Experimental Study on Permeability Evolution of Bituminous Coal Under High Temperature and Volumetric Stress

Studies on coal permeability at high temperatures (> 300 celcius) under the thermo-hydro-mechanical-chemical (THMC) coupling effect are few and their conclusions are divergent. Herein, the influence of different volumetric stresses (12, 24, and 36 MPa) on bituminous coal permeability is discussed. Moreover, the permeability evolution law of bituminous coal under different pore pressures (0.2-0.8 MPa) and temperatures (300 celcius-600 celcius) are studied using a self-made triaxial test machine with the THMC coupling effect. The experimental findings and micro-computed tomography (micro-CT) test results of coal samples under different temperatures in situ are reported. The obtained results show that (1) bituminous coal permeability underwent a negative logarithmic change with pore pressure (0.2-0.8 MPa) at different temperatures because of gas adsorption-expansion, gas slippage, and shear deformation. (2) Bituminous coal permeability varied with temperature in stages; this result differs from traditional research results. At 300 celcius-450 celcius, the permeability continuously decreased and slightly increased at 450 celcius-600 celcius; however, it was still lower than that at 300 celcius. The micro-CT results show consistent porosity evolution with permeability. (3) At 300 celcius-450 celcius, the permeability attenuation of bituminous coal mainly resulted from coal softening deformation. The continuous increase in the coefficient of linear thermal expansion increased the permeability decay rate. With the increasing temperature from 450 celcius to 600 celcius, the accelerated pyrolysis of the coal sample enhanced the permeability, the coefficient of linear thermal expansion exhibited a negative correlation with temperature, and the inhibition effect of coal compression on permeability was weakened, slightly increasing the permeability at this stage. However, the continuous compression accumulation effect of stress still played a principal role, inducing low permeability. (4) Under the same temperature and pore pressure, the correlation between the permeability and volumetric stress of the coal samples decreased exponentially.