Increasing production mechanism and key technology of gas extraction in coal mines

Gas extraction is the main way to control gas disasters in coal mines. For a long time, a variety of gas extraction technologies have been developed, which provide a powerful guarantee for the safe production of coal mines in China. However, for some serious outburst low-permeability coal seams, there are still problems of low extraction efficiency and high cost. There is an urgent need to form a system that can realize the transformation of gas extraction from " regular extraction to the quick, economical, pure and clean extraction" . Therefore, the quantitative characterization of coal and rock mass structure, gas migration and production mechanism, and gas migration and production process based on the structure of coal and rock mass were introduced firstly in this paper. The mechanism of gas extraction and production-increasing mechanism was clarified from the aspects of physical, biochemistry and surface modification. The research results of the team engaged in gas control work in the past 20 years are combined, a systematic analysis of six gas extraction technologies including hydraulic fracturing, integrated drilling and stamping, old hole repair, solid-liquid two phase hole sealing device and technology for three blocks and two injections, downhole automatic drainage, slag discharge and gas flooding, ground hole replacement for roadway were systematically analyzed, and self-developed equipment, devices and field application results were briefly introduced. It is pointed out that the coal body structure determines the transformability of the coal seam, determines the migration and production flow pattern of gas, and then puts forward the choice of gas extraction and production-increasing technology, which is the basis for the establishment of gas treatment technology system. The multiple production increase mechanisms of gas extraction were revealed, including permeability improvement from fracture generation, compacity enhancement, gas flooding, gas dilation, fracturing, microbial degradation, strong oxidant oxidation and surface modification, etc. It is indicated that the three technologies of hydraulic fracturing, drilling and stamping integration and the ground hole replacement for roadway are the hydraulic strengthening measures with permeability improvement and production increase as the main approaches. The three technologies of solid-liquid two phase hole sealing device and technology for three blocks and two injections, old holes repairing, downhole automatic drainage, slag discharge and gas flooding are based on multiple production-increasing mechanisms to extend the service life of the extraction boreholes and improve the quality and quantity of the extracted gas. The essence of gas flooding technology is to increase gas pressure gradient and realize gas extraction and increase production in low-pressure coal seam at the later stage of gas extraction. The completed engineering tests and application have fully demonstrated that the potential advantages of this technical system. With the further standardization and proceduralization of this technical system, it will become a universal technology for high-efficiency gas extraction in coal mines, and its popularization and application will greatly reduce cost of gas control.