Heterogeneous Transport of Free CH4 and Free CO2 in Dual-Porosity Media Controlled by Anisotropic In Situ Stress during Shale Gas Production by CO2 Flooding: Implications for CO2 Geological Storage and Utilization.

Regarding shale gas production by CO2 flooding, few existing reports explain the performance of free CH4 and free CO2 in shale reservoirs controlled by anisotropic in situ stress, partly restricting the integrated recognition of shale-based CO2 geological storage and utilization (CGSU). In this work, a self-developed model embedded with thermo-hydro-mechanical coupling relationships is introduced to investigate how the anisotropic in situ stress determines the transport of free gases (CH4 and CO2) after CO2 is injected into the shale. Therefore, the stronger anisotropy of in situ stress enables more CO2 in the free phase to be trapped in the shale reservoir and is insignificant for the content of residual free CH4 compared to the situation under isotropic in situ stress. Along with CO2 injection into the shale, the matrix porosity decreases invariably, while the fracture porosity decreases first and then increases gradually. Therein, the variation amplitude of the matrix/fracture porosity is more distinct under a stronger anisotropic in situ stress. The simulations also suggest that the ratio of free CO2 relative to all free gases in shale is ∼65% at most after sufficient CGSU operation. Hopefully, this comprehensive work is helpful in enhancing the knowledge on the promising shale-based low-carbon CGSU technique.