consuming and carries inherent risks; even if a particular inhibitor performs well under laboratory conditions, if the mechanisms by which it works are not fully appreciated, then it is difficult to foresee problems that may occur once in field use (i.e., unexpected changes in system composition or operating conditions). Compounding this problem are differences in the ways that LDHIs are tested; methods and equipment vary greatly between laboratories, and often results are neither repeatable nor transferable, particularly in the case of KHI induction time data. Thus, if KHI-based hydrate control strategies are to become a trusted industry standard flow assurance solution, it is vital that the above problems be addressed. This requires the development of proven, reliable, rapid, methods for inhibitor testing/screening. Techniques should produce results that are repeatable and transferable between different laboratories/ experimental setups, giving confidence in the test results. To achieve this goal, a better understanding of the fundamental mechanisms of LDHI inhibition and the effect of pressure, temperature, system type (gas, condensate, oil), and the presence of other chemicals (e.g., salts, thermodynamic hydrate inhibitors (THIs), scale and corrosion inhibitors) is very important.

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EXPERIMENTAL SECTION

Tests using the crystal growth inhibition (CGI) method have been carried out on a wide range of gas−aqueous and gas−aqueous−liquid hydrocarbon systems in the presence of KHI polymers; example results are reported here. Tests were carried out on in-house (Hydrafact/Heriot−Watt University) designed/built 280-mL-volume high-pressure (maximum pressure of 41 MPa) stainless steel or titanium autoclave cells. Cell temperature is controlled by circulating coolant from a programmable cryostat through a jacket that is surrounding the cells. Temperature is determined using a platinum resistance thermometer (PRT, ±0.1 K), with the pressure being measured by either strain standard gauge (±7 KPa) or precision Quartzdyne (±0. 07 KPa) transducers; these being regularly calibrated against a dead weight tester. A more-detailed description of the experimental setup and procedure has been given in another paper by Anderson et al.4

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RESULTS AND DISCUSSION Recent Findings with Respect to KHIs. Kinetic Hydrate Inhibitors (KHIs) are now seeing increased use in production operations as an alternative to traditional thermodynamic inhibitors (e.g., glycols, methanol), giving considerable CAPEX/OPEX benefits. Recent studies at Centre for Gas Hydrate Research at Heriot−Watt University (HWU) have elucidated on the mechanisms involved in hydrate prevention characteristics of KHIs.4 A novel, pressure−volume−temperature (PVT) phase behavior/crystal growth inhibition-based approach has been developed and standardized over the past five years; in contrast to traditional induction time studies, this method yields consistently reliable, repeatable, and transferable results. In-depth studies demonstrated that the effects of KHI polymer extend well beyond the nucleation process, inducing several specific, well-defined growth/inhibition regions, as a function of subcooling. Investigation of these regions provides a novel means to examine the fundamentals of KHI inhibition mechanisms in detail, as a function of various parameters (gas 8255

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Energy & Fuels five years of an ongoing HWU Joint Industry Project (JIP), with considerable steps forward having been made in terms of understanding the effects of various parameters on KHI performance. The method is also gaining acceptance as a standard KHI evaluation technique in the oil and gas industry. It has been used as a screening method for several commercial KHI formulations, and the results from some consultancies in Hydrafact have been successfully used under field conditions.5−8 Based on the encouraging results obtained from its field application, the method has a bright prospect for greatly increased use in the future. Figure 1 shows the predicted methane hydrate stability zone in the presence of distilled water and experimental pressure and

Figure 2. Methane hydrate phase boundary in the presence of distilled water and experimental pressure/temperature (P/T) profiles during cooling, hydrate formation, heating, and melting in the presence of 0.25% poly(vinylcaprolactam) (PVCap) (by mass) in the aqueous phase. Abbreviations: SDR, slow dissociation region; CIR, complete inhibition region; SGR(S), slow growth region (slow); SGR(M-R), slow growth region (moderate); and RGR, rapid growth region. Reproduced from our previous work4 (copyright 2011).

