Prediction of Gas Hydrate Formation Conditions in Aqueous Solutions of Single and Mixed Electrolytes
- You-Xiang Zuo (Dept. of Chemical Engineering, Technical U. of Denmark) | Erling H. Stenby (Dept. of Chemical Engineering, Technical U. of Denmark)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 1997
- Document Type
- Journal Paper
- 406 - 416
- 1997. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 4.6 Natural Gas, 5.9.1 Gas Hydrates, 5.2.2 Fluid Modeling, Equations of State, 4.3.1 Hydrates, 4.1.5 Processing Equipment, 5.2 Reservoir Fluid Dynamics
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In this paper, the extended Patel-Teja equation of state1 was modified to describe non-ideality of the liquid phase containing water and electrolytes accurately. The modified Patel-Teja equation of state (MPT EOS) was utilized to develop a predictive method for gas hydrate equlibria. The new method employs the Barkan and Sheinin hydrate model2 for the description of the hydrate phase, the original Patel-Teja equation of state3 for the vapor phase fugacities, and the MPT EOS (instead of the activity coefficient model) for the activity of water in the aqueous phase. The new method has successfully predicted the gas hydrate formation conditions in aqueous solutions of single and mixed electrolytes. The agreement between experimental data and predictions was found to be excellent.
Gas hydrates are ice-like clathrate components that may be formed when light hydrocarbons and/or some other light gases and water are contacted under certain conditions of temperatures and pressures. The formation of hydrates during the oil and natural gas production, transportation and processing can lead to serious problems. For preventing the formation of hydrates, inhibitors can be used to lower the formation temperature or to increase the formation pressure. On the other hand, however, the formation of gas hydrates offers the possibility for the development of sea water desalination process as well as processes for gas storage and separation.
Electrolytes have the ability to suppress the hydrate formation conditions. It is important to obtain hydrate equilibrium conditions in the presence of electrolytes since the drilling muds and naturally occurring water contain electrolytes. Holder et al.4 and Sloan5 have compiled the sources of hydrate equilibrium data available in the literature for natural gas components in pure water and in the presence of inhibitors like alcohols and single electrolytes. Recently, Bishnoi and his coworkers6-10 have reported experimental hydrate equilibrium data for methane, carbon dioxide, ethane, propane and methane-carbon dioxide mixtures in aqueous single and mied electrolyte solutions.
Menten et al.11 were the first to present an empirical method, based on freezing point depression data, for calculating light hydrocarbon hydrate formation conditions in single salt solutions.
Englezos and Bishnoi12 coupled the statistical thermodynamic model of van der Waals and Platteeuw13 with the available activity coefficient model14-15 for electrolyte solutions to calculate the hydrate equilibrium conditions in aqueous solutions of single and mixed electrolytes. Although this method produced excellent results for systems with substances sparingly soluble in water (e.g. light hydrocarbons, nitrogen), it cannot be suitable for carbon dioxide and hydrogen sulfide hydrate formation in aqueous electrolyte solutions because the solubilities of carbon dioxide and hydrogen sulfide in water are significant. On the other hand, as pointed out by Dholabhai7, the deviation increases with an increase of ionic strength for the method of Englezos and Bishnoi12 since salt concentrations are close to or exceed the maximum molality limits of the used activity coefficient model. Englezos16 used an equation of state for electrolytes to calculate hydrate formation conditions of only carbon dioxide in aqueous NaCl solutions. Satisfactory results were obtained. However, the Englezos16 method is complicated because the solubilities of carbon dioxide in aqueous NaCl solutions were required to be taken into consideration.
Recently, Barkan and Sheinin2 proposed an improved general technique for the calculation of gas hydrates. In their model gas solubilities in water are not needed to be taken into account and the model reproduces within experimental error the main body of known experimental data obtained over a wide range of temperatures and pressures. However, the inhibiting effect of electrolytes on gas hydrate formation was not reported.
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