Hydraulic horizontal slit-stress synergistic unloading fracturing in coal seams

Solving the problem of gas extraction from low-permeability coal seams is an urgent issue in the global coal industry, and exploring green and efficient mining methods has potential applications in this field. In this study, the No. 9 coal seam of the Guizhou Longfeng Coal Mine was selected as the research object. The method of using a hydraulic horizontal slit to create a compensation space and then synergizing it with the coal body's own stress to decompress and increase the permeability of the coal body was studied through similarity simulation and numerical simulation experiments. This study showed that the slit can cause the overall development, expansion, and penetration of coal seam fissures. Thereby, a large number of fissures are produced in the coal seam and its upper rock layer, which form, as a whole, a "positive trapezoid" shape. The three stages of the slit produced 165, 487, and 1572 fissures, which unpressurized and increased the penetration of the coal and rock body. The slit causes the upper coal body to lose its support, and the original stress balance of the coal and rock is broken, forming a decompression zone. At the same time, the coal body below starts to expand and deform after losing the vertical stress. The decompression zone gradually expands up and down, and the stress on both ends of the compensation space becomes larger, forming a stress concentration area. After the overall displacement of the coal seam caused by the slit, a "positive triangle" area is formed. In the upper part of the area, six displacement dividing lines are created, which form five areas. The displacement shows the distribution characteristics of a large displacement in the middle and a small displacement at both ends. The results of the similarity simulation and numerical simulation experiments were verified. The study provides a new way of thinking for effectively preventing coal and gas protrusion and improving mining efficiency.