Optimal X-ray micro-CT image based methods for porosity and permeability quantification in heterogeneous sandstones

Abstract

3D X-ray micro-CT (XCT) is a non-destructive 3D imaging method, increasingly used for a wide range of applications in Earth Science. An optimal XCT image-processing workflow is derived here for accurate quantification of porosity and absolute permeability of heterogeneous sandstone samples using an assessment of key image acquisition and processing parameters: Image resolution, segmentation method, representative elementary volume (REV) size and fluid-simulation method. XCT image-based calculations obtained for heterogeneous sandstones are compared to two homogeneous standards (Berea sandstone and a sphere pack), as well as to the results from physical laboratory measurements. An optimal XCT methodology obtains porosity and permeability results within ± 2% and vary by one order of magnitude around the direct physical measurements, respectively, achieved by incorporating the clay fraction and cement matrix (porous, impermeable components) to the pore-phase for porosity calculations and into the solid-phase for permeability calculations. Two Stokes-flow finite element modelling (FEM) simulation methods, using a voxelised grid (Avizo) and tetrahedral mesh (Comsol) produce comparable results, and similarly show that a lower resolution scan (~5 µm) is unable to resolve the smallest intergranular pores, causing an underestimation of porosity by ~3.5 %. Downsampling the image-resolution post-segmentation (numerical coarsening) and pore network modelling both allow achieving of a representative elementary volume (REV) size, whilst significantly reducing fluid simulation memory requirements. For the heterogeneous sandstones, REV size for permeability (≥ 1 cubic mm) is larger than for porosity (≥ 0.5 cubic mm) due to tortuosity of the fluid paths. This highlights that porosity should not be used as a reference REV for permeability calculations. The findings suggest that distinct image processing workflows for porosity and permeability would significantly enhance the accurate quantification of the two properties from XCT.

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ORCID for Ben James Callow: orcid.org/0000-0003-2296-1702

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