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

Optimal X-ray micro-CT image based methods for porosity and permeability quantification in heterogeneous sandstones
Optimal X-ray micro-CT image based methods for porosity and permeability quantification in heterogeneous sandstones
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.
Core, Microstructure, Numerical modelling, X-ray micro-CT, image processing, permeability and porosity
0956-540X
1210-1229
Callow, Ben James
19f1a5fe-cabd-4c49-a84f-dc7de0e3c462
Falcon-Suarez, Ismael Himar
f14858f6-d086-4761-9dc5-ba09bd89d95b
Marín‐Moreno, Héctor
f8b611ad-3be4-44da-989a-274e2dd2ca66
Bull, Jonathan
974037fd-544b-458f-98cc-ce8eca89e3c8
Ahmed, Sharif
ddc6bab1-9d76-4391-b7ea-ae68d6f3924d
Callow, Ben James
19f1a5fe-cabd-4c49-a84f-dc7de0e3c462
Falcon-Suarez, Ismael Himar
f14858f6-d086-4761-9dc5-ba09bd89d95b
Marín‐Moreno, Héctor
f8b611ad-3be4-44da-989a-274e2dd2ca66
Bull, Jonathan
974037fd-544b-458f-98cc-ce8eca89e3c8
Ahmed, Sharif
ddc6bab1-9d76-4391-b7ea-ae68d6f3924d

Callow, Ben James, Falcon-Suarez, Ismael Himar, Marín‐Moreno, Héctor, Bull, Jonathan and Ahmed, Sharif (2020) Optimal X-ray micro-CT image based methods for porosity and permeability quantification in heterogeneous sandstones. Geophysical Journal International, 223 (2), 1210-1229. (doi:10.1093/gji/ggaa321).

Record type: Article

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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Callow_et_al_2020_GJI-S-19-1155.R1 - Accepted Manuscript
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More information

Accepted/In Press date: 19 June 2020
Published date: 27 June 2020
Keywords: Core, Microstructure, Numerical modelling, X-ray micro-CT, image processing, permeability and porosity

Identifiers

Local EPrints ID: 441855
URI: http://eprints.soton.ac.uk/id/eprint/441855
ISSN: 0956-540X
PURE UUID: 632b5eaa-372f-4141-945c-a13fcf5a3b6e
ORCID for Ben James Callow: ORCID iD orcid.org/0000-0003-2296-1702

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Date deposited: 30 Jun 2020 16:31
Last modified: 25 Nov 2021 20:35

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Contributors

Author: Ismael Himar Falcon-Suarez
Author: Héctor Marín‐Moreno
Author: Jonathan Bull
Author: Sharif Ahmed

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