Influence of Diffusion, Adsorption, Connate Water, and Salinity on Enhanced Gas Recovery and Carbon Storage Reservoir Simulations

We develop a compositional model based on the electrolyte cubic-plus-association (e-CPA) equation of state (EoS) to study the effects of diffusion, adsorption, connate water, and salinity on enhanced gas recovery reservoir simulations. The proposed e-CPA EoS is very accurate for modeling the phase equilibria of various gas–H2O–salt systems. The developed free-volume model can accurately calculate the viscosity of gas–H2O systems. The diffusion accelerates the mixing of the CO2–CH4 gas, which leads to early CO2 breakthroughs. As a result of competitive adsorption between CO2 and CH4, CO2 is adsorbed in rock and CH4 is desorbed from rock. Hence, the time of CO2 breakthrough and CH4 cumulative production increase. Adsorption is beneficial to enhanced gas recovery (EGR) and CO2 storage. The adsorption improves by more than 10% of the CH4 recovery factor, while the diffusion reduces by 3% of the CH4 recovery factor. For low-permeability reservoirs, diffusion is an important way to accelerate the mixing of CO2 and CH4 gas for considering convection and diffusion mechanisms, which reduces CH4 gas recovery. To consider adsorption, the time of production of pure CH4 gas is longer. Moreover, the early CO2 breakthrough leads to a lower CH4 gas recovery. The low permeability reduces the CH4 gas recovery factor by more than 20%. Because a portion of CO2 dissolves in the aqueous phase, connate H2O slightly increases the CH4 gas recovery factor. In low salt concentrations, the salt concentration in connate H2O has less effect on EGR, and we suggest neglecting salt effects for CO2 injection in depleted gas reservoirs.