Mechanisms and Multi-Field-Coupled Responses of CO2-Enhanced Coalbed Methane Recovery in the Yanchuannan and Jinzhong Blocks Toward Improved Sustainability and Low-Carbon Reservoir Management

Supercritical CO2 modifies deep coal reservoirs through the coupled effects of adsorption-induced deformation and geochemical dissolution. CO2 adsorption causes coal matrix swelling and facilitates micro-fracture propagation, while CO2–water reactions generate weakly acidic fluids that dissolve minerals such as calcite and kaolinite. These synergistic processes remove pore fillings, enlarge flow channels, and generate new dissolution pores, thereby increasing the total pore volume while making the pore–fracture network more heterogeneous and structurally complex. Such reservoir restructuring provides the intrinsic basis for CO2 injectivity and subsequent CH4 displacement. Both adsorption capacity and volumetric strain exhibit Langmuir-type growth characteristics, and permeability evolution follows a three-stage pattern—rapid decline, slow attenuation, and gradual rebound. A negative exponential relationship between permeability and volumetric strain reveals the competing roles of adsorption swelling, mineral dissolution, and stress redistribution. Swelling dominates early permeability reduction at low pressures, whereas fracture reactivation and dissolution progressively alleviate flow blockage at higher pressures, enabling partial permeability recovery. Injection pressure is identified as the key parameter governing CO2 migration, permeability evolution, sweep efficiency, and the CO2-ECBM enhancement effect. Higher pressures accelerate CO2 adsorption, diffusion, and sweep expansion, strengthening competitive adsorption and improving methane recovery and CO2 storage. However, excessively high pressures enlarge the permeability-reduction zone and may induce formation instability, while insufficient pressures restrict the effective sweep volume. An optimal injection-pressure window is therefore essential to balance injectivity, sweep performance, and long-term storage integrity. Importantly, the enhanced methane production and permanent CO2 storage achieved in this study contribute directly to greenhouse gas reduction and improved sustainability of subsurface energy systems. The multi-field coupling insights also support the development of low-carbon, environmentally responsible CO2-ECBM strategies aligned with global sustainable energy and climate-mitigation goals. The integrated experimental–numerical framework provides quantitative insight into the coupled adsorption–deformation–flow–geochemistry processes in deep coal seams. These findings form a scientific basis for designing safe and efficient CO2-ECBM injection strategies and support future demonstration projects in heterogeneous deep coal reservoirs.