Modeling coupled gas flow and geomechanics process in stimulated coal seam by hydraulic flushing

Abstract

Hydraulic flushing (HF) is an efficient technique for improving coal mine gas extraction. Although much research has been conducted in recent decades, many problems on its mechanism of enhancing permeability still exist. For example, how does coal strength affect the enhanced permeability by HF? Can a larger borehole generate a higher gas flowrate? Therefore, a coupled gas flow-geomechanics model was developed based on an equivalent fractured-coal model to solve these issues. Plastic volumetric strain was introduced in this model to quantify the coal structure, which can improve the numerical models for coal permeability, gas flow in fracture and diffusion in matrix, respectively. The model was used to describe gas flow in the virgin and disturbed coal seams. Results show that neglecting coal failure can cause erroneous stress and permeability distributions, and lead to an overestimation of gas pressure and an underestimation of gas extraction amount. Effects of coal strength and borehole diameter on the distributions of stress, plastic zone, permeability and gas pressure were systematically studied. It indicates that the stress relief, plastic and enhanced-permeability zones around the boreholes in soft coal are obviously larger than those in hard coal. The stress-concentration level and permeability reduction in soft coal are greater than those in hard coal. After 500 d extraction, gas pressure in soft coal has a higher decrease than that in hard coal, indicating that the enhanced permeability by HF of soft coal is larger than that of hard coal. Besides, the investigation on the effects of borehole diameter shows that there exists an optimal borehole diameter. Gas pressure between the boreholes decreases before the optimal diameter, and increases sharply after the optimal diameter due to the high stress concentration. For the Pingmei 8th mine, the optimal borehole diameter is about 1.0 m. Field tests indicate that the optimized HF can considerably eliminate the risk of coal and gas outburst.