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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2017-52
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
13 Jun 2017
Review status
This discussion paper is a preprint. It has been under review for the journal Solid Earth (SE). A final paper in SE is not foreseen.
Imaging and quantification of the pore microstructure of gas shales using X-ray microtomography
Mozhdeh Mehrabi1, Mehrdad Pasha1, Ali Hassanpour1, Paul W. J. Glover2, and Xiaodong Jia1 1Institue of Particle Science and Engineering, School of Chemical and Process Engineering, University of Leeds, Leeds, LS2 9 JT, UK
2School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
Abstract. Optimisation of gas production from shale gas reservoirs depends critically upon a good understanding of the porosity and pore microstructure of the shale. Conventionally surface area measurements or mercury porosimetry have been used to measure the porosity in gas shales. However, these conventional methods have limited accuracy and only provide a bulk measurement for the samples. More recently, scanning electron micrography (SEM) and Focussed Ion Beam SEM (FIB-SEM) techniques have been applied in an attempt to address these limitations. Unfortunately, these two methods destroy the samples. In this research three-dimensional x-ray micro tomography (XRMT) imaging techniques were used to capture the structure of three samples and also compared to data from mercury porisimetry. The resulting data have been segmented in order to recognize individual pores down to a resolution of about 1 µm. Distributions of pore volume, pore size, pore aspect ratio, surface area to pore volume ratios and pore orientations were calculated from the XRMT data. It was found that the porosity obtained from XRMT measurements is smaller than that obtained using mercury porisimetry, the reason for which might be displacement of kerogen by the high pressures generated in the mercury technique, but is unlikely to be due to both techniques not being able to measure pores smaller that about 900 nm. Pore volume and size distributions showed all of the shales tested in this work to be multimodal with similar major modal values for volume and pore size. The pores also have a range of pore aspect ratios and surface area to pore volumes, including values indicating the presence of significant oblate spheroidal pores where the major axis is up to 330 times bigger than the minor axis. This has implications both for the connectedness of pores and the resultant gas permeability and the effectiveness of gas desorption processes into the gas shale's pores. These high aspect ratio pores were oriented both in dip and azimuth in preferential directions making it likely that the shale gas itself has significant anisotropy both for permeability and in its mechanical properties. Permeabilities calculated from the XRMT distribution data matched very well with permeabilities obtained by scaling considerations and typical values for similar gas shales.

Citation: Mehrabi, M., Pasha, M., Hassanpour, A., Glover, P. W. J., and Jia, X.: Imaging and quantification of the pore microstructure of gas shales using X-ray microtomography, Solid Earth Discuss., https://doi.org/10.5194/se-2017-52, 2017.
Mozhdeh Mehrabi et al.
Interactive discussionStatus: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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RC1: 'se-2017-52 review', Andrew Aplin, 14 Jun 2017 Printer-friendly Version Supplement 
 
RC2: 'Comments on "Imaging and quantification of the pore microstructure of gas shales using X-ray microtomography"', Anonymous Referee #2, 02 Aug 2017 Printer-friendly Version 
Mozhdeh Mehrabi et al.
Mozhdeh Mehrabi et al.

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Short summary
The paper demonstrates a method for evaluation of gas production capacity from shale gas reservoirs by gathering a good understanding of the porosity and pore microstructure of the shale. In this research three-dimensional x-ray micro tomography (XRMT) imaging techniques were used to capture the internal structure of three samples and also were compared to the data from mercury porosimetry.
The paper demonstrates a method for evaluation of gas production capacity from shale gas...
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