The rate of hydrate formation increases as the system is cooled further, and finally, the KHI begins to fail ∼5 K inside the hydrate stability zone. A series of detailed heating and cooling runs with different fractions of hydrate present and at different rates have allowed the identification/mapping with pressure of the range of CGI regions, as shown in Figure 2. Figure 3 shows experimental CGI regions determined for 1.0% PVCap (by mass) with methane. The CIR is where

Figure 1. Predicted methane hydrate stability zone (solid black line) in the presence of distilled water and experimental pressure/temperature (P/T) profiles during cooling, hydrate formation, heating, melting, and reformation of hydrates. Data points are plotted in 5 min intervals. Reproduced from our previous work4 (copyright 2011).

temperature profiles during cooling, hydrate formation, heating and melting, then reformation of hydrates in a KHI-free system. As can be seen, on initial cooling, at some degree of subcooling, hydrates are formed. An exothermic reaction results in a slight increase in the system temperature and significant pressure drop to the hydrate stability zone (according to the phase rule, there is only one degree of freedom). Increasing the system temperature to a temperature slightly higher than the methane hydrate stability zone results in hydrate dissociation and pressure increase. Before dissociating all hydrates, the system is cooled to re-enter the hydrate stability zone. As shown, the hydrates are formed again without any subcooling (this is expected, because of the presence of hydrate crystals). Figure 2 shows the methane hydrate phase boundary in the presence of distilled water and experimental pressure and temperature profiles during cooling, hydrate formation, heating, melting, then recooling in the presence of 0.25% PVCap (by mass) in the aqueous phase. As can be seen, the initial cooling and associated hydrate formation is very similar to that seen for distilled water (Figure 1). However, for normal hydrate dissociation rates, the system temperature must be increased 4−5 K outside the hydrate stability zone. Furthermore, upon recooling and re-entering the hydrate stability zone, most of the remaining hydrates dissociate (instead of regrowing, as observed in Figure 1). Upon subsequent slow cooling (data points taken every 5 min), the fraction of hydrates increases only when the system is ∼2.5 K inside the hydrate stability zone (in other words, hydrates regrows only upon re-entering the SGR(S) region).

Figure 3. Experimentally determined crystal growth inhibition (CGI) regions for 1.0% by mass PVCap with methane. Abbreviations: SDR, slow dissociation region; CIR, complete inhibition region; SGR(VS), slow growth region (very slow); SGR(S), slow growth region (slow); and RGR, rapid growth region.

hydrate formation can be effectively prevented indefinitely. The SGR is where the amount of hydrates formed is a function of time, and this time difference (Δt) is dependent on the degree of subcooling (ΔT). This region can be further divided into very slow (VS) and slow (S) regions, as shown in the figure. The RGR is where the KHI can no longer prevent or delay hydrate formation. The SDR is where hydrate dissociation is abnormally slow, with hydrates showing a considerable degree of metastability, because of the presence of PVCap. The data shown in Figures 1−3 are for methane−water systems, where structure I (sI) is the stable structure. Figure 4 8256

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Figure 4. Comparison of determined subcooling extents of 0.5% PVCap (by mass) aqueous induced hydrate CGI regions from sI phase boundary for methane (CH4) and standard North Sea natural gas (NG) at 10 and 25 MPa. Abbreviations: SDR, slow dissociation region; CIR, complete inhibition region; SGR(VS), slow growth region (very slow); SGR(S), slow growth region (slow); and SGR(M), moderate growth region.

Figure 6. HydraFLASH model predicted pressure/temperature (P/T) for hydrate stability zones of a lean natural gas where sI is more stable at high pressure conditions and sII at low pressures.10 The gap between sI and sII at low pressure is very small, hence little sII-related increase in the CIR is observed.

shows a comparison between determined subcooling extent for 0.5% PVCap (by mass) in a sI forming system with methane and a standard natural gas where sII is the stable structure. As can be seen, in the natural gas sII forming system, the extent of all the CGI regions is larger, compared to methane. Our investigations have shown that, for systems where structure II (sII) is the stable structure, for higher polymer fractions (e.g., at least 0.25% or greater (by weight)), the degree of subcooling will be commonly dependent on the position of the sI hydrate phase boundary (Figures 5−7), because sI seems to be the

Figure 7. HydraFLASH model predicted pressure/temperature (P/T) for hydrate stability zones of a very lean gas where sI is more stable than sII, hence no sII-related increase in the CIR is observed.10

