Kerogen deformation upon CO2/CH4 competitive sorption: Implications for CO2 sequestration and enhanced CH4 recovery

Abstract

The low permeability of kerogen governs the storage and production of shale gas. The flexible kerogen constantly experiences mechanical deformation induced by reservoir environment and complex interplay with geofluids. However, the kerogen deformation associated with CH4/CO2 competitive sorption remains poorly understood. In this work, the effect of preloaded moisture on the deformation of kerogen with different organic types was investigated with molecular dynamics simulation. The kerogen deformation upon CH4/CO2 competitive sorption was quantified with the combination of grand canonical Monte Carlo simulations and poromechanics theory. The effects of various factors and their corresponding contributions were discussed in detail. The effects of kerogen deformation on CH4/CO2 diffusion were studied. Some implications for CO2 sequestration and enhanced gas recovery (CS-EGR) were proposed. Our results verify the theoretical feasibility of CS-EGR in shale gas reservoir. CO2 is observed to have a higher affinity with kerogen and a lower diffusion coefficient compared with CH4, facilitating it to replace CH4 and retain in the kerogen matrix. The rising CO2 composition can induce larger kerogen swelling, thus opening fluid flow pathways and increasing shale gas production. There are optimum moisture content and reservoir pressure corresponding to the maximum effective pore size in kerogen. It could be feasible to enhance the efficiency of CS-EGR by manipulating the reservoir moisture and CO2 injection timing. Thermal stimulation in deep shale reservoir may not be efficient for CS-EGR. CH4/CO2 competitive sorption can induce significant swelling of kerogen. The flexible nature of kerogen should be considered to improve the evaluation on both gas-in-place and CO2 storage capacity.