Due to a surge in oil demand over the past years and its impact on prices, major heavy oil accumulations are gaining special attention in the Middle East. Knowledge of the fluid characteristics is critical to determine a suitable recovery process for heavy oil reservoirs. Oil viscosity is the property that most affects producibility and ultimate recovery in these settings, but it is the most difficult property to determine. This is due to the challenges associated with obtaining representative oil samples in increasingly heavy oil reservoirs. Additionally, the oil viscosities in these reservoirs exhibit large variations within the same formation. Hence, downhole in-situ measurements are key in establishing viscosities and their variation within the reservoir.
Current practice in heavy oil accumulation is to extract oil samples through crushing of cores at surface, or sampling through changing the temperature of the near wellbore in the hope that the fluid will flow. These techniques provide viscosity ranges that help in uncertainty testing of various recovery processes but are only point-data measurements. Nuclear magnetic resonance (NMR) logging is also used to determine in-situ oil viscosity accurately in conventional light oil reservoirs. However, this technique has limitations in heavy oil settings because the extremely fast relaxivity of the heavy oil components results in very short transverse or relaxation time (T2) values that are not detectable by the NMR tool. The short T2 time also means that the heavy oil and bound-water signals, overlap resulting in an ambiguous oil signal.
A novel technique combines data from 3D NMR and dielectric dispersion tools to determine in-situ oil viscosity. This was based on recently published NMR viscosity correlations. The primary output from the dielectric tool is the total water volume. Using this output combined with the 3D NMR data makes it possible to accurately derive bound-water, free-water, and oil volumes. These are necessary inputs into NMR-based viscosity correlations. This technique yielded continuous logs of heavy oil viscosities within the same viscosity range as measurements obtained from the core data.