Simulation and optimization of high-pressure air injection promoting underground coal seam gas drainage

Abstract

ABSTRACT In the soft coal seam with ultra-low permeability, the effect of permeability enhancement by hydraulic fracturing or stress relief methods is not ideal due to the closure of fractures. Injecting foreign gas into the coal seam can replace part of the gas adsorbed in the coal matrix and drive the coal seam gas to the drainage borehole, so as to achieve the purpose of promoting gas drainage. In this paper, we established a fluid-geomechanics coupling model for the high-pressure air injection promoting gas drainage. This mathematical model contains governing equations of ternary gases competitive sorption and transport, and deformation response of coal material, reflecting the interactions among gas mixtures and coal seam. The model is solved by the finite element method to simulate the high-pressure air injection promoting coal seam gas drainage through floor roadway drillings in studied Coal Mine. The gas pressure of CH4, N2 and O2 is comparatively analyzed, and the effective drainage area is obtained. It is confirmed that the air injection can enhance gas drainage by the competitive adsorption and pushing effect. Meanwhile, numerical simulations are carried out to optimize the borehole arrangement. When the operation continues 120 days, the effective areas in studied case are 10.1, 11.23, 11.35, and 6.23 × 103 m3 for borehole spacing of 6 m, 8 m, 10 m and 12 m respectively. The borehole spacing of 10 m is the optimal arrangement in the studied case, owing to its greater effective drainage volume and fewer boreholes. These provide a foundation for guiding the operation of air injection promoting coal seam gas drainage in the field.