Effects of Stress and Temperature on the Permeability of Gas-Saturated Wet Coal

Fracturing and heat injection are promising methods for in situ modification of low-permeability reservoirs to enhance coalbed methane (CBM) recovery. The technical processes enable substantial water residence in the reservoirs and cause rebalance and redistribution of the stress and temperature fields around the wells or boreholes. In this study, a dual-porosity permeability model is established. The coupling effects of variable stresses and temperatures (below 100 °C) on the permeability evolution of gas-saturated wet coal are also studied. The results show that (1) the mean effective stress and temperature enabled complicated nonlinear decreasing trends in permeability, and sensitivity analysis of input parameters by the random forest method indicates that the effect of stress on permeability was much greater than that of temperature; (2) the relationship between incremental effective stress and permeability can be well expressed by a new function, while initial permeability and variations in thermal cracking and stiffness of the matrix during heating will affect fit coefficients of the function; and (3) as signified by substantial breakthrough in thermal cracks and fractures, permeability evolution during heating consists of phase I and phase II, and the evolution performance in phase I was more complicated as it is controlled by external stress conditions and also related to mutual competition between positive effects (due to thermal volatilization and sorption-induced matrix shrinkage) and negative effects (due to thermal expansion and thermal cracking). Moreover, variability of permeability versus temperature at low deviatoric stress (0 MPa) differed from that at high deviatoric stresses (3–12 MPa). These results would be beneficial for permeability prediction and CBM extraction design.