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

Abstract

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.