Molecular investigation on CO2-CH4 displacement and kerogen deformation in enhanced shale gas recovery

CO2-enhanced gas recovery (EGR) is a promising method that can not only improve the production of adsorbed shale gas but also achieve the geological storage of CO2. During the injection of CO2, shale is expected to deform due to both CH4 desorption-induced shrinkage and CO2 adsorption-induced swelling. Regardless of the potential effects on gas permeability and transport, the sorption-induced deformation remains poorly understood and is generally overlooked in large-scale simulations of CO2-EGR. In this paper, Monte Carlo and molecular dynamics simulations are performed to study the CO2-CH4 displacement in the type II-D overmature kerogen (i.e., the primary organic matter in shale). Our results indicate that the CO2-CH4 displacement efficiency increases with the increase of the CO2 injection pressure but decreases linearly with the increase of the reservoir depletion pressure. Moreover, the sorption-induced deformation is measured as volumetric strain and analyzed after initial CH4 saturation, pressure drawdown, and CO2 injection. It is observed that CO2 injection can induce a volume expansion of the original CH4-saturated kerogen up to 5%. In addition, a novel non-linear adsorption-strain model is derived to provide a theoretical basis for kerogen deformation by taking the gas adsorption and deformation coupling into consideration. Different from the conventional linear theory, the increment of volumetric strain induced by gas adsorption becomes much faster when close to adsorption saturation. The derived model can accurately capture the observed non-linearity and predict deformation caused by single-component gas and gas mixtures. Since swelling can further reduce shale permeability, these results imply that CO2 injectivity and CH4 production rate may both decline faster than expected in CO2-EGR.