Evolutionary Model and Experimental Validation of Gas-Bearing Coal Permeability under Negative Pressure Conditions

Coal bed methane drainage is the main approach to lower risks of coal seam while raising the efficiency in natural resource utilization. The negative pressure used for extraction in coal mines is largely determined empirically due to a lack of experimental research on how coal permeability changes under the combined influence of effective stress and negative pressure. This results in low gas extraction efficiency and concentration. In this paper, to study the effect law of complex stress and extraction on coal permeability during coal and gas co-mining, a test system was specially designed to determine the gas flow and coal permeability of coal samples under different stress paths and negative pressure conditions in the lab. The study analyzed the correlation between coal permeability, effective stress, and negative pressure and subsequently developed a permeability evolution model for gas-bearing coal under negative pressure conditions. The results showed that the permeability of coal increases with the increase in negative pressure and decreases with the increase in effective stress; the permeability of coal can be abruptly changed by changes in stress loading patterns; the established model of permeability evolution of gas-bearing coal can better reflect the correlation between permeability, effective stress, and negative pressure. The research outcomes offer a valuable theoretical foundation for the efficient extraction and utilization of methane in coal mines.