Over 12000 Gt carbon, stored as methane-hydrates (MH) in subsea sediments, makes MH a potential huge energy resource. Carbon dioxide (CO2) injection is a promising method for methane recovery from hydrate reservoirs. The injection process utilizes CO2 displacement by methane to shift the equilibrium and form a hydrate of CO2 in the place of MH. Therefore, not only methane can be recovered, but also the CO2 gas would be stored in the hydrate, which maintains subsea hydrate stability. Knowledge of the multiphase hydrate-liquid-vapor equilibrium conditions of CO2-H2O and CO2-CH4-H2O is essential for the efficient design of displacement process. Classically, models are based on an equation of state to describe the liquid and vapor phases and van der Waals-Platteeuw theory to describe hydrate phase behavior. Cubic equations, such as PR and SRK, have been employed in the past to model the multiphase V-LWH equilibrium for CO2-H2O and CO2-CH4-H2O systems. Cubic equations do not take into consideration the effect of association interactions and are expected to perform poorly in prediction of phase equilibrium conditions of the systems containing associating molecules, e.g. H2O. In the present work, three-phase hydrate-liquid water-vapor equilibrium of CO2-H2O and CO2-CH4-H2O systems are modeled based on the updated statistical association fluid theory (SAFT2) equation of state to describe the vapor and liquid phases along with the statistical model by van der Waals-Platteeuw to describe the hydrate phase behavior. The modeling results for CO2-H2O system are depicted for the temperature range between 273.15 to 292.15 K and the pressures up to 1600 bar. The binary interaction parameter was adjusted and formulated in terms of temperature. Three-phase V-Lw-H equilibrium conditions for CO2-CH4-H2O system were also modeled at various temperatures from 273.15 to 286.15 K and CO2 vapor mole fractions of 80, 60, 40, 20, 10 and 0% in the mixture. The calculated equilibrium conditions for both systems are in a good agreement with experimental equilibrium data in the literature.
July 27, 2016
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