Therefore, while it is possible to estimate the degree of inhibition for a given concentration of thermodynamic inhibitor in the aqueous phase, it is not possible to predict the degree of inhibition offered by KHIs without due consideration to the composition of the hydrocarbon phase and the relative position of sI hydrate phase boundary. This could help explain the mixed fortunes observed in various parts of the world, with respect to application of KHIs; rules of thumb about inhibition subcoolings have historically been applied to the sII phase boundary when inhibition often more closely relates to the sI boundary. Clearly, the emphasis on hold time (i.e., including the time required for nucleation) has historically contributed significantly to uncertainties reported/observed in the application of KHIs (in contrast to THIs). It should be mentioned that there are several other factors affecting the performance of KHIs and these are currently being investigated at the Centre for Gas Hydrate Research at HWU through a multisponsor Joint Industry Project. Two other common concerns, with respect to KHIs, included the following:11 (1) Their performance under shut-in conditions. The results of extensive tests show that, as long as the system is inside the CIR, there is no risk of hydrate blockage in the bulk of aqueous

Figure 5. HydraFLASH model predicted pressure/temperature (P/T) for hydrate stability zones of a typical natural gas where sII is the most stable structure.10 The gap between sI and sII phase boundaries is ∼6.2 K at 10 MPa, which can result in an increase in the CIR by 6.2 K.

hydrate structure that generally forms first in the presence of KHIs (typically existing KHIs are not very efficient in preventing sI hydrate formation, compared to sII).1,4,9 This can help explain the discrepancies observed in the performance KHIs in various fluid systems, in addition to other reasons that are detailed in the study by Anderson et al.4 The technique has been successfully applied to the evaluation of commercial KHIs.5−8 Therefore, unlike thermodynamic inhibitors, where the degree of inhibition is not a strong function of the hydrocarbon system, the degree of KHI hydrate inhibition is a strong function of the hydrocarbon system. 8257

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Energy & Fuels phase during shut-in. Even in the SGR(VS), KHIs can successfully inhibit during both shut-in and restart, notably where warm up occurs during the latter. However, under certain circumstances (such as shut-in where a warm section of the pipeline can cool down as a result of heat loss to a cold environment) hydrates can form at the top of pipeline from condensed water (where there is no KHI due to low vapor pressure of KHIs) under these conditions.11 (2) Hydrate formation at the top of a pipeline under stratif ied f low. Under certain circumstances, such as the one mentioned above, hydrates could form at the top of a pipeline, where there is water condensation. However, the results of recent studies show that the amount of hydrates formed at the top of the pipeline is limited by water mass transfer with potentially low rate of growth. Furthermore, the initial results show when these hydrates come into contact with the aqueous phase (containing KHIs within CIR), their growth is prevented and could even dissociate,11 however, this aspect requires further investigation. The above results have greatly improved the repeatability of test results and industrial confidence in the performance of KHIs. This has resulted in a significant increase in the uptake of KHIs in preventing gas hydrate problems; this is a trend that is expected to increase as the industry becomes more aware of the latest findings in this area. Combination of Kinetic Hydrate Inhibitors (KHIs) and Thermodynamic Inhibitors (THIs). Another interesting prospect is combination of KHIs and THIs. The main applications in the past have focused on reducing the degree of subcooling for KHI applications. As mentioned previously, KHIs can completely prevent gas hydrate formation to certain degrees of subcooling (i.e., CIR). If the expected subcooling is high, then one option is to add a suitable thermodynamic inhibitor (e.g., MEG) and shift the hydrate phase boundary to the left, hence reducing the effective degree of subcooling for KHIs. However, recent findings, with respect to KHIs (as detailed above), provided another opportunity for KHI application, i.e., introducing KHI with the objective of reducing the dose rate of THIs. Recently, we applied the latest KHI inhibition concept to several case studies where MEG was used as the primary hydrate inhibitor. The results showed that low doses of KHI can replace 20%−40% MEG (by weight).12 A typical calculation is shown in Figure 8 for demonstration purposes. Other investigators have reported similar results.13,14 It should be mentioned (based on our results so far) that the CIR is a function of the type of KHI, hydrocarbon composition, pressure, and other chemicals in the system. Therefore, if, e.g., 60 wt % MEG is required to avoid gas hydrate problems in a field, this could potentially be reduced to 20 wt % MEG plus 0.5%−1.0 wt % KHI (active polymer).12 This could provide significant advantages, including (i) a significant reduction in the size of MEG regeneration/ reclamation units, pipelines, and pumps, hence, CAPEX; (ii) a significant reduction in the volume of the inhibitor in circulation and, hence, pump/pipeline sizes, which would also result in a reduction in OPEX, because of a reduction in the volume of fluid required for heating/boiling (in regeneration/ reclamation units); and (iii) an ability to cope with higher water cuts and, hence, longer reservoir life and hydrocarbon recovery factor (particularly unexpected, after considering the necessary modifications in rich-MEG stream to remove KHI before introduction to the MEG unit).

