A modified Kozeny-Carman model for estimating anisotropic permeability of soft mudrocks

This paper quantifies the effects of clay fraction and fabric on the permeability of soft mudrocks with emphasis on micro-level mechanisms. Soft mudrocks are treated as a mixture composing the non-clay minerals and the clay-water composite as two constituents. A simplified approach to estimate the fabric orientation distribution of soft mudrocks based on readily measured parameters such as clay fraction and porosity is presented. A modified Kozeny-Carman model is proposed to estimate the anisotropic permeability of soft mudrocks. An effective porosity is used in the proposed model to consider the effect of clay-bound water on the flow behavior. The anisotropy in permeability is studied by using an anisotropic tortuosity which is quantitatively linked to the data of fabric orientation distribution and porosity. In the proposed theoretical model, the clay fraction data is included in parameters of effective porosity, anisotropic tortuosity, and external specific surface area. An increase in clay fraction results in a significantly decrease in vertical permeability which is due to the increases of external specific surface area and tortuosity along the vertical direction (perpendicular to the bedding plane), and the decrease of effective porosity. The proposed permeability model has been validated using published permeability data on soft mudrock samples with different geological origins. The model can be used to predict anisotropic permeability in soft mudrocks or hard clay soils from conventional geophysical logging data with minimal core analysis.