Macrolithotype-Controlled Permeability Variations in Anthracitic Coal under High-Pressure and High-Temperature Conditions

The permeability of deep coal reservoirs is sensitive to high-temperature and high-pressure environments, while the responses of coals with different material compositions are not clear. Here, coals with four different macrolithotypes, i.e., bright, semibright, semi-dull, and dull, were measured under temperatures of 60, 70, 80, and 90 °C and confining pressures from 5 to 35 MPa. Then the sensitivity-influencing factors were discussed by Pearson's correlation analysis. The results show that the permeability of all the four coals exhibits a negative power-law relationship with increasing effective stresses, with the bright and dull coals decreasing from 0.148 to 0.00005 mD and from 0.3472 to 0.0055 mD under 60 °C, separately. The bright and semibright coals are of high-permeability damage ratio and stress sensitivity compared to the semi-dull and dull coals. The permeability of bright and semibright coals gradually decreases with increasing temperature, while the semi-dull and dull coals exhibit no significant variation. Under high effective stresses, the temperature shows enhanced influences on permeability, e.g., the bright coal shows temperature-sensitivity coefficient decreases from 0.04 to 0.02 under 5 to 35 MPa under 60 °C. The Pearson correlation coefficients between the permeability and effective stress range from -0.631 to -0.856, while -0.021 to -0.221 between permeability and temperature, which also indicates that the effective stresses are of higher influences compared to temperature. The results provide theoretical support for the efficient exploration and development of deep coalbed methane with different macrolithotype compositions.