Figure 8. HydraFLASH model predicted pressure/temperature (P/T) profiles for stability zone for sII and sI hydrates of a typical gas condensate.10 Studies show 0.5 wt % PVCap gives a CIR of ∼10 K for this system. This is equivalent to 25 wt % MEG at ∼22 MPa and 37.5 wt % MEG at ∼3.5 MPa. As shown in the figure, the inhibition offered by 0.5 wt % PVCap is equivalent to 32.5 wt % MEG at ∼8 MPa. Considering the fact that MEG is an excellent synergic for KHI and is excluded from hydrate structure, the above estimates are conservative.

Another potential application that will require further investigation is in well testing (and similar operations), where there are limited provisions for produced water treatment and/ or disposal, hence the produced water (containing hydrate inhibitor/methanol) is normally disposed overboard. Adding KHI to conventional thermodynamic inhibitors (e.g., methanol) can potentially reduce the volume of thermodynamic inhibitor required.12 In addition to significant reduction in logistical and storage/pumping issues, the reduction in the volume of disposed harmful chemicals (e.g., methanol) to the sea could help in protecting the environment and perhaps, in some cases, extending the well-test duration (hence gathering more information for field development). Problems Associated with Kinetic Hydrate Inhibitors (KHIs). While KHIs are now experiencing increasingly widespread successful use in production operations, problems are beginning to emerge in terms of their handling with respect to produced water processing/treatment/disposal.15 Most of the commonly used KHI polymers precipitate out of solution at elevated temperatures. This is sometimes said to be the cause of fouling problems in pumps and processing equipment such as MEG regeneration units. Similarly, in the case of produced water reinjection, KHI dropout associated with high downhole/ reservoir temperatures may cause fouling of perforations/pore space, reducing injection efficiency. Furthermore, most current KHI formulations are not regarded as “green inhibitors”; hence, there are strict limitations with respect to their use and/or disposal to the environment. In summary, the main operational/environmental problems associated with KHI are: (1) KHI coming out of solution in produced water reinjection (PWRI) schemes, if the downhole temperature is above system cloud point. This could result in fouling and/or reduced injectability and associated problems. (2) KHIs coming out of solution in combined KHI + THI applications where the thermodynamic inhibitor is regenerated/reused, for example, in MEG regeneration/reclamation units, where the MEG-rich solution is heated to high temperatures. This could result in KHIs coming out of solution and depositing onto various hot surfaces. 8258

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must be researched, in both cases, it might be possible to install a KHI removal stage prior to sending the produced water for reinjection or MEG regeneration. • Addressing issues associated with the disposal of nonbiodegradable KHI polymers and/or THIs into the sea in well-testing applications where there are limited/no facilities for produced water treatment. Clearly, the industry is investigating various options for addressing the above problems, including: • Developing high cloud point KHIs. • Developing green/environmentally friendly KHIs. • Developing new KHIs that can provide better protection, with respect to the degree of subcooling (i.e., larger CIR) and/ or rate of growth. This could result in significant reduction (and even elimination) in the usage of thermodynamic inhibitors (e.g., MEG). The recent advances in understanding the mechanisms involved in KHI hydrate inhibition, as well as their testing and evaluation techniques, are believed to address the main concerns in KHI application, including: • Perceived complex mechanisms of inhibition and apparent inconsistencies in performance. • Environmental issues associated with KHIs. • Problems associated with KHI disposal, produced water handling, and MEG regeneration. Therefore, the usage of KHI is expected to increase. This could result in a significant increase in the number of fields that use KHI as their main hydrate inhibition strategy. It is believed that KHI removal will be a major factor in future KHI design/ applications/research. Further research is ongoing within two joint industry projects at HWU and Hydrafact.

