Numerical simulations of enhancing permeability and gas extraction of thick coal seams through static fracturing

This study seeks to improve the underground operation efficiency and optimize the borehole arrangement parameters for static fracturing of coal seams. To this end, this study conducted tests in the belt roadway of the No. 12316 fully mechanized mining face in the Wangjialing Coal Mine, China Coal Huajin Group. By combining the equations for coal seams’ deformation and failure, gas diffusion and seepage, and coal seams’ permeability evolution, this study constructed a model of the damage and permeability evolution of coal seams. Based on the numerical simulations of the process of static fracturing and permeability enhancement of coals, as well as its influencing factors, using FlAC^3D and COMSOL Multiphysics, this study revealed the laws of the stress distribution, plastic expansion, and gas pressure transfer and evolution of coal seams under the action of static fracturing. Furthermore, through field experiments using fracturing parameters selected through optimization, this study quantitatively analyzed the changes in gas flow extracted from coal seams during static fracturing under different borehole spacings. The following findings are obtained. During the static fracturing, the expansion stress in coals was evenly transferred outward along the radial directions of the borehole for fracturing. An annulus stress zone and a plastic zone were formed during single-hole fracturing. In the case of double-hole fracturing, the expansion stress in both holes exhibited better superposition effects in the horizontal direction than in the vertical direction, leading to more significant damage to coals in the horizontal direction than in the vertical direction. Furthermore, zones between both holes were damaged earlier than other zones. After permeability and gas extraction were enhanced by static fracturing, the gas pressure and permeability of coal seams were positively and negatively correlated with the hole spacing, respectively due to the limited static fracturing range. During field tests, static fracturing and permeability enhancement were conducted under hole spacings below 1.6 m. After gas extraction for 30 days under negative pressure of 20 kPa, the pure gas flow extracted after fracturing doubled. This indicates that static fracturing has significant pressure-relief and permeability-enhancement effects on gas extraction.