Effects of Image Resolution on Sandstone Porosity and Permeability as Obtained from X-Ray Microscopy

Abstract

Estimating porosity and permeability for porous rock is a vital component of reservoir engineering, and imaging techniques have to date focused on methodologies to match image-derived flow parameters with experimentally identified values. Less emphasis has been placed on the trade-off between imaging complexity, computational time, and error in identifying porosity and permeability. Here, the effect of image resolution on the permeability derived from micro-X-ray microscopy (micro-XRM) is discussed. A minicore plug of Bentheimer sandstone is imaged at a resolution of $$1024 \times 1024 \times 1024$$ voxels, with a voxel size of 1.53 $${\upmu }\hbox {m}$$ , and progressively rebinned to as low as 32 voxels per side (voxel size 48.96 $${\upmu }\hbox {m}$$ ). Pore-scale flow is modeled using the finite volume method in the open-source program OpenFOAM®. A sharp drop in permeability between images with a voxel size of 24 and 12 $${\upmu }\hbox {m}$$ suggests that an optimal speed/resolution trade-off may be found. The primary source of error is due to reassignment of voxels along the pore–solid interface and the subsequent change in pore connectivity. We observe the error in permeability and porosity due to both image resolution and thresholding values in order to find a method that balances an acceptable error range with reasonable computation time.