GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 93-6
Presentation Time: 9:30 AM


GUO, Bo, Department of Hydrology & Atmospheric Sciences, University of Arizona, Tucson, AZ 85721, MA, Lin, School of Earth, Atmospheric, and Environmental Sciences, University of Manchester, Manchester, M13 9PL, United Kingdom and TCHELEPI, Hamdi, Department of Energy Resources Engineering, Stanford University, Stanford, CA 94305

Digital rock analysis that utilizes high-resolution 3D digital images of rock pore structures as input for flow simulations is becoming a popular tool to understand shale hydrocarbon transport at the pore-scale. A widely recognized issue with shale rock images (e.g., FIB-SEM, nano-/micro-CT images) is the so-called “cutoff length”, i.e., pores and heterogeneities below the resolution cannot be resolved, which leads to two length scales—resolved features and unresolved sub-resolution features—that are challenging for flow simulations. Here we develop a micro-continuum model that naturally couple the resolved pores and the unresolved nano-porous regions. Hydrocarbon transport in the resolved macropores is modeled by the Stokes equation with two options: one directly discretizes the pore space and solves the Stokes equation while the other approximates the pore space as interconnected nodes (i.e., pores) and bonds (i.e., throats) and uses a pore-network formulation. For the unresolved regions where the pore sizes are below the image resolution, we treat them as a continuum and develop an apparent permeability model considering non-Darcy effects at the nanoscale including slip flow, Knudsen diffusion, adsorption/desorption, and surface diffusion. This leads to a hybrid micro-continuum pore-scale modeling framework that can simulate hydrocarbon transport in 3D shale images. We present case studies to demonstrate the applicability of the model, where we apply the new micro-continuum model to 3D segmented FIB-SEM shale images that include four material constituents: organic matter, clay, granular minerals, and macropore. We populate the model with experimental measurements (e.g., pore size distribution of the sub-resolution pores) and parameters from the literature, and identify the relative importance of different physics on hydrocarbon production. Overall, the micro-continuum modeling framework provides a novel and computationally efficient tool for digital rock analysis of organic-rich shale rocks.