Experimental Research into the Initiation and Propagation of Hydraulic Fractures in Coal Combinations with Different Strengths

The fracture initiation location and expansion mode are important indexes to evaluate the fracturing effect during the hydraulic fracturing of a coal seam. However, the initiation location and propagation of hydraulic fractures in different-strength coal assemblages are still unclear. For this reason, physical simulation experiments on the initiation and propagation of hydraulic fractures in coal composite with different strength were carried out. According to the difference of fracture initiation position, pumping pressure curve, and propagation trend, three fracture initiation modes, three fracture propagation modes, and four fracture morphology are classified. The formation mechanism of different fracture forms is revealed. The results show that with different-strength coal combinations, the strength difference leads to joint crack initiation, and the single main crack is formed only in low-strength coal with crack propagation and weak structural plane crack propagation. The strength difference leads to joint cracking, and only in low-strength coal, high-strength coal expansion, and structural plane expansion is a single cross crack formed. Multiple clusters of independent fractures are formed by strength difference leading to coinitiation, stress difference leading to coinitiation, low-strength coal expansion, and weak structural plane expansion. Strength difference leads to joint cracking, and stress difference leads to joint cracking, lows trength coal expansion, and high-strength coal expansion to form multiple clusters of network cracks. When the vertical stress difference is 3 to 4 MPa and the stress difference coefficient is 0.3 to 0.4, the complex fracture network can be formed. It is concluded that reducing the number of perforations of low-strength coal can improve the influence of strength difference on fracturing effect. The results provide experimental support for the initiation and propagation behavior of hydraulic fractures in coal combinations with different strengths.