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Effects of competitive adsorption on production capacity during CO2 displacement of CH4 in shale

During CO2 displacement of CH4 in shale, competitive adsorption results in reduced pore space used for gas flow in shale, which is closely associated with the production capacity of shale-gas reservoirs. Thus, the present work investigates the effects of CO2–CH4 competitive adsorption on production capacity. Herein, a slit–pore model is developed in terms of gas storage (CO2 and CH4) and graphene pores using molecular dynamics and implemented via large-scale atomic/molecular massively parallel simulator. The effects of CO2 injection pressure, temperature, and velocity and of pore size on CO2–CH4 displacement and competitive adsorption properties are simulated and examined. Hence, the displacement efficiency of CH4 and the adsorption layer thickness of the CO2–CH4 binary mixture are determined. Moreover, based on a basic seepage model of planar linear flooding, the effect of CO2–CH4 competitive adsorption on production capacity is analytically investigated. Results demonstrate that the production capacity with consideration of adsorption layer thickness is less than that without consideration of adsorption layer thickness, illustrating that CO2–CH4 competitive adsorption behaviors are closely connected with permeability, flow rate, and production capacity of shale-gas reservoirs, especially for shale-gas reservoirs containing large numbers of pores and slits.