When an offshore oilfield enters its secondary development stage, the formation residual oil saturation is the crucial information for development planning. Pulsed-neutron logs can accomplish the task of residual oil saturation investigation in cased hole. Building on the various types of nuclear reactions between neutrons and formation, multiple pulse neutron logging modes were developed and are applied in different logging environment. How to choose the most appropriate log mode to improve work efficiency is particularly important for offshore operations.

The capture cross-section log (SIGMA) is the best choice in high water salinity reservoirs. It can be recorded with relatively faster speed and need single pass only, saving lots of operation time. But sometimes the formation behind casing may be polluted by the low-salinity water injected during the past oilfield development, the uncertainties of the water salinity always act an important role effecting the accuracy of interpretation results. The carbon/ oxygen ratio log (C/O) can overcome the water salinity problem, because it is salinity-independent log. However, due to the measurement physics, a C/O log usually requires multiple passes with lower logging speeds, cost significant amounts of operation time.

This paper introduces a novel pulsed-neutron log design that was successfully practiced in North Asia, where it took the advantages from both Sigma and C/O log modes to efficiently overcome the challenges of residual oil investigation in an injection polluted formation. The core method of this log design is "C/O calibrated Sigma". We use C/O log results to calibrate the formation sigma water, then apply it to calculate the water saturation of full interval. Field engineer log a short interval in C/O mode with multiple passes. Then log only a single pass with full target formation in SIGMA mode. Once the geoscientist received the data, firstly process C/O data to work out the water saturation of that short interval. Secondly, use the saturation result of C/O along with SIGMA log data to inverse-calculate the formation water sigma. Finally, apply the water sigma to calculate the water saturation of full interval.

By applying this methodology, the calibrated formation water salinity was calculated and proved to be lower than the original formation water. The full scope formation water saturation was successfully determined within an acceptable operating time (6 hours), compared with conducting the C/O log across the full interval which required more than 18 hours, increased the offshore operations efficiency as well as minimized the interpretation result uncertainties.

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