RESEARCH PROGRESS ON CO₂ FRACTURING, REPLACEMENT AND DISPLACEMENT TO ENHANCE THE SHALE GAS RECOVERY

CO₂ fracturing, replacement, and displacement are critical methods for enhancing shale gas recovery. This technology enables reservoir fracturing and CH₄ displacement, thereby improving shale gas production while achieving CO₂ geological storage. It is therefore essential to summarize and understand existing technical advances in CO₂ fracturing, replacement, and displacement to guide future research in this field. This paper presents a review from the perspective of scientific mechanisms, covering supercritical CO₂ fracturing, CO₂ replacement of CH₄, CO₂ displacement of CH₄, and key scientific issues related to enhancing shale gas recovery via the integrated CO₂ fracturing-replacement-displacement technology. The results indicate that this integrated technology is characterized as green, clean, environmentally friendly, and resource-efficient. Supercritical CO₂ fracturing offers several advantages over hydraulic fracturing, including reduced rock strength, maintained stability of clay minerals, increased pore pressure, phase transition-induced fracturing, and thermal stress cracking. Furthermore, compared to CH₄, CO₂ exhibits a smaller molecular diameter, lower adsorption temperature, smaller self-diffusion coefficient, and quadrupole moment characteristics. CO₂ molecules can replace adsorbed CH₄ through competitive adsorption, and CO₂ can displace CH₄ under pressure—a process influenced by factors such as temperature, pressure, and pore diameter. Currently, further research is needed in areas such as viscosity enhancement of supercritical CO₂, the microscopic mechanism of CO₂/CH₄ competitive adsorption, quantification of adsorption sites, flow and mechanical mechanisms of CO₂ displacement of CH₄, reservoir modification, and CO₂ geological storage and monitoring. The combined CO₂ fracturing-replacement-displacement technology shows great promise as a crucial method for alleviating energy pressure and contributing to the achievement of carbon peaking and carbon neutrality goals.