The use of horizontal wells for directional perforation fracturing in the coal seam roof can effectively avoid defects such as structural fragmentation, poor mechanical properties, and construction difficulties in fragmented-soft coal seams with low-permeability characteristics, which improves the efficiency of gas extraction from fragmented-soft coal seams. Based on the rock samples from a coal mine site in northern Anhui Province, China, physical and mechanical parameters were obtained by indoor rock mechanics experiments, a roof-interface-coal seam similar fracturing specimen was prepared. According to the measured ground stress range of the sampling section, three sets of ground stress conditions of 16-12-12, 30-16-12, and 30-16-6.4 MPa were selected. The self-developed indoor rock mechanics true triaxial hydraulic fracturing simulation system was used to carry out hydraulic fracturing true triaxial physical simulation tests of a coal seam roof. The characteristics and causes of each stage of the fracturing curve were analyzed, and the influence laws of coal seam tensile strength, coal-rock interface tensile strength, and ground stress conditions on the expansion of hydraulic fractures in the roof were clarified. The morphologies of hydraulic fracturing fractures were summarized. The experiment used fracturing fluid with a viscosity of 1 mPa·s and mixed with red tracer, with a constant pumping rate of 15 mL/min. The results indicate that: (1) Low tensile strength of coal seams is conducive to the formation of complex fracture networks. When the tensile strength of coal seams is low and there is a significant difference from the tensile strength of overlying rock layers, cracks are sensitive to weak cementation at interlayer joints and are prone to deflection when extended to this point. (2) The lower tensile strength of the coal-rock interface leads to a thicker weak layer under the roof, and after the accumulation of fracturing fluid, it infiltrates along the joint planes to form horizontal bedding fractures, which is not conducive to the formation of longitudinal cracks that penetrate through. (3) Cracks always have a tendency to propagate along the direction of the maximum principal stress, and larger vertical stresses are conducive to the formation of longitudinal main cracks that connect the internal cracks of each layer. The key to forming a complex crack network is high vertical stresses and the small difference of stresses between two horizontal directions.
Zhang et al. (Tue,) studied this question.