Thermal Displacement with CO2 for E-CBM Recovery: Mechanisms and Efficacy of Temperature–Pressure Synergy in Permeability Enhancement

The efficient development of coalbed methane (CBM) faces persistent challenges due to low recovery rates. While CO2 thermal displacement offers a promising approach, the pore–fracture structure (PFC) evolution and gas displacement mechanisms under temperature–pressure coupling remain insufficiently clear. To address this knowledge gap, the in situ, dynamic quantification of pore–fracture evolution during CO2 displacement was achieved by an integrated system with NMR and CT scanning, revealing the expansion, connection, and reconfiguration of coal PFC under temperature–pressure synergy and establishing the intrinsic relationship between supercritical CO2 (ScCO2)-induced permeability enhancement and methane displacement efficiency. Experimental results identify an observed transition in permeability near 80 °C under the tested conditions as a critical permeability transition point: below this value, permeability declines from 0.61 mD to 0.49 mD, reflecting pore structure adjustment; above it, permeability rises markedly to 1.18 mD, indicating a structural shift toward fracture-dominated flow. A “pressure-dominated, temperature-assisted” mechanism is elucidated, wherein pressure acts as the primary driver in creating macro-fractures and forming percolation pathways, while temperature—mainly via thermal stress—promotes micro-fracture development and assists gas desorption, offering only limited direct contribution to permeability. Although elevated injection pressure enhances permeability and establishes fracture networks, displacement efficiency eventually reaches a physical limit. To transcend this constraint, a synergistic production mechanism is proposed in which pressure builds flow channels while temperature activates microporous desorption. This study provides an integrated, in situ quantification of the pore–fraction evolution under high-temperature ScCO2 conditions. The elucidated synergy between pressure and temperature offers insights and an experimental basis for the design of deep CBM recovery and CO2 storage strategies.