Diffusion-weighted nuclear magnetic resonance imaging (DWI) for fluid flow direction and intensity recognition in carbonates – examples from Permian reefs
Many petroleum companies worldwide encounter significant problems when investigating carbonate reservoirs, in which complex diagenesis and the wealth of former fauna may lead to porosity complication. This often takes place in bioclastic reef carbonates. Therefore, one of such objects located in the Polish part of the Southern Permian Basin (the Brońsko Reef) was evaluated. The purpose of this contribution is to utilize the diffusion-weighted Nuclear Magnetic Resonance imaging (DWI) in order to resolve the specifics of naturally bendy carbonate pore networks consisting of narrow dissolution channels. The paper explains how to determine a major fluid flow direction using the DWI. Seventy meters (230 feet) of drill cores were carefully described in order to choose the sample bearing both fractures and dissolution channels. Macro- and transmitted-light microscopic observations were followed by 0.6 T DWI, performed on a cylindrical, roughly 25 mm (1 inch)-wide and long plug sample. 150, 3 mm (0.12 inch)-thick sample plasters were polarized separately. Polarization took place under three different 0.35 T/m magnetic field gradient configurations. Each gradient (x, y and z directions, respectively) was involved separately to induce diffusion in a corresponding direction. Porosity was verified using 0.05 T low field spectrometer, whereas vertical permeability was derived from nitrogen-based measurements. As shown by the obtained results, the studied rocks contain fracture, channel and mouldic porosity. Both moulds and channels originated through the dissolution of numerous ostracod shells being very common among reef packstones. As suggested by diffusion measurements, the majority of the channels are oriented horizontally with respect to well’s axis, as deduced from increased diffusion coefficient values. They ensure a moderate permeability of roughly 100 mD and effective porosity equaling 10.5 %. It can, thus be concluded that the methodology presented allows for ameliorating the reservoir evaluation, provided that sufficient amount of data is gathered.