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SPATIAL PERMEABILITY ESTIMATION FOR CARBONATES - X-RAY MICROTOMOGRAPHY IN ZECHSTEIN LIMESTONE STRATA ASSESSMENT

P. J. Strzelecki, A. Fheed
Thursday 11 October 2018 by Libadmin2018

ABSTRACT

X-ray microtomography (μCT) has been increasingly popular among geoscientists interested in reservoir rocks’ pore space, as it enables studying much more parameters than solely the porosity. Because carbonate rocks can be dramatically complex with respect to their permeability, porosity, and mineral composition – they need advanced tools like μCT to be studied in details. Petrographically and petrophysically variable carbonate sample withdrawn from the Brońsko Reef, West Poland was examined by means of the μCT-based image analysis. The sample comes from the Reńsko 1 well, penetrating the Zechstein Limestone (Ca1, Permian) strata. With the help of polarized, transmitted-light microscopy and X-ray diffraction, the main challenge was to assess its spatial permeability, pore size distribution and tentative mineral composition, using μCT as a leading method. μCT-derived permeability was verified by standard nitrogen test. The images were processed with Fiji software via filtering, thresholding and segmenting procedures. In order to estimate the permeability, Kozeny-Carman model, involving porosity, pore area and pore perimeter was applied. The formula was used for μCT images with voxel resolution equal to 15×15×15 μm3. The calculations were made for each 2D slice of the μCT images in the same direction in which nitrogen permeability test was carried out. The bryozoan-rich, porous dolograinstone studied appeared to be built of 4 phases detectable in the μCT images. Mineral phases were verified on the basis of X-ray diffraction analysis (XRD) and microscopic observations. The rock is composed of carbonate cements (calcite and dolomite) and calcified skeletons of former organisms, occupying at least 59-73% of its mineral composition, anhydrite – taking about 6-19%, and little admixture of barite (around 1%). The voids make up roughly 13-24% of rock’s volume. Their segmentation revealed that they are highly connected and thus ensure high effective porosity, reaching 93% of the total porosity. Nitrogen test yielded permeability of 1.5 D, while μCT-derived results indicate that the permeability ranges between 15.7 and 0.8 D (6.1 D on average). Three zones differing in permeability coefficients were distinguished: (1) 5-10 D, (2) over 10 D, and (3) 0.8-2.5 D. Importantly, the last zone matches the nitrogen-derived permeability value. The permeability drop inside this zone is associated with a general porosity decrease, reduced pore throat sizes and anhydrite cementation. Summing up, it was concluded that simple calculations performed on μCT images using the open-source software are capable of providing numerous details about the pore space in a relatively time- and cost-efficient manner. The results show that the obtained permeability values are generally of the same order of magnitude in comparison to the nitrogen-based test. Therefore it appears that this method can be successfully applied for highly porous carbonate rocks.

Keywords: microtomography, image analysis, permeability, Permian, petrophysics


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