Digital evaluation of nanoscale-pore shale fractal dimension with microstructural insights into shale permeability

Abstract

Permeability significantly affects the production of shale oil and shale gas, and shale microstructures characterized by pore and fracture spaces innately affect the permeability. However, the quantitative characterization of permeability related to pore and facture spaces is not fully understood. This work aims to propose 3D spatial fracture-pore fractal dimensions to predict shale permeability and their effects on fluid flow behaviors. First, triaxial compressive stress and X-ray CT imaging tests are conducted on shale samples to establish fractural models. The digital surface roughness segmentation (DSRS) method is then proposed to obtain the fracture-pore microstructures. Next, spatial fractal dimensions of self-similarity microstructures are proposed to predict the microstructural permeability. Finally, two-phase fluid flows are simulated to study the hydrocarbon flow behaviors in fractural microstructures using the level set method. The results show that the average relative errors between the microstructural spatial dimensions and theoretical fractal dimensions are all less than 3%, highlighting the accuracy of the proposed method. The numerical results for permeability are very close to the analytical solutions, in which fracture permeability is almost 100 times the order of magnitude of the pore structure permeability in the nanoscale pore shale, and the facture and pore structure permeabilities both increase with increasing spatial fractal dimension. The changes of fluid flow behaviors are similar to the permeability variations, and the fluid phase fraction increases with increasing fractal dimension.