Shale permeability model considering bedding effect under true triaxial stress conditions

Abstract

The permeability anisotropy of shales can be attributed largely to bedding planes. It is critical to study the bedding effect on permeability evolution for shales. We conducted an experiment on shale permeability anisotropy under true triaxial stress conditions. Even though the mean stress of shales was different, the permeability still exhibits obvious bedding dependent. In this study, we present a new permeability model considering bedding planes. The introduction of characteristic functions and parameters characterizes the transformation process from bedding to non-bedding during compression. Compared with the experimental data, we obtained a good fitting result. Owing to the increase in the resistance of the pressurized gas into the fracture system, Biot's coefficient α decreases with increasing mean stress. The porosity sensitivity exponent s parallel to the bedding planes are larger than those perpendicular to the bedding planes. In our opinion, owing to the smaller stiffness of the bedding plane, the relatively large amount of pore deformation is caused by the loading on the vertical bedding planes. Furthermore, we introduce η to characterize the effect of non-bedding and bedding on permeability. The permeability of shale or bedding is more sensitive at lower stress than at higher stress and the fluid flow through the non-bedding in shales cannot be ignored.