A heat transfer model of high-temperature nitrogen injection into a methane drainage borehole

While pre-draining the coalbed methane from the high gassy coal seams would significantly reduce the hazard in the stage of coal mining, gas recovery from coal seams of low permeability and strong adsorption potential would be an extremely difficult practice. To improve the methane recovery, injection of high-temperature nitrogen (N2) into the coal seams prior to borehole drainage is attempted. According to technical specifications of the Chinese coal mine gas drainage, the permeability that corresponds to the diffusion radius of high-temperature N2 is 0.0025 mD. Triaxial tests were conducted on coal samples from the No. 8 Coal Mine in Pingdingshan. The effective stress that corresponds to a N2 permeability of 0.0025 mD was found to be 21.5 MPa. Numerical simulations of the stress distribution around boreholes were conducted, and the results show that the N2 diffusion radius increases linearly with the borehole radius. During the N2 injection, the distribution of temperature allows affected coal seams to be divided into three zones along the radial direction of the borehole: the N2 heating zone, the heat transfer heating zone and the non-heating zone. A heat transfer model of high-temperature N2 injection into a methane drainage borehole has been established, and the results indicate that either by increasing the difference of temperatures between high-temperature N2 and the coal mass or increasing the injection time, the thermal diffusion radius would increase significantly, but the rate at which it does so decreases. Thermal diffusion radius increases linearly with the borehole radius. The heat transfer model could be considered as a guide for the high-temperature N2 injection into boreholes.