Modeling apparent permeability of coal: Incorporation of slippage effect and gas pressure decline

This study investigates the underlying processes that govern coal seams' seepage and slippage effects during the extraction of coalbed methane. A coal rock seepage test under the influence of constant external stress and gas pressure was conducted by using a gas-containing coal thermal–fluid–solid triaxial servo seepage device. On that premise, combined with the effective stress coefficient equation that considered the adsorption strain, a dynamic change model representing apparent permeability was established. Following which, the response characteristics of coal rock's permeability, in addition to the important control mechanisms relating to the slippage effect during the discharge of the gas pressure process, were quantitatively explored. The results revealed that (1) permeability rose as the coal seam's gas pressure fell, a consequence of the coal rock's contraction. (2) The results from a newly developed model corresponded closely with empirical observations, thereby offering a more accurate portrayal of permeability dynamics during the exploitation of coalbed methane. (3) Effective stress would increase as gas pressure fell. (4) The new model was also suitable for describing the seepage law relating to coal rock under three-way stress, with the slippage effect gradually increasing under a rise in effective stress, that proved the rationality of the dynamic slippage coefficient equation's correction. (5) By analyzing the impact of the gas's adsorption-induced strain on the slippage effect, it was observed that higher values of εmaxi and pεi correlated with greater permeability. The insights obtained from those results have been instrumental in comprehending the progression relating to coal-rock's permeability during the decompression of gas extraction, which would contain substantial implications for the economic viability of coalbed's methane operations.