Reassessment of coal permeability evolution using steady-state flow methods: the role of flow regime transition

Steady-state flow methods are widely applied in the laboratory where permeability evolution is typically evaluated assuming a uniform pressure gradient along the sample. The accuracy of this approach is questionable for tight geomaterials when gas is used as the injecting fluid, due to: (i) the nonlinear distribution of pore pressure and gradient along the sample, suggesting that (ii) both slip and viscous flow may occur concurrently within samples containing nano-pores. The following presents laboratory permeability measurements integrated with numerical analysis to investigate the evolution of coal permeability under different flow regimes. Measured coal permeability first decreases and then rebounds as gas injection pressure is reduced, indicating the transition in flow regime from viscous to slip dominant. Numerical results chart the nonlinear distribution of pore pressure along the sample and the spatial transition of flow regimes determined by controlling the magnitude of the injection pressure. When injection pressure is below the threshold for slip flow, then slip flow dominates throughout the entire coal sample and apparent permeability increases significantly along the gas flow direction. The relative contribution of the slip flow to total flow increases with the reduction in pore pressure, increasing from 0.02 to 0.18 for the tested sample. Results also show that the conventional method of plotting apparent permeability against the mean experimental pressure always gives a greater permeability than the two alternate methods proposed in this work, and the discrepancy increases with increasing injection pressures (up to 7.66% when pore pressure = 6 MPa). Uncertainty analysis is strongly recommended when measuring permeability of tight rocks using this experimental method