Resolving Uncertainty in Oil Quality and Extension of Field Producible Area Using Latest LWD NMR Technology

Abstract

This paper describes how the uncertainty in oil quality in an offshore carbonate reservoir was resolved using the latest slim logging-while-drilling (LWD) nuclear magnetic resonance (NMR) tool. This tool was included in the bottomhole assembly (BHA) during drilling of the sidetrack well with the objective of confirming the presence of immovable heavy-oil/tar mat below the light/heavy-oil contact (LHOC). However, the tool showed that the reality was different from the expectations as movable/light hydrocarbon was confirmed below LHOC thereby leading to an extension of the field producible area. The NMR interpretation methodology followed two scenarios based on an offset well with a rich NMR dataset. In the first scenario, because of the absence of porosity deficit and low apparent bound fluid volume, no heavy oil was identified, and the tool provided permeability and irreducible water saturation. In the second scenario, heavy oil was identified primarily from the presence of high apparent bound fluid and secondarily by a porosity deficit, and oil viscosity was computed. The high-angle well intersected the same carbonate reservoir layer multiple times. The NMR measurement showed that Zones A–1, A–3, and B contained light oil, while the expectation for Zone B was to contain a heavy oil layer below the LHOC. The NMR identified heavier oil in each crossing of Zone A–2. An oil viscosity gradient was observed on both crossings of the reservoir layers. The reservoir was otherwise at irreducible water saturation in the subsequent interceptions, and the heavy oil or tar were not identified again. All the observations from the tool were supported by related observations from the mud logging gases. The first application of this service in this field was a success. The NMR provided unique fluid typing information, confirming that most reservoir sections contained light oil and quantified the heavy oil viscosity variations along the reservoir layer, which could not be quantified from the density, neutron, or resistivity measurements. Finally, the NMR measurement showed that the reality of heavy oil distribution in this part of the reservoir was quite different from what was expected (i.e. the presence of immobile heavy oil below the LHOC) based on the reservoir model, enabling the prospect of light oil production from this part of the reservoir.