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

DOI: 10.1093/gji/ggaa321 DOI Help

Authors: Ben Callow (University of Southampton) , Ismael Falcon-Suarez (National Oceanography Centre, University of Southampton) , Hector Marin-Moreno (National Oceanography Centre, University of Southampton) , Jonathan M. Bull (University of Southampton) , Sharif Ahmed (Diamond Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Geophysical Journal International

State: Published (Approved)
Published: June 2020
Diamond Proposal Number(s): 18758

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 per cent 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 per cent. 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.

Journal Keywords: Microstructure; Permeability and porosity; Numerical modelling; Core; Image processing

Subject Areas: Earth Science, Technique Development

Instruments: I13-2-Diamond Manchester Imaging

Added On: 02/07/2020 09:46

Discipline Tags:

Earth Sciences & Environment Geology Geophysics Technique Development - Earth Sciences & Environment

Technical Tags:

Imaging Tomography