Laboratory Measurement of Permeability Evolution Behaviors Induced by Non-Sorbing/Sorbing Gas Depletion in Coal Using Pulse-Decay Method

Gas permeability in coal plays a critical role in predicting coalbed methane (CBM) production. The permeability evolution induced by gas depletion in low-permeability coal is complicated and affected by multi-mechanistic flow components. This study runs a series of permeability tests using the pulse-decay method for helium and CH4, CO2 depletions in coal under both the constant stress boundary (CSB) and uniaxial strain boundary (USB) conditions. With the measured pulse-decay curves, the gas desorption effect on pore pressure depletion can be clearly noticed and gas permeability change can be estimated. The result shows that the helium permeability under the CSB condition is slightly lower than that under the USB condition, and it decays nonlinearly with pressure drawdown and does not rebound in the low-pressure region, which indicates that the helium permeability evolution is mainly controlled by the effective stress in the tested coal. For the sorbing gas CH4/CO2, the permeability profile under the two boundary conditions behaves somewhat similarly; it initially declines with the pressure depletion and then starts to rebound in low-pressure region. The permeability ‘rebound’ of CH4 is comparatively less than that of CO2 due to the larger adsorption capacity of CO2 in coal. With the comparison of permeability behaviors of the different test fluids, it is inferred that the sorbing gas permeability ‘rebound’ should be mainly caused by the matrix shrinkage. The result of this study reveals that coal reservoirs produce CBM using a multi-mechanism approach, and the effect of matrix flows on the permeability behavior and the overall CBM production should be highly emphasized.