Quantification of Rock Structures with High Resolution X-Ray µ-CT for Laboratory SIP Measurements Matthias Halisch
Sabine Kruschwitz
Leibniz- Institut für Angewandte Geophysik (LIAG) Stilleweg 2, D-30655 Hannover, Germany
[email protected]
Bundesanstalt für Materialforschung und –prüfung (BAM) Unter den Eichen 87, D-12205 Berlin, Germany
[email protected]
Mayka Schmitt
Andreas Weller
Federal University of Santa Catarina 88040-900 Florianópolis, Brazil
[email protected]
Institut für Geophysik, Technische Universität Clausthal Arnold-Sommerfeld Str. 1, D-38678 Clausthal-Zellerfeld, Germany
[email protected]
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SUMMARY Spectral Induced Polarization (SIP) measurements are used in many different ways to characterize natural rocks and soils. Main foci of interest are the enhanced characterization of the causes of IP-effects in clastic rocks (especially sandstones), the interactions between the matrix-fluid-system and within the electrical double layers as well as the correlation with “classical” petrophysical parameters, such as specific surface area, permeability, mercury intrusion capillary pressure (MICP) and others. Nevertheless, for all of these investigations, knowledge of the inner structure of the sample material is essential in order to create reliable and validated models as well as to interpret and to assess the data most completely. Unfortunately, many of the methods used, to get access to the inner structure of rocks are destructive (e.g. MICP, thin sectioning, etc.) and the valuable sample is lost. In addition, data is either of volume integrated nature or only available for the 2D case and the usage of sister cores does not necessarily lead to reliable results. In this paper, the authors showcase the possibilities of non-destructive and three dimensional X-ray computed tomography and of enhanced image analysis capabilities for the quantification of rock structures at the pore scale. Key words: µ-CT imaging, rock structure, digital image analysis, pore geometry, grain geometry, SIP
INTRODUCTION Spectral induced polarization measurements are used in many ways to characterize natural porous rocks and soil material. In the last couple of years, there have been some efforts to correlate IP spectra and IP related data towards petrophysical and structural, i.e. pore scale quantities, such as: -
specific surface area (e.g. Börner et al., 1996; Slater et al., 2006; Weller et al., 2010); permeability and hydraulic conductivity (e.g. Börner et al., 1996; Weller et al., 2015);
IP2016 – 6-8 June, Aarhus, Denmark
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pore and pore throat sizes (e.g. Scott & Barker, 2003; Revil et al., 2014); general textural structures (Kruschwitz et al., 2010); fractal dimension of pore space geometries (Zhang & Weller, 2016).
Nevertheless, in many cases valuable core material is either destroyed during the measurements (e.g. by MICP experiments), or sister core plugs are used, which might not feature the same pore scale structures, or exact mineralogical composition. Hence, results of SIP and other measurements necessarily do not need to fit or correlate. This is where the imaging and image analysis techniques can contribute with an important part to pore scale research. In the following, the authors are giving a brief introduction of X-ray micro computed tomography (µ-CT), Digital Image Analysis (DIA) and Digital Rock Physics (DRP) at the pore scale. Afterwards, a selected variety of results from these different methods are showcased, in order to give an overview on the possibilities of non-destructive and three dimensional (3D) imaging procedures.
METHODS In this chapter, we would like to introduce the main technical background of high resolution X-ray computed tomography, followed by the extensive DIA and DRP workflow. X-Ray Computed Tomography Figure 1 showcases the basic principle of the µ-CT measurements. X-rays are emitted from a high power nanofocus source (Figure 1, left hand side) in form of a so called cone beam. As soon as they hit the sample material, which rotates stepwise in pitches
