Multiscale Digital Rock Imaging and Modeling for Measuring the Heterogeneous Carbonate and Conglomerate Permeability at the Laboratory Plug Scale

Rock permeability is an essential petrophysical property for evaluating the hydrocarbon exploitation and CO2 geological sequestration. The large-scale rock heterogeneity by single-scale digital rock analysis (DRA) poses a challenge in predicting the permeability of the centimeter-scale core plug for fractured carbonates, vuggy carbonates, and conglomerates. The present study developed two multiscale workflows to upscale the permeability from the millimeter scale to the near centimeter scale. The basic two-step workflow consists of multiscale imaging and multiscale modeling to integrate the Darcy flow in the microporous matrix representative element volume and the Stokes flow in larger-sized pores/vugs/fractures. The basic two-step workflow was then extended to the four-step workflow to speed up the computation and reduce the computer memory. With experimental permeability as the benchmark, this study compared single-scale simulation methods and multiscale workflows to quantify the best permeability analysis method for the three rock types. The basic two-step workflow with binary segmentation is accurate for the fractured carbonate due to fully modeling the preferential flow and the fluid exchange between the fracture and matrix. The extended four-step workflow with ternary segmentation is optimal for the conglomerate to simulate fluid resistances of the gravel and matrix with satisfied computational efficiency. The permeability of some vuggy carbonates can be well evaluated by the single-scale simulation of the matrix flow where the vugs are isolated and cannot facilitate fluid communication. The acceptable agreement between the experimental and simulated permeabilities for 18 core plugs improves the confidence in using the developed DRA to predict the permeability for complex rocks.