(3) In well-testing applications where the produced water is discharged to the environment and KHIs can cause environmental concerns. Kinetic Hydrate Inhibitor (KHI) Removal from Produced Water. To help address KHI fouling problems in produced water processing, various different treatment methods have been investigated, such as using membranes or ozone oxidation of produced waters to remove KHIs, albeit with limitations, in terms of efficiency/practicality.16 Recently, a simple treatment method for the removal of KHIs from produced water streams has been investigated by Hydrafact, which shows significant promise.17 The technique is based on adding a treatment chemical (TC) to the aqueous phase, which extracts all or most of the KHI polymer from the aqueous phase. The method was originally developed for a different purpose, namely, the determination of KHI content of produced waters, particularly for low polymer concentrations, where accurate measurements are problematic using typical laboratory methods (e.g., high-performance liquid chromatography (HPLC), Fourier transform infrared spectroscopy (FTIR)). The concept was to find a chemical that caused a significant displacement of polymer from the aqueous phase, concentrating it in the TC. With the polymer enriched in the TC, measurement of the polymer content of the TC could be carried out accurately. With a known mass of TC, a known mass of produced water, and calibration for partitioning as a function of polymer content, then the polymer content of the original solution could be determined, in theory, from measurement of the polymer content of the TC, following contact and separation from the produced water phase. Various treatment chemicals were examined. During this process, one was found that apparently displaced all PVCap (which is one of the most widely used and effective KHI polymers) from the aqueous phase, i.e., when calibrations were undertaken, it was found that up to 100% of the polymer had been displaced into the TC. Subsequent drying of the separated aqueous phase confirmed this: within accuracy, up to 100% of the polymer had been removed from the aqueous phase under ambient conditions. Various degrees of success were achieved with other KHI formulations. At this point, it was realized that the TC offered a potential solution to the problem of KHI removal from produced waters: KHI polymers causing problems with respect to produced water disposal (nonbiodegradability, precipitation/fouling in the case of produced water reservoir reinjection) and processing (e.g., avoiding polymer fouling of MEG regeneration units where KHIs were being used in addition to MEG), as detailed above. Since the initial discovery of KHIs, the research work has focused on confirming the ability of the TC family to displace PVCap from aqueous solutions at different concentrations and in the presence of MEG, salts, etc. A new family of TCs has been discovered recently with improved recovery factors for some KHI formulations. Furthermore, a novel approach has been formulated for dry gas systems that could have a significant effect on the reliability and economy of KHI-based hydrate inhibition techniques. It is believed that removal of KHIs has huge potential, including: • Addressing problems associated with produced water reinjection and MEG regeneration where KHI could come out of solution and precipitate/deposit. While the cost of installation of the process units may be case dependent and

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CONCLUSIONS Kinetic hydrate inhibitors (KHIs) are considered to be among the low dosage hydrate inhibitors and have been used in the industry for the past 20 years, despite limited knowledge on the mechanisms involved. New findings at Heriot−Watt University and Hydrafact have resulted in better understanding and reliable/repeatable test results. Furthermore, a new technique has been developed for removing KHIs from produced water. This could open up new opportunities for using KHI as the sole inhibitor or in combination with thermodynamic hydrate inhibitors (THIs). The latest developments are expected to result in a significant increase in the usage of KHIs in the industry, and we should expect a powerful return for KHIs.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +44 (0)131 451 3672. Fax: +44 (0)131 451 3127. Email: [email protected] (B. Tohidi). *E-mail: [email protected] (R. Anderson). *E-mail: houra.mozaff[email protected] (H. Mozaffar). *E-mail: [email protected] (F. Tohidi). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. § These authors contributed equally. 8259

