Analysis of the Upper and Lower Boundaries of Permeability Evolution during Shale Rock Shear Deformation

Abstract

Understanding the evolution of permeability during rock shear displacement plays an important role in many geosciences and geo-engineering, such as shale gas exploitation. In previous studies, permeability data were obtained under different experimental conditions with varied in situ stress environments, fracture apertures, or loading stresses. However, the mechanisms controlling the permeability evolution during the shear process have not been well explained. In this study, two special conditions, fractures with constant stiffness (CS) and constant volume (CV), are proposed to define the upper and lower boundaries of permeability evolution. An inverted "L-shaped" model containing a matrix and fracture was established to investigate the permeability evolution and internal morphological changes in the fractures. The impacts of the initial fracture aperture and normal stress in the shear process are discussed together with the apparent fracture aperture, effective flow area, and pore throat radius. Under the CS condition, the self-supporting effect produced by the contact between the upper and lower fracture surfaces plays a dominant role in changing the permeability, and three stages of permeability evolution with shear displacement are observed: rapid increase, slow increase, and decrease. Under CV conditions, the submergence effect of the fracture closed the effective flow channels, thereby reducing the permeability in three stages: rapid decrease, slow decrease, and recovery. The model results show that both a smaller initial fracture aperture or a greater normal stress lead to a greater variation in the permeability and the roughness of fracture surface determines the permeability evolution range. The new model was applied to generate a series of shale permeability evolution boundaries under different limit conditions. These boundaries are consistent with experimental observations reported in other literatures. This work provides new insights into the permeability evolution of rock strata under shear action.

A Review of Swelling Effect on Shale Permeability: Assessments and Perspectives

“…For instance, Cai et al 82 proposed an easy-toimplement model and disclosed that shale permeability could be influenced by heterogeneous pore size distribution. Sun et al 83 pointed out that shale permeability could be influenced by shear deformation. The impacts of these factors are beyond the scope of this review and will not be further discussed.…”