Phase Behavior of CO2-CH4-Water Mixtures in Shale Nanopores Considering Fluid Adsorption and Capillary Pressure

The accurate description of adsorption and phase behavior of gas-water mixtures in nanopores is essential for CO2enhanced gas recovery and geological sequestration in water-containing shale gas reservoirs. However, the distribution and phasebehavior of shalefluids in the presence of the water phase remain unclear. In this work,first, based on molecular dynamicssimulation, a new method to describe the adsorption of gas-water mixtures is proposed using adsorption thickness and reducedadsorption density. Second, the two adsorption parameters are utilized to establish an adsorption-dependent equation of state that isspecified for gas-water mixtures (GWA-PR-EOS). Third, a two-phase behavior calculation model is developed by combiningcapillary pressure with GWA-PR-EOS. Finally, the model is tested on a CO2-CH4-water mixture to evaluate its reliability andaccuracy and then used to calculate the phase equilibrium of CO2-CH4-water mixtures under different confinement andfluid feedconditions. Results indicate that the molecules of the vapor phase are inclined to move into the aqueous phase under theconfinement effect. Without the nanopore confinement effect, the molar fractions of CO2and CH4in the aqueous phase changeslightly; however, under the nanopore confinement effect, the molar fractions of CO2and CH4in the aqueous phase increase withthe water feed. Overall, the effect of confinement on the mutual solubility is more significant under smaller pore size, lower pressure,smaller CO2feed, and larger water feed conditions