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(9) Peytavy, J.-L.; Glénat, P.; Bourg, P. Qualification of |Low Dose Hydrate Inhibitors (LDHIs): Field Cases Studies Demonstrate the Good Reproducibility of the Results Obtained from Flow Loops. In Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, July 6−10, 2008. (10) http://www.hydrafact.com/software_hydraflash.html. (11) Nazeri, M.; Tohidi, B.; Chapoy, A. An evaluation of Risk of Hydrate Formation at the Top of a Pipeline. Presented at the SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, Oct. 22−24, 2012; Paper No. SPE 160404. (12) Mozaffar, H.; Anderson, R.; Tohidi, B. Effect of Ethylene Glycol Ethanol and Methanol on PVCap-induced Hydrate Crystal Growth Inhibition in Methane Systems. In Proceedings of the 8th International Conference on Gas Hydrates, Beijing, China, July 28−Aug. 1, 2014. (13) Cha, M.; Shin, K.; Kim, J.; Chang, D.; Seo, Y.; Lee, H.; Kang, S. Thermodynamic and Kinetic Hydrate Inhibition Performance of Aqueous Ethylene Glycol Solutions for Natural Gas. Chem. Eng. Sci. 2013, 99, 184−190. (14) Allenson, S. J.; Scott, A. Evaluation and Field Optimisation of Kinetic Hydrate Inhibitors for Application within MEG Recovery Units. Presented at the Gas Condensate Field, Mediterranean Sea, North Africa Technical Conference and Exhibition, Cairo, Egypt, Feb. 14−17, 2010; Paper No. SPE 127421. (15) Al-Shaabi, M.; Emadaddhi, K.; Roquet, D. Produced Water Management for Sustainable ReinjectionBench Scale Tests to Remove and Destroy KHI. Presented at the International Petroleum Technology Conference, Doha, Qatar, Jan. 19−22, 2014. (16) Hussain, A.; Gharfeh, S.; Adham, S. Study of Kinetic Hydrate Inhibitor Removal Efficiency by Physical and Chemical Processes. Presented at the SPE International Production and Operations Conference & Exhibition, Doha, Qatar, May 14−16, 2012; Paper No. SPE 157146. (17) Anderson, R.; Mazloum, V.; Tohidi, B. Water Treatment. World Patent No. WO 2013121217 A2, Aug. 22, 2013.

ACKNOWLEDGMENTS The authors would like to thank Hydrafact, Ltd., for their permission to submit this work.

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ABBREVIATIONS Δt = time difference ΔT = temperature difference AA = anti-agglomerant CAPEX = capital expenditure CGI = crystal growth inhibition CIR = complete crystal growth inhibition region HWU = Heriot−Watt University JIP = joint industry project KHI = kinetic hydrate inhibitor LDHI = low dosage hydrate inhibitor MEG = monoethylene glycol MeOH = methanol OPEX = operational expenditure PT = pressure−temperature PVCap = poly(vinylcaprolactum) PVT = pressure−volume−temperature PWRI = produced water reinjection RGR = rapid growth region sI = structure I sII = structure II SDR = slow dissociation rate region SGR = slow growth rate region ti = induction time TC = treatment chemical VS = very slow S = slow THI = thermodynamic hydrate inhibitor wt % = weight percent

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REFERENCES

(1) Kelland, M. A. History of the Development of Low Dosage Hydrate Inhibitors. Energy Fuels 2006, 20 (3), 825−847. (2) Klomp, U. The world of LDHI: From Conception to Development to Implementation. In Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, 2008. (3) Sloan, E. D.; Koh, C. Clathrate Hydrates of Natural Gases, Third Edition; CRC Press: New York, 2007. (4) Anderson, R.; Mozaffar, H.; Tohidi, B. Development of a Crystal Growth Inhibition Based Method for the Evaluation of Kinetic Hydrate Inhibitors. In Proceedings of the 7th International Conference on Gas Hydrates, Edinburgh, U.K., July 17−21, 2011. (5) Glénat, P.; Anderson, R.; Mozaffar, H.; Tohidi, B. Application of a New Crystal Growth Inhibition Based KHI Evaluation Method to Commercial Formulation Assessment. In Proceedings of the 7th International Conference on Gas Hydrates, Edinburgh, U.K., July 17− 21, 2011. (6) Bourg, P.; Glénat, P.; Bousque, M. L. Selection of Commercial Kinetic Hydrate Inhibitors Using a New Crystal Growth Inhibition Approach Highlighting Major Differences between Them. Presented at the SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, March 10−13, 2013; Paper No. SPE-164258-MS. (7) Luna-Ortiz, E.; Healey, M.; Anderson, R.; Sorhaug, E. Crystal Growth Inhibition Studies for the Qualification of a Kinetic Hydrate Inhibitor under Flowing and Shut-In Conditions. Energy Fuels 2014, 28, 2902−2913. (8) Rithauddeen, M. A.; Al-Adel, S.; Anderson, R.; Tohidi, B. A New Approach to Evaluate KHI Performance. In Proceedings of the 8th International Conference on Gas Hydrates, Beijing, China, July 28−Aug. 1, 2014. 8260

DOI: 10.1021/acs.energyfuels.5b01794 Energy Fuels 2015, 29, 8254−8260