Experimental study on the expansion law of hydraulic fracturing cracks in coal seam roof

The hydraulic fracturing of coal seam roof is one of the frequently used techniques for increasing production of soft crushed coal seam. Finding out the crack propagation law of coal seam roof in hydraulic fracturing can lay a foundation for the optimization of fracturing parameters. In this paper, similar materials of coal and rock are made, and physical simulation experiments of coal seam roof hydraulic fracturing under different stress states and different coal + roof strata are carried out by using true triaxial hydraulic fracturing experimental device. Based on rock fracture theory, the criterion of crack propagation is established. The effects of the combination of mechanical properties of coal and roof rock and the difference of horizontal stress on crack propagation are discussed, and the fracture propagation model of hydraulic fracturing is proposed. The results show that the fracture morphology of hard roof + hard coal and hard roof + soft coal is more complex, and the crack propagation distance is longer and can occur through layer propagation. The fracture morphology of soft roof + hard coal is relatively simple, the crack propagation distance is short and it is not easy to cross layer propagation. When the horizontal stress difference is small, it is difficult for fracturing fractures with the stress state of extrusion type to propagate through the layer. The hydraulic fracturing fractures of the specimens with small horizontal stress difference, the specimens with large horizontal stress difference and the specimens with large horizontal stress difference all spread through the layer, and the fracture morphology is “1” font, “V” font and “7” font, respectively. Fracture propagation of hydraulic fracturing is controlled by fracture stress intensity factor and rock fracture toughness. Whether or not cross-layer propagation can occur can be judged by the steep drop of construction pressure. During hydraulic fracturing, there are four crack propagation modes: fracturing fracture roof extension, roof and bedding surface extension, through layer extension, roof extension + manual intervention through layer extension. The research results provide the experimental basis for the research on the mechanism of seam roof penetration in hydraulic fracturing.