Evolution of Pore–Fracture Structure in Deep Coal During CO2 Energy Storage Fracturing

Abstract

CO2 fracturing is an important stimulation method for deep coal seams, but the relationships among coal structural changes during CO2 exposure, pressure response, and post-fracturing network development require further characterization. High-temperature, high-pressure CO2 soaking, pore structure and mechanical tests, energy storage fracturing, and paired three-dimensional computed tomography (CT) reconstruction were combined in this study. Uniaxial compressive strength was lower by 23.8% and 51.5% in specimens exposed for 3 days than in those exposed for 1 day under the 52 °C and 18 MPa and 58 °C and 23 MPa conditions, respectively, whereas the elastic modulus changed in the opposite direction. Selective pore–fracture reorganization associated with the initial coal structure was revealed by CT. Breakdown pressure was reduced from 18.1 to 16.6 MPa, and time to breakdown was shortened from 7.65 to 1.42 min along an operating path in which storage duration and repressurization rate were increased simultaneously. The largest connected component accounted for 90.2% to 99.2% of the segmented fracture volume in the cylindrical specimens after fracturing. Network evolution was characterized by opening of existing fractures, coalescence of dispersed fractures, and branch development, while fracture volume and connectivity did not increase proportionally. CO2 exposure was accompanied by changes in pore–fracture structure and load-bearing behavior, while fracture extension, coalescence, and reconnection were identified following pressurization and rapid pressure release. These findings link storage-stage structural evolution to fracture network formation and provide an experimental basis for CO2 fracturing design and fracture network evaluation in deep coal seams.