Effects of image resolution and numerical resolution on computed permeability of consolidated packing using LB and FEM pore-scale simulations

Abstract

Image-based pore-scale modeling has become an important tool for studying fluid transport and other phenomena in porous media. Spatial resolution of the digital images used for modeling is critical not only because it dictates the scale of features that can be resolved, but also because for most techniques there is at least some relationship between voxel size in the image data and numerical resolution applied to the computational simulations. In this work we investigate this relationship using a computer-generated consolidated porous medium, which was digitized at voxel resolutions in the range 2–10μm. These images, which are free of experimental and segmentation errors, are then used to compute permeability and tortuosity using lattice Boltzmann (LB) and finite elements methods (FEM). Results show how changes in computed permeability are affected by image resolution (which dictates how well the pore geometry is approximated) versus grid or mesh resolution (which changes numerical accuracy). For LB, the image and grid resolution are usually taken to be the same; we show at least one case where effects of grid and image resolution appear to counteract one another, giving the mistaken appearance of resolution-independent results. For FEM, meshing can provide certain attributes (such as better conformance to surfaces), but it also adds an extra step for error or approximation to be introduced in the workflow. Results show that performing grid coarsening on the FEM mesh caused a reduction in computed permeability, but in this particular case the effect is related to tightening of the pore space rather than loss of numerical accuracy.