Fracture–Seepage Behavior of Coal under Pulsating CO ₂ Foam Fracturing with Varying Injection Flow Rates: Implications for CO ₂ Sequestration

Pulsating CO2 foam fracturing (PCFF) is emerging as a promising technology for enhancing coalbed methane (CBM) recovery while achieving efficient CO2 geological sequestration. However, the impact of injection flow rate on the fracture–seepage behavior of coal subjected to this method remains unclear. To address this issue, a series of experiments were conducted using a self-built PCFF apparatus, varying the injection flow rates (1–5 L/h). Real-time recordings of pressure-time curves and AE signals were taken, while the postfracturing fracture morphology was visualized and reconstructed using 3D-XRM. Fracture permeability and surface roughness of the fractured coal were characterized. The results demonstrated that higher injection flow rates elevated the breakdown pressure of coal while shortening the fracturing duration. Simultaneously, AE cumulative energy, fracture volume, surface area, permeability, and surface roughness all decreased, resulting in simpler crack formations. When the injection flow rate increased from 1 to 5 L/h, the breakdown pressure rose by 46.22%, the fracturing duration decreased by 72.60%, and the fracture permeability was reduced by 91.05%. As injection flow rate increased, the proportion of tensile failure increased, whereas shear failure decreased. Lower injection flow rates promoted more shear failure, which helped form a more complex fracture network with higher heterogeneity, thus enhancing both CBM recovery and CO2 sequestration efficiency. The AE-improved b value showed a significant decrease before forming macroscopic cracks in the coal, serving as a predictive criterion for evaluating fracturing effectiveness in practical applications. The research findings address a critical gap in understanding the fracture–seepage behavior of coal subjected to PCFF at varying injection flow rates. These results aid in advancing the engineering application of PCFF technology, offering significant benefits for enhancing CBM recovery and achieving efficient CO2 geological sequestration.