Abstract
For low‐permeability coal seam, the gas extraction rate is relatively low. The repeated low‐intensity shock wave can improve the permeability of coal and raise the rate of coal seam gas drainage. A simulation test platform was set up to carry out repeated low‐intensity shock wave simulation experiment. Under the effect of repeated low‐intensity shock wave, the development process of the macrofracture, pore evolution, and the law of microcrack propagation was analyzed to study the law of coal fracture propagation. Research shows that the expansion of the pore of the coal is made by the development of large, medium, and micropores by the repeated low‐strength shock wave. The main contribution of the total pore volume increase comes from the micropore growth. The microcrack of the coal mainly begins to sprout and develops from the telocollinite where the fracture is more developed. With the increase of impact times, the microcracks extend to other components. Under the impact of different times, the fractal dimension of the coal sample increases with the increase in the number and length of cracks.
Highlights
Improving the permeability of coal seam and increasing gas drainage have always been the main research direction of gas disaster control and gas resource extraction technology [1, 2]
Experimental Device. e low-intensity explosion impact simulation experiment device mainly consists of three parts, such as a water tank (1.2 m × 1.2 m × 1.2 m in size, water depth 0.8 m), a simulated shock wave generator, and a shock wave pressure sensor (Figure 1). e simulated drilling casing size is 139.7 mm, the hole density is 60 holes per metre, and the hole diameter is 10 mm. e shock wave generating device triggers the energetic bomb with electric energy to form a low-intensity shock wave. e shock wave pressure sensor is fixed on the water tank. e probe position is parallel to the center point of the shock wave generating window, and the distance is 300 mm
In order to prevent the coal from falling into the water due to cracking during the impact, a metal mesh with a mesh size of 10 mm × 10 mm was used to wrap the coal block. en, the coal block was immersed in the water tank along the parallel bedding direction, and the occurrence window of the shock wave was consistent with the central position of the coal sample. e distance between coal sample and shock wave source is 100 mm
Summary
Improving the permeability of coal seam and increasing gas drainage have always been the main research direction of gas disaster control and gas resource extraction technology [1, 2]. Hydraulic sand fracturing is the main measure to improve permeability of coal seam at present [3,4,5,6]. It is an effective method to improve the efficiency of gas extraction by adopting explosions increasing permeability of coal seam [11,12,13]. Strong pressure shock waves were generated by underwater electrical discharges or high energy explosion, which served to damage the associated solids, creating fractures and cracks. The secondary disasters such as roof fall were caused by strong pressure shock waves [16,17,18]
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