Low- and High-Resolution X-Ray Tomography Helping on Petrophysics and Flow-Behavior Modeling

Abstract

Summary Reservoir studies involve spatial scales ranging from pore models to field-simulation grids. X-ray tomography is a nondestructive technique of inspection and characterization of samples that works from microns in the pore scale to centimeters in drill cores, which makes it a useful tool for modeling porous systems in reservoir-rock samples. The generated models serve to simulate physical phenomena helpful to determine both static and dynamic properties. Various techniques of preprocessing, porous-medium modeling, and simulation are applicable depending on the degree of heterogeneity of the sample. This work presents a workflow to make the porosity and permeability models from images acquired with axial computed tomography (CT) at different resolution scales. The workflow was used in sandstone and carbonate samples from Brazilian outcrops. The density and porosity of the sample were determined at the millimeter scale using a medical scanner and the distributions of pore size, grain size, and absolute permeability were determined using a synchrotron beamline, which provided high-resolution data at the micrometer scale. Integrating the data obtained from both scales, the combined description of the drill-core porosity served as a base to build a permeability model for the sample with the dimensions of that one scanned by the medical CT. The approach to generate the model depends on the type of heterogeneities associated with the sample. The homogeneous sandstone sample was modeled as a matrix with undefined porosity at the resolution of the medical tomography. The carbonate sample showed defined porosity at different scales: Moldic and vugular porosity were defined on the medical scale, while interparticle porosity was set on the synchrotron scale. Permeability was simulated in the micrometric model using an approach that integrates Stokes law and Darcy’s law in a percolating porous medium. Subsequently, the permeability was transferred to the millimetric model, creating the 3D distribution of porosity and permeability. Finally, the upscaled value for the samples was determined. This study contributes to the approaches regarding techniques of rock upscaling and digital rock physics and presents a workflow and some discussions of the multiscale approach.