A Thermo–Hydro–Mechanical Model: Capturing the Effects of Initial Permeability and Gas Pressure on Outburst-Prone Indicators

Abstract

Coal and gas outbursts often result in a large number of casualties and economic losses. Its complex processes involve the multi-field coupling of gas flow, coal stress, and temperature variation. A coal temperature drop and an initial rate of the gas emission (qm) of boreholes are often used as indicators for the outburst-proneness of the heading face during the excavation of a coal roadway. However, the threshold values of these two indicators are empirical; few theoretic studies have been reported on their influence factors and laws. In order to investigate the influence laws of the initial gas pressure and permeability on these two indicators, a coupled thermal–hydraulic–mechanical (THM) model is proposed. This model considers the heat of absorption by gas from coal due to desorption and expansion. The presented THM model is verified using experimental data from the literature. Based on the verification of the model, the THM model is applied to a local prediction of outburst-proneness during the heading face advancement. The study indicates that the temperature drop and the qm both decrease as the initial permeability and gas pressure decrease; more specifically the relationship between the temperature drop and qm is a power function. The temperature drop of the borehole wall reduces with elapsed time, but the larger variation rate of the temperature drop occurs at the beginning of forming the borehole. Therefore, when temperature drop is adopted to indicate the outburst-proneness of heading faces, the temperature measurement moment needs to be pre-specified, a value of 20–30 min after borehole forming is recommended. Adopting the variation magnitude of qm during the advancement of the roadway heading face as an indication of coal and gas outburst-proneness may be more reasonable than qm itself. This study provides a reference for the prevention of coal and gas outbursts and a new idea for the application of a multi-field-coupled model of coalbed gas flow in practical engineering.