Experimental Study of Supercritical CO₂ Fracturing Across Coal–Rock Interfaces

Indirect fracturing with supercritical carbon dioxide (CO₂) is a promising technology of coalbed methane exploitation in soft and low-permeability coal seams. The key to its success is to ensure the cracks produced by supercritical CO₂ fracturing can penetrate the coal–rock interface and enter the coal seam. In this work, the crack propagation behaviors at coal–rock interfaces in supercritical CO₂ fracturing is studied through laboratory experiments, and the experimental results are compared with those of hydraulic fracturing. The results show that whether the cracks can penetrate the coal–rock interface is dependent on the vertical stress and interface cohesion. Only when the stress difference between the vertical stress and the minimum horizontal principal stress exceeds a certain threshold value will the crack extend across the coal–rock interface. The higher the interface cohesion is, the easier it is for cracks to cross the coal–rock interface. The threshold values are negatively related to the interfacial cohesion. For interfacial cohesion values of 0.1, 0.3, and 0.7 MPa, the threshold values are 12, 10, and 8 MPa, respectively. Unlike indirect fracturing with water, in supercritical CO₂ fracturing, the injection pressure curve does not present a significant secondary rise when cracks cross the coal–rock interface. Compared with hydraulic fracturing, in supercritical CO₂ fracturing, the stress difference between the vertical stress and the minimum horizontal principal stress for cracks to penetrate through the coal–rock interface is larger under the same conditions.