In-Situ Oil Carbon Density Characterization for Enhanced Reservoir Saturation Monitoring Using Carbon-Oxygen Logs

Abstract

Carbon/Oxygen (C/O) log is the most commonly used measurement for reservoir saturation monitoring (RSM), especially in fresh water and mixed salinity environments. In interpreting C/O logs, oil carbon density (OCD) is a required input parameter, where a single averaging number from such as oil pressure-volume-temperature (PVT) tests is commonly used. An in-situ determined OCD, taking into account OCD variety areally as well as vertically across a reservoir, would improve the accuracy of CO RSM, the objective of this paper. In a previously published work, regions of different OCDs are identified based on available crude oil PVT data across the reservoir, and each of the regions is assigned a corresponding average OCD. Although this coarse regioning can provide improvements in determinations of oil saturation (So) from C/O logs, it can be further enhanced by taking into account variations of OCD across each region. In this paper, we discuss a new approach intended to increase the accuracy of the calculated So from C/O logging data, through the integration of a continuous oil density curve into the C/O data processing workflow. The new approach utilizes oil viscosity acquired from nuclear magnetic resonance (NMR) logs, in addition to temperature logs and PVT data, to develop a localized relationship between oil viscosity and oil density. The application of the optimum correlation shall yield an accurate oil density log, which is then used as a modular dynamic input of OCD in C/O data processing. The new workflow was applied to several wells across a heavy oil carbonate reservoir, with proven vertical change in oil properties. The comparison of the new with the original saturation profile, obtained by using the conventional C/O data interpretation workflow, showed a significant increase in accuracy. Where the new approach induced a better match to openhole – resistivity derived – water saturation log across heavy oil, with both good and moderate porosities, unperforated zones. Unlike the original data processing scheme which has usually over-estimated water saturation across the same zones, because of the lack of the required sensitivity towards the heavy hydrocarbon fraction. This new technique has been proven to closely capture the changes in reservoir oil properties, increasing the accuracy of water saturation profiling across reservoirs with varying oil properties, thus provides a means to maximize the benefit of C/O logging across reservoirs of varying hydrocarbon properties and optimize oilfield development.