The initial reservoir fluid distribution in the Prudhoe Bay Reservoir, Alaska, consisted of relatively planar gas-oil and oil-water contacts. These contacts were soon perturbed by production as well as water and gas reinjection. Understanding the current reservoir fluid distribution and the movement of fluids in response to reservoir depletion mechanisms is key to optimizing and maximizing recovery.

Historically, the focus of much of the successful cased hole surveillance effort in the Gravity Drainage area of Prudhoe Bay has been directed at tracking the movement of the gas-oil contact and quantifying the remaining oil bypassed by gravity drainage processes. Forward modelling of pulsed neutron tool attributes has been employed to enable gas saturation quantification. Recent introduction of memory Multi-Detector Pulsed Neutron technology has enabled the quantification of bypassed oil in horizontal wells using coiled-tubing. Selective perforation has been used to access undrained oil.

In the Waterflood area of Prudhoe Bay, it is a challenge to use conventional sigma logs to distinguish between oil and water due to the relatively low and variable formation and injection water salinity. Consequently, continuous and multiple passes and stationary Carbon-Oxygen logs have been employed to identify bypassed oil.

In areas of the field where gas, oil and water columns exist in a single wellbore, both Carbon-Oxygen and Multi-Detector Pulsed Neutron nuclear attributes are combined together using a novel three phase interpretation technique to quantify oil, gas and water saturation. The technique has been applied in a number of Prudhoe Bay wells to enhance understanding of the fluid distribution and to design perforation strategies to maximize off take in existing cased hole wells.

Case studies of each scenario illustrate the use and integration of Carbon-Oxygen and Multi-Detector Pulsed Neutron attributes to identify bypassed oil.

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