Apparent Permeability of Gas Shales-Separation of Fluid-Dynamic and Poro-Elastic Effects

Gas permeability coefficients measured on low-permeable rocks are affected over a large pore pressure range by both, fluid-dynamic (slip flow) and poro-elastic effects (stress-dependent pore compressibility). In this contribution we demonstrate an apparent permeability model, which accounts for both effects. The procedure was applied to permeability data determined with different gases (He, Ar, N-2, CH4, C2H6) on an unfractured Bossier Shale (matrix) sample and a fractured Haynesville Shale sample. Additionally, we applied our model to seven data sets previously published in literature. Our analysis indicates that pore pressure has a smaller influence on effective stress than confining pressure, resulting in effective stress coefficients chi <1. Fluid-dynamic gas slippage effects are significant up to pore pressures of 20 MPa. Not accounting for gas slippage at these pressures will result in too low chi values, and therefore incorrect predictions of the fluid-dynamic effects at high pore pressures.