Reservoir Crude-Oil Viscosity Estimation from Wireline NMR Measurements—Rajasthan, India

Abstract

Abstract In 2004, the Mangala, Aishwariya, and Bhagyam fields were discovered in Rajasthan, India. Oil viscosity is a main factor controlling performance from these high-permeability paraffinic reservoirs. PVT data show areal and vertical variations in crude properties. Meter-by-meter geochemical core data corroborates vertical variations in oil composition. Continuous wireline measurements of nuclear magnetic resonance (NMR) properties and station NMR properties from wells drilled with both water- based (WBM) and synthetic oil-based muds (OBM) were also used to calculate a viscosity profile. This paper correlates results from all techniques, and shows how NMR measurements can provide oil viscosity profiles in compositionally-complex pools. Black oil PVT samples typically test several meters of reservoir, while Rajasthan geochemical data is available at meter-scale. NMR logs provide continuous data, and calibrated to PVT and geochemistry, can provide the most detailed picture of in situ viscosity variations. Results were used to construct a detailed spatial description of the reservoir's in situ oil viscosity. The NMR data helped define a zone of biodegraded oil up to ~25 meters thick above the oil- water contact (OWC), and showed thin accumulations of higher- viscosity oil on top of minor shale layers within oil columns. The major conclusion is that detailed in situ oil viscosity profiles can be developed from conventional wireline T2 measurements. Techniques presented allow quick and accurate calculation of oil viscosity profiles from wireline logs, and can reduce the need for time-consuming detailed geochemical measurements. Results directly impacted the static and dynamic modelling of the Rajasthan fields and proposed waterflood designs (different for each field). Procedures described are generally applicable to reservoirs with similar datasets. Finally, the wealth of data presented allows a detailed examination of the viability of several approaches advocated for estimating in situ oil viscosity from wireline NMR measurements.