| At present, the borehole drainage is the main measure for gas control in the coal mines of China. However, the gas drainage in China is very difficult for the micro-pores, low-permeability and high-adsorption of the coal seams, which severely restricts the efficient exploitation of coal and gas. The borehole permeability-enhancement technique using water jet is one of the efficient ways to improve the gas drainage efficiency, which has been widely used in the field. However, the research about characteristics of breaking coal-rock mass and mechanism of permeability enhancement by high-pressure water jet was mainly conducted by numerical simulation, therefore the theoretical research drops behind the field application and the technique improvement is lack of theoretical support, which are the urgent problems to be solved. In the current work, taking the application of the permeability-enhancement technique using water jet in the coal mine gas drainage as the engineering background, we further studied the water jet impingement characteristics in borehole and the mechanism of breaking coal-rock mass, proposed a new permeability enhancement method by water jet and analyzed the permeability-enhancement mechanism and the affecting range. The main findings achieved are as follows:(1)Pore structure and adsorption properties of outburst-prone coal seam, as well as the effect of the water jet impingement on coal seam were investigated by the Mercury Intrusion Porosimetry, methane isothermal adsorption and nuclear magnetic resonance technique. The results show that the development degree of pores in the outburst-prone coal seam is relatively low, the connectivity of pore is poor and the pore structure is dense. The large number of micro-pores and mini-pores lead to a high adsorption capacity of coal seam, while the small number of meso-pores and macropores block gas migration and increase gas drainage difficulty. The impingement of water jet can reform pore structure, reduce the proportions of micro-pores and minipores and increase the proportions of meso-pores and macro-pores.(2)The impingement characteristics of water jet were revealed. Compared with the impingement of the plane, the fluid form and structure characterization are significantly different. At the moment of impacting the borehole by water jet, a “water cushion” formed at the contact point, the thickness of the “water cushion” and area of the reflected fluid increases when the impingement is stable, and the reflection angle is obviously lager than that of the plane. The velocity and pressure of water jet on borehole surface are relatively small, but they reduced slowly when water jet intrudes into coal mass. Based on the experimental results, the affecting area length of water hammer and the acting time of water jet were derived by theoretical analyses. In addition, the impact force of water jet exerted on borehole surface was modeled from the perspective of momentum.(3)The mechanism of breaking coal-rock mass by water jet in borehole was revealed. A higher pressure led to a higher impact force, which resulted in bigger impact depth and area. With an increase of the impact target distance, the impact force decreased, and the impact depth and area increased, indicating the increase of rupture degree and change of rupture mode. Compared with plane, the impact depth is smaller, the impact area is larger, and the time needed to break the coal is longer in borehole. Based on the investigations of timeliness and heat effect of water jet impact, the process and mechanism of breaking coal-rock mass by water jet in borehole were described from the perspective of damage and energy. The process of breaking coal-rock mass by water jet in borehole is also the process of damage evolution of coal caused by water jet, which was a combined result caused by impact load and quasi-static pressure. The essence of the process is converting kinetic energy of water jet to internal energy of coal-rock mass.(4)The model of borehole radial permeability was established, and the effect of borehole radius on the radial permeability was analyzed. It was proved that increasing the borehole radius was an efficient way to improve radial enhanced permeability. Based on the research result, the method of forming large diameter cross borehole by water jet was proposed. The crack evolution rules of coal containing borehole was numerically investigated, and the influence radius and optimum spacing were analyzed. The result showed that the increase of borehole diameter increased the tensile stress around borehole, enhanced the interaction among boreholes, increased the secondary fractures and expanded the effective influence range of borehole. The effective influence radius can be fitted by the expression as0 0 01.108 ln() 3.612IR(28)-r r(10)r, or estimated as 2 times of the plastic zone radius.(5)Based on the loading test system for specimen containing circular hole, the loading characterizations and damage mechanism of the coal containing hole were investigated and the permeability enhancement mechanism of large diameter borehole was described from the perspectives of increasing range of pressure relief, improving pore structure, forming fracture network and enhancing effect among boreholes. Aiming at the high outburst-prone coal seam in 12 th Coal Mine of Pingdingshan Coal Group Co., Ltd., we proposed the gas drainage mode using large diameter borehole. The field test results showed that after the adoption of the gas drainage mode, the number of boreholes used for draining gas in coal roadway strip reduced by 32.5%,the length of the cross boreholes decreased by 42.9%, and the rate of coal roadway driving improved by nearly 100%.During the research, 20 papers were published, 5 of which were published as first author(1 SCI and 3 EI). 19 national invention patents were applied, 14 of which had been authorized. The author was also received 3 provincial science and technology award, namely one first prize and two second prize. |
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