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.

Freezing and thawing cycles occur with cyclic liquid nitrogen (LN 2 ) injection in coal. The freeze−thaw treatment damages the pore structure of coal and thus increases its permeability. In this study, NMR and strain monitoring were employed to investigate the changes in coal structure when the coal specimens were under cryogenic treatment using LN 2 . We classified freeze−thaw process into four stages; stages I and III are dominated by seepage pore development, and stages II and IV are dominated by adsorption pore development. It was found that LN 2 freeze−thaw cycles can cause structural deterioration in the coal so as to improve both fracture density and overall permeability. The results demonstrate that the rate of increase of both the effective porosity and total porosity of the coal are positively correlated with the LN 2 freezing time and the number of freezing cycles but negatively correlated with the residual porosity. For the same absolute LN 2 freezing time, cyclic freeze−thawing has a greater effect on the rate of growth of pore spaces and reduction of P-wave velocity in the coal compared with single freeze−thaw treatment. It was also found that the number of freeze−thaw cycles is a very important factor for the creation of larger pores, pores that can connect the fracture network. The results suggest that appropriate control of the number of freeze−thaw cycles can result in effective fracturing of coal.

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