Mechanistic analysis of coal permeability evolution data under stress-controlled conditions

At present, two types of experiments under stress-controlled conditions were normally conducted to measure coal permeability: constant confining pressure (CCP) tests and constant effective stress (CES) ones. The original rationale of this situation was to assume that the impacts of effective stresses and gas sorption-induced matrix swelling/shrinking on coal permeability could be separated and investigated individually. In this study, we collected coal permeability data measured under both conditions with a purpose to see if this original rationale was appropriate. This goal was achieved through collection of experimental permeability data under the CCP conditions; collection of experimental permeability data under the CES conditions; and comparison of those experimental data with solutions of the poroelastic theory. For CCP tests, the permeability ratios change from reductions (less than 1.0) to enhancements (greater than 1). These changes are bounded by an upper envelope and a lower one. The upper envelope is corresponding to the solution of free-swelling while the lower one zero-swelling. For CES tests, the permeability ratios also change within an upper envelope and a lower one. The upper envelope is equal to 1.0 corresponding to the solution of free-swelling while the lower one zero-swelling. Through these comparisons, we found that permeability data for both types of tests are confined within the poroelastic solutions for two extreme boundary conditions: free-swelling and zero-swelling. These findings suggest that permeability ratios for both constant confining tests and constant effective stress tests are primarily determined by the matrix-fracture interactions, including sorption-induced swelling/shrinking, through transient effective stresses in matrixes and fractures.