Time-dependent coal permeability: Impact of gas transport from coal cleats to matrices

Coal seams are usually fractured reservoirs where the factures/cleats are fluids flow channels, and the matrix blocks serve as the storage space. The gas transport within the matrix has crucial impacts on the variation of cleat aperture and thus affects the reliable determination of coal permeability. Most literature on laboratory measurements and the development of relevant analytical models focus on response gas pressure dependent coal permeability change, under the assumption that gas transport in coal is in equilibrium. This means that these models are not suitable for the assessment of transient permeability when gas transport in coal is unsteady flow. In this paper, we real-time measured the entire process of the coal deformation induced by gas injection, and further obtained the diffusion-dependent curve of coal permeability against time by modifying the typical pressure-pulse-decay approach. An improved permeability model was then proposed by incorporating an Internal Deformation Coefficient into the pore compressibility of coal seams. The results show that the gas inflow can decline the pressure differential between the downstream pressure and the upstream pressure, during which the deformation of coal experiences a transition from early rapid expansion to latter slow expansion. As the gas was diffusing into coal matrices, the coal permeability was observed to first decrease and then became stable. It is also found that Internal Deformation Coefficient increases with the injection gas pressure. This indicates that the matrix blocks preferentially swell toward cleats. The modified permeability model achieves a much better fit of the coal permeability-pore pressure relationship when compared with two widely used permeability models. This approach can be extended to evaluate the deformation compatibility between coal cleats and matrices.