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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

3-D X-ray micro-CT (XCT) is a non-destructive 3-D 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 voxelized 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 REV size, whilst significantly reducing fluid simulation memory requirements. For the heterogeneous sandstones, REV size for permeability (≥1 mm3) is larger than for porosity (≥0.5 mm3) 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

3-D X-ray micro-CT (XCT) is a non-destructive 3-D 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 voxelized 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 REV size, whilst significantly reducing fluid simulation memory requirements. For the heterogeneous sandstones, REV size for permeability (≥1 mm3) is larger than for porosity (≥0.5 mm3) 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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Accepted/In Press date: 19 June 2020
Published date: 1 November 2020
Additional Information: Funding Information: We acknowledge funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 654462 - STEMM-CCS. We acknowledge Diamond Light Source for time on Beamline I13-2 under Proposal MT18758. We give our thanks to Hans Deyhle, Laurence North, Christina Reinhard, Shashi Marathe, Andrew Bodey and the Diamond Support Scientists at I13-2 for their support during the experiment. We are also very grateful for the advice and guidance with image reconstruction provided by Kaz Wanelik. We acknowledge the use of the IRIDIS High Performance Computing Facility, and associated support services at the University of Southampton, in the completion of this work. We would like to thank our colleagues at the ?-VIS X-Ray Imaging Centre, University of Southampton for their helpful advice and support. We would also like to thank Andrew Hurst and Antonio Grippa (University of Aberdeen, Sand Injection Research Group) for their assistance and advice with fieldwork and rock sampling in California. We would like to thank the FEI Visualization Sciences Group for providing the use of the Avizo 9.3.0 software for XCT image processing, Simpleware for the use of their ScanIP meshing software and Comsol for use of theirMultiphysics software. Finally, we would like to sincerely thank Samuel Jackson and two anonymous reviewers for their constructive comments, which greatly improved the manuscript. Author contribution statement: B.C., I.F.S.: Conceptualization, Formal Analysis, Validation. B.C.: Data curation, Methodology, Software, Visualization. B.C., J.M.B., S.A.: Funding acquisition. B.C., I.F.S., H.M.M., S.A.: Investigation. B.C., I.F.S., H.M.M., J.M.B., S.A: Writing. The data set associated with this work is available online at https://doi.org/10.5258/SOTON/D1441. Here we provide the raw and segmented XCT tomographic image data, and the associated image processing files. Publisher Copyright: © 2020 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.
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
ORCID for Jonathan Bull: ORCID iD orcid.org/0000-0003-3373-5807

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Date deposited: 30 Jun 2020 16:31
Last modified: 29 Oct 2022 01:34

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Contributors

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

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