Influence of anisotropic and heterogeneous permeability coupled with in-situ stress on CO₂ sequestration with simultaneous enhanced gas recovery in shale: Quantitative modeling and case study

CO₂ injection into shale facilitates a dual-purpose utility in enabling geological CO₂ sequestration with enhanced gas recovery (CS-EGR). Unfortunately, the CS-EGR responses under anisotropic/heterogeneous permeability and in-situ stress remain unclear. This study presents a thermal-hydraulic-mechanical coupled model to numerically determine how the anisotropic/heterogeneous permeability and/or in-situ stress affect the efficiency of CS-EGR in shale. The modelling results indicated that the permeability anisotropy was promoted significantly during the CS-EGR process, which determines the asymmetrical CO₂/CH₄ distribution in the modeling reservoir. It is also concluded that the anisotropic/heterogeneous permeability has a greater influence than in-situ stress on the CS-EGR. For a single shale layer, higher horizontal permeability or lower horizontal in-situ stress provide positive contributions in obtaining a better CS-EGR performance. For two contiguous shale layers, 1) the CS-EGR outputs are mainly controlled by the horizontal permeability or in-situ stress of the lower layer where the CO₂ injection well located; 2) greater horizontal permeability difference between the two layers enhance the magnitude of CO₂ accumulation and CH₄ desorption. Furthermore, the implication of this work is applied by an optimization of the location of the production well focusing on different CS-EGR objectives, under the background of the anisotropic/heterogeneous Silurian Longmaxi formation. In summary, the anisotropic/heterogeneous permeability and in-situ stress distributions, which exists in real shale reservoirs, provide significant influence in predicting a responsible CS-EGR outcome.