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FOSSIL-RELATED POROSITY SCANNING BY MEANS OF THE NUCLEAR MAGNETIC RESONANCE AND COMPUTED MICROTOMOGRAPHY - THE PERMIAN BRONSKO REEF CARBONATES, WESTERN POLAND
Abstract
Both complicated dissolution processes and different patterns of fossilsпїЅ distribution may be expected in carbonate hydrocarbon reservoirs. Therefore, this paper aims to precisely locate such places using few independent analytical tools. Five carbonate samples from the Permian Bro?sko Reef (Western Poland) were studied in a form of both thin sections and cylindrical core samples. The latter were subjected to Computed Microtomography (?CT) and Nuclear Magnetic Resonance (NMR) experiments. While the ?CT helped to preliminarily localize the most valid pores, the NMR provided significant information on petrophysical attractiveness of the reservoir. NMR research consisted of: (1) 9.4 [T], high-field, Zero Echo Time (ZTE) imaging; and (2) 0.05 [T], low-field, spatially resolved T2 profiling. As illustrated by the obtained results, joining together both low-field and high-field data, seems to be very desirable. The low-field approach showed the general trends in pore-size distribution, whereas the ZTE imaging allowed for the most permeable zones to be caught. Hence, numerous mouldic, cavernous and intraparticle porosity networks could have been located and investigated. Moreover, one more advantage of combining low- and high-field data should be considered. Although the high-field experiments may, in fact, yield better signal to noise ratio (SNR), they tend to show a relatively high vulnerability to generate the internal magnetic field gradients. The gradients may be present particularly when there is a significant difference between the magnetic susceptibility of the fossils and pore fluids, respectively. A much weaker magnetic field, utilized in the low-field NMR, enables omitting such signal disruptions, but this happens at the expense of methodпїЅs resolution. The exactness of such measurements can be additionally ameliorated by applying the spatially-resolved T2 experiments. Chosen slices of the same sample are then polarized separately and improve the accuracy of any experiment. It was hence shown, that a specific set of methods can significantly improve the effectiveness of the petrophysical analyzes, being a valid point of hydrocarbon prospection.
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References12
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