Gas migration in the reservoirs of ultra-low porosity and permeability based on an improved apparent permeability model

Abstract An apparent permeability model considering the solid-fluid coupling is established to analyze the gas transport in the reservoirs of ultra-low porosity and permeability. In the new model, multiple flow mechanisms including viscous flow, the dynamic effects induced by gas molecules collision with the pore walls and surface diffusion are taken into account according to the flow distributions. The effect of solid deformation is embodied with the stress dependence model of the intrinsic permeability which is based on the poroelasticity theory, discards the assumption of constant total stress and accounts for the influence of the initial deformation state. Numerical results show that the dynamic effects and surface diffusion can promote the apparent permeability, especially when the pore pressure drops to a low level. The geomechanical deformation generally results in the reduction of the intrinsic permeability while the large sorption strain may induce rebound of the intrinsic permeability. The assumption of constant total stress only slightly affects the permeability, but it eliminates the instantaneous equilibrium stage in the gas migration process. Correct determination of the initial deformation state is critical to evaluate the gas flow capacity. Finally, parametric analysis is conducted to identify the key factors in gas transfer.