Abstract

A new and innovative high-performance laboratory system for performing low-field nuclear magnetic resonance (NMR) relaxation times and diffusion measurements was built to enable a study of reservoir fluids at the high temperatures and pressures that occur in many oil and gas reservoirs. The primary objective of the study was to determine which reservoir fluid properties can be predicted from NMR measurements and the accuracy of the predictions. A secondary objective was to determine if combining NMR and near-infrared (NIR) optical absorption data improved the predictions.

Previous industry publications on reservoir fluid studies using NMR are based on measurements acquired using commercial NMR systems. The temperature and pressure specifications and signal-to-noise ratio (SNR) of the new system represent a quantum leap in the technology compared with those of commercially available systems. For the first time, high-quality NMR data on live reservoir fluids can be rapidly acquired at the high temperatures and pressures of worldwide oil reservoirs. The SNR of our system is more than a factor of 15 higher than that of commercial systems so that data acquisition is more than 225 times faster than previously possible. Moreover, the system is easy to maintain and pressure compensation is not required to achieve high pressures. The heart of the system is a compact sensor consisting of an NMR magnet with a low static magnetic field gradient, a radio frequency (RF) antenna, and a pair of pulsed field gradient (PFG) coils used for NMR diffusion measurements.

The new system was used to acquire a database of hundreds of NMR and optics measurements at different temperatures and pressures on a representative suite of oils typical of those sampled by fluid sampling tools. In addition, to the NMR and optics measurements, the database contains measured fluid properties including molecular compositions; saturate, aromatic, resin and asphaltene (SARA) fractions; gas/oil ratio (GOR); viscosity; compressibility; formation volume factor; and density measurements for each oil. The NMR, optics, and fluid properties measurements were measured at pressures up to 25,000 psi and at temperatures up to 175 degC.

The database was used to determine how accurately fluid properties including molecular composition, SARA fractions, viscosity, GOR, density, and compressibility can be predicted from NMR measurements. We discuss the laboratory system, sample preparation, measurements, and accuracy of the predictions. We show that each of the aforementioned reservoir fluid properties can be accurately predicted from NMR measurements given the pressure and temperature of the reservoir fluid.

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