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

Abstract

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.