A diffuse layer model for hydrocarbon mass transfer between pores and organic matter for supercritical CO2 injection and sequestration in shale

Abstract

Abstract CO2 injection in shale oil reservoirs is a feasible method for CO2 geological sequestration and enhanced oil recovery. However, the mechanisms of mass transfer in inorganic pores and organic matter (kerogen) are still ambiguous. In this study, the mechanisms of diffusion and extraction were investigated. A novel pore-kerogen diffuse layer (PKDL) model was proposed for mass transfer between kerogen matrix and pores (inorganic pores and organic pores). Mathematical models for hydrocarbon mass transfer in spherical and cylindrical shaped rocks were derived. The predicted responses of the mathematical models closely matched the experimental data of CO2 injection experiments performed on various shales and tight rocks at high pressure and high temperature conditions. Hydrocarbon recovery of shales shows a delayed effect compared to tight rocks due to the additional extraction process between supercritical CO2 and kerogen. Hydrocarbons were extracted out of the kerogen matrix and then diffused through the inorganic and organic pores. Hydrocarbon diffusion coefficients ranged from 0.61 to 3.3\xa0×\xa010−7 m2/s and extraction rate coefficients varied from 1.7 to 4.9\xa0×\xa010−5 s−1 for shale rocks. Hydrocarbon diffusion coefficients varied from 0.49 to 4.8\xa0×\xa010−7 m2/s for tight rocks. Both diffusion coefficients and extraction rate coefficients decreased exponentially with carbon numbers. The extraction rate coefficients decreased exponentially because the diffusion coefficients of hydrocarbons in kerogen also decreased exponentially with carbon numbers. This research advances the diffusion and extraction theories in shale rocks and promotes the injection and sequestration of CO2 in shale formations.