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Volume 3, issue 1
Solid Earth, 3, 71-86, 2012
https://doi.org/10.5194/se-3-71-2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.
Solid Earth, 3, 71-86, 2012
https://doi.org/10.5194/se-3-71-2012
© Author(s) 2012. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 08 Mar 2012

Research article | 08 Mar 2012

Pore formation during dehydration of a polycrystalline gypsum sample observed and quantified in a time-series synchrotron X-ray micro-tomography experiment

F. Fusseis1, C. Schrank2, J. Liu4,3, A. Karrech6,5, S. Llana-Fúnez7, X. Xiao8, and K. Regenauer-Lieb5,4,3 F. Fusseis et al.
  • 1Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Germany
  • 2School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, Australia
  • 3Western Australian Geothermal Centre of Excellence, Perth, Australia
  • 4School of Earth and Environment, University of Western Australia, Crawley, Australia
  • 5CSIRO Earth Science and Resource Engineering, Kensington, Australia
  • 6School of Mechanical Engineering, University of Western Australia, Crawley, Australia
  • 7Departamento de Geología, Universidad de Oviedo, Oviedo, Spain
  • 8Advanced Photon Source, Argonne, USA

Abstract. We conducted an in-situ X-ray micro-computed tomography heating experiment at the Advanced Photon Source (USA) to dehydrate an unconfined 2.3 mm diameter cylinder of Volterra Gypsum. We used a purpose-built X-ray transparent furnace to heat the sample to 388 K for a total of 310 min to acquire a three-dimensional time-series tomography dataset comprising nine time steps. The voxel size of 2.2 μm3 proved sufficient to pinpoint reaction initiation and the organization of drainage architecture in space and time.

We observed that dehydration commences across a narrow front, which propagates from the margins to the centre of the sample in more than four hours. The advance of this front can be fitted with a square-root function, implying that the initiation of the reaction in the sample can be described as a diffusion process.

Novel parallelized computer codes allow quantifying the geometry of the porosity and the drainage architecture from the very large tomographic datasets (20483 voxels) in unprecedented detail. We determined position, volume, shape and orientation of each resolvable pore and tracked these properties over the duration of the experiment. We found that the pore-size distribution follows a power law. Pores tend to be anisotropic but rarely crack-shaped and have a preferred orientation, likely controlled by a pre-existing fabric in the sample. With on-going dehydration, pores coalesce into a single interconnected pore cluster that is connected to the surface of the sample cylinder and provides an effective drainage pathway.

Our observations can be summarized in a model in which gypsum is stabilized by thermal expansion stresses and locally increased pore fluid pressures until the dehydration front approaches to within about 100 μm. Then, the internal stresses are released and dehydration happens efficiently, resulting in new pore space. Pressure release, the production of pores and the advance of the front are coupled in a feedback loop.

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