AN IMAGE-BASED MICRO-CONTINUUM MODELING FRAMEWORK FOR HYDROCARBON TRANSPORT IN ORGANIC-RICH SHALE ROCK

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.