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.

Automated summary

The study found that in shale, the permeability anisotropy increases significantly during CS-EGR, and has a greater influence on CS-EGR than in-situ stress.