Interaction behavior between hydraulic fractures and interface in coal-rock complex rock layer: Experimental study and application exploration

To achieve long-term and efficient gas extraction in soft, low-permeability coal seams, this study conducted hydraulic fracturing experiments on coal-rock complexes under true triaxial conditions. The pattern of hydraulic fractures (HFs) was reconstructed based on the fractal dimension concept. The results indicate that the tendency of the complex rock layers to initiate fractures toward the coal weakens the trend of increasing fracture initiation pressure with rising geostress. When HFs interact with the interface, the extension pressure significantly decreases. With the lateral pressure coefficient decreasing, HFs tend to extend toward the coal and be captured by the interface, transitioning from a single-wing to a double-wing shape and approaching a symmetrical conjugate state. Only when the vertical principal stress is sufficiently large can HFs separate from the interface. Based on the derived distribution function of induced stress in the coal-rock matrix around the HFs, the displacement conditions of the coal, rock, and interface were examined. The interaction process of rock layer HFs and the interface was divided into three stages: deflection, capture, and separation. The applicability of this study to high-gas soft coal seams was discussed, and a gas management plan involving roof fracturing and full-period extraction was proposed, with the aim of providing a theoretical foundation for the co-extraction and efficient utilization of coal and gas in mines.