Evaluation of Variable Oil Volume and Rock Types With NMR and Dielectric Dispersion Logs in a Carbonate Reservoir High Deviation Drain

Abstract

Abstract We examine the along-hole profiles of oil and water in the highly deviated drain of a light oil carbonate reservoir offshore UK, and analyse the discrepancy between the volumes calculated from LWD resistivity-porosity (Archie) and from magnetic resonance and dielectric dispersion wireline tools conveyed by open hole tractor. We also correlate the petrographic analysis of these bioclastic (ostreiid) grainstones and packstones to the logs responses and rock types. Following the acquisition of LWD resistivity and porosity logs, additional formation evaluation logs – a combined tool string of NMR, dielectric dispersion, borehole images and formation pressure – was recorded in a single run conveyed by an open hole tractor between the toe and heel of the highly deviated (60-70 deg.) drain. The water volume profile calculated from resistivity-porosity Archie analysis – the only data type available in earlier wells – is compared to the volumes of capillary bound water from NMR and to the dielectric dispersion water-filled porosity. In a well drilled with oil base mud, we expect that oil filtrate invades the formation, so that the water volume measured by dielectric dispersion represents capillary-bound plus clay-bound water volume and should be equal to the bound fluid volume measured by a NMR tool. Differences between these water volumes represent either formation evaluation anomalies or the presence of free (moveable) water. Guided by the petrographic analysis of core samples from the same formation and the textural information from the NMR and dielectric dispersion logs, 3 main rock types are identified, to be propagated onto the other field wells with conventional porosity, resistivity, and GR logs. The evaluation of LWD resistivity-porosity logs provides moderate and almost constant water volumes along the length of the drain, seemingly independent of total porosity variations. In contrast, the water volumes from NMR and dielectric dispersion are more variable, correlating well with total porosity and with the laminated features observed on the borehole images. We also observe that the NMR bound fluid water volume matches the dielectric water volume, and that they are both larger than the resistivity-porosity Archie water volumes. We propose that the NMR and dielectric water volumes are correct and correspond to variations in reservoir properties in the different rock types, that the more accurate hydrocarbon volume profile is provided by the difference between total porosity and NMR bound fluid volume or dielectric porosity, and that water is at irreducible saturation along the whole drain section. Although the NMR and dielectric dispersion logs have been used before to resolve carbonate formation evaluation problems, they have rarely been used in highly deviated drains and it is likely that the tractor conveyance and tools combination is unique to this project. Bioclastic (ostreiid) grainstone and packstone reservoirs are also rare, as is the correlation of petrographic analysis to NMR and dielectric dispersion logs in this environment.