Research advances in enhanced coal seam gas extraction by controllable shock wave fracturing

With the continuous increase of mining in depth, the gas extraction faces the challenges of low permeability, great ground stress, high temperature and large gas pressure in coal seam. The controllable shock wave (CSW), as a new method for enhancing permeability of coal seam to improve gas extraction, features in the advantages of high efficiency, eco-friendly, and low cost. In order to better utilize the CSW into gas extraction in coal mine, the mechanism and feasibility of CSW enhanced extraction need to be studied. In this paper, the basic principles, the experimental tests, the mathematical models, and the on-site tests of CSW fracturing coal seams are reviewed, thereby its future research directions are provided. Based on the different media between electrodes, the CSW can be divided into three categories: hydraulic effect, wire explosion and excitation of energetic materials by detonating wire. During the process of propagation and attenuation of the high-energy shock wave in coal, the shock wave and bubble pulsation work together to produce an enhanced permeability effect on the coal seam. The stronger the strength of the CSW is, the more cracks created in the coal is, and the greater the length, width and area of the cracks being. The repeated shock on the coal seam is conducive to the formation of complex network fracture system as well as the reduction of coal seam strength, but excessive shock frequency will also damage the coal structure, resulting in the limited effect of the enhanced gas extraction. Under the influence of ground stress, the crack propagation in coal seam will be restrained. The difference of horizontal principal stress has a significant impact on the shape, propagation direction and connectivity of the CSW induced cracks. The permeability enhancement effect of CSW is affected by the breakage degree of coal seam. The shock wave is absorbed by the broken coal, which may hinder the propagation of CSW, resulting in a poor effect of permeability enhancement. When arranging two adjacent boreholes for CSW permeability enhancement test, the spacing of boreholes should not be too close, which may lead to negative pressure mutual pulling in the early stage of drainage. At present, the accurate method for effectively predicting the CSW permeability enhanced range should be further investigated.