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Coal bed Methane (CBM), a primary component of natural gas, is a relatively clean source of energy.Nevertheless, the impact of considerable coal mine methane emission on climate change in China has gained an increasing attention as coal production has powered the country's economic development. It is well-known that coal bed methane is a typical greenhouse gas, the greenhouse effect index of which is 30 times larger than that of carbon dioxide. Besides, gas disasters such as gas explosive and outburst, etc. pose a great threat to the safety of miners. Therefore, measures must be taken to capture coal mine methane before mining. This helps to enhance safety during mining and extract an environmentally friendly gas as well. However, as a majority of coal seams in China have low-permeability, it is difficult to achieve efficient methane drainage. Enhancing coal permeability is a good choice for high-efficiency drainage of coal mine methane. In this paper, a modified coal-methane co-exploitation model was established and a combination of drilling-slotting-separation-sealing was proposed to enhance coal permeability and CBM recovery. Firstly, rapid drilling assisted by water-jet and significant permeability enhancement via pressure relief were investigated, guiding the fracture network formation around borehole for high efficient gas flow. Secondly, based on the principle of swirl separation, the coal-water-gas separation instrument was developed to eliminate the risk of gas accumulation during slotting and reduce the gas emission from the ventilation air. Thirdly, to improve the performance of sealing material, we developed a novel cement-based composite sealing material based on the microcapsule technique. Additionally, a novel sealing-isolation combination technique was also proposed. Results of field test indicate that gas concentration in slotted boreholes is 1.05-1.91 times higher than that in conventional boreholes. Thus, the proposed novel integrated techniques achieve the goal of high-efficiency coal bed methane recovery.

Chinese coal seams are characterized by high gas content and low permeability. The permeability of coal seams should be improved to achieve maximum extraction of coalbed methane. This study explores how coal and gas behave when subjected to hydraulic slotting. A fluid−solid coupling experimental system of gas-bearing coal subjected to hydraulic slotting was first established. Then, the fluid−solid coupling property of gas-bearing coal subjected to hydraulic slotting was revealed using the established experimental system. Meanwhile, indicators used to describe the process of hydraulic slotting were derived, and the factors affecting the process of hydraulic slotting were analyzed using the aforementioned indicators. The research achievements indicate that the gas pressure response of the monitoring points in the coal sample shows different characteristics at different stages. Corresponding to the change trend of the gas pressure, the vertical and parallel strains demonstrate the five-stage change characteristics. With the increase of gas pressure, the final deformation amount before slotting gradually increases, and the gas diffusion parameter increases exponentially. With the increase of the slot radius, the gas diffusion parameter shows a similar change tendency with the ultimate deformation amount, i.e., it tends to become flat after a rapid increase. The research achievements can provide certain theoretical and practical references for the revelation of the enhanced coalbed methane recovery mechanism through hydraulic slotting and the rational selection of the key parameters in the field test, respectively.

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