Laboratory Evaluation of Kinetic Hydrate Inhibitors: A Procedure for Enhancing the Repeatability of Test Results Christophe Duchateau,* ,† Jean-Louis Peytavy, ‡ Philippe Gle ´nat, ‡ Tong-Eak Pou, § Manuel Hidalgo, § and Christophe Dicharry † Laboratoire des Fluides Complexes, UMR CNRS 5150, UniVersite ´ de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, France, TOTAL, CSTJF, AVenue Larribau, 64018 Pau Cedex, France, and Arkema-Ceca, CRRA, Rue Henri Moissan, BP 63, 69493 Pierre-Be ´nite Cedex, France ReceiVed August 27, 2008. ReVised Manuscript ReceiVed NoVember 28, 2008 Evaluation of kinetic hydrate inhibitors (KHIs) in laboratory high-pressure cells generally yields scattered results. In this work we propose a procedure that consists of characterizing the effectiveness of KHI on second hydrate formation. This procedure limits the stochastic character of hydrate crystallization using the persistence of precursory hydrate structures in water that has previously experienced hydrate formation and decomposition. It is shown that the presence of these precursors strongly increases the repeatability of results compared with systems containing “fresh water” and allows unambiguous discrimination between blank (uninhibited system) and KHI tests. 1. Introduction Gas hydrates can form at pressures and temperatures com- monly encountered in oil and gas production systems. 1 They can block flow lines, valves, wellheads, and pipelines, causing huge production losses and posing safety problems. The conventional treatment for prevention of hydrate plug formation is based on injection of thermodynamic hydrate inhibitors (THIs) such as methanol or ethylene glycol, which shift the hydrate equilibrium conditions to higher pressures and lower temperatures. As the search for oil and gas goes into colder and/or deeper regions, the quantity of THI required to prevent installations from plugging increases, thereby inducing prohibitive expenses. 2 Injection of low-dosage hydrate inhibitors can be an alternative and cost-effective method of hydrate prevention. 3,4 This class of molecules can be separated into two groups: antiagglomerants (AAs) and kinetic inhibitors (KHIs). AAs are surfactants that allow formation of transportable hydrate slurry, whereas KHIs are polymers with surfactant properties which delay hydrate nucleation and/or crystal growth. Only KHIs are discussed in this paper. Before being used on production fields, KHIs have to be evaluated in the laboratory. 5 Generally, oil companies use flow loops to simulate field conditions. 6 Because such equipment is rather expensive to run, KHI suppliers prefer using laboratory high-pressure cells to evaluate the effectiveness of their addi- tives. However, this more convenient and cost-effective way for testing KHIs generally yields scattered results due to the stochastic nature of hydrate crystallization. 7 Improving the repeatability of high-pressure cell test results is necessary to increase confidence in the evaluation of KHIs and develop better KHIs. In this paper, we propose an experimental procedure for laboratory evaluation of KHIs that allows producing repeatable results. This procedure is based on use of the residual hydrate structures remaining in a water phase that has previously experienced a hydrate formation/decomposition cycle. 8-10 The procedure is derived from the protocol that has been imple- mented by Total for 15 years in their flow loop. 11 2. Experimental Section 2.1. Chemicals. The feed gas was a mixture of 98 mol % methane and 2 mol % propane supplied by Linde. This binary gas mixture is predicted to form structure II hydrates. The hydrate phase diagram 12 for this gas mixture and for the range of pressures and temperatures of interest for this study is shown in Figure 1. The oil phase was a degasified condensate from the Frigg field (North Sea) sampled during a no treatment production phase. * To whom correspondence should be addressed. Phone: +33-5-59-40- 76-86. Fax: +33-5-59-40-76-95. E-mail: christophe.duchateau@univ-pau.fr. † Universite ´ de Pau et des Pays de l’Adour. ‡ TOTAL. § Arkema-Ceca. (1) Sloan, E. D. Clathrate hydrates of natural gases, 2nd ed.; Marcel Dekker: New York, 1998. (2) Sloan, E. D. Fluid Phase Equilib. 2005, 228-229, 67–74. (3) Phillips, N. J.; Kelland, M. A. The application of surfactants in preventing gas hydrate formation. In Industrial application of surfactants IV; Proceedings of the 4th Congress on Industrial Application of Surfactants, Cambridge, 1998. (4) Varma-Nair, M.; Costello, C. A.; Colle, K. S.; King, H. E. J. Appl. Polym. Sci. 2007, 103 (4), 2642–2653. (5) Leporcher, E. M.; Fourest, J. M.; Labes-Carrier, C.; Lompre, M. Multiphase transportation: a kinetic inhibitor replaces methanol to prevent hydrates in a 12-inc. pipeline. In Proceedings of the European Petroleum Conference, The Hague, 1998. (6) Peytavy, J. L.; Gle ´nat, P.; Bourg, P. IPTC 11233. International Petroleum Technology Conference, Dubai, Dec 4-6, 2007. (7) Kashchiev, D.; Firoozabadi, A. J. Cryst. Growth 2002, 243 (3-4), 476–489. (8) Vysniauskas, A.; Bishnoi, P. R. Chem. Eng. Sci. 1983, 38 (7), 1061– 1072. (9) Herri, J. M.; Gruy, F.; Pic, J. S.; Cournil, M.; Cingotti, B.; Sinquin, A. Chem. Eng. Sci. 1999, 54 (12), 1849–1858. (10) Lee, J. D.; Susilo, R.; Englezos, P. Chem. Eng. Sci. 2005, 60 (15), 4203–4212. (11) Peytavy, J. L.; Gle ´nat, P.; Bourg, P. Qualification of low dose hydrate inhibitors (LDHIS): Field cases studies demonstrate the good reproducibility of the results obtained from flow loops. 6th International Conference on Gas Hydrates, Vancouver, 2008. (12) Tohidi, B.; Danesh, A.; Todd, A. C. Chem. Eng. Res. Des. 1995, 73 (A4), 464–472. Energy & Fuels 2009, 23, 962–966 962 10.1021/ef800710x CCC: $40.75  2009 American Chemical Society Published on Web 01/13/2009