Abstract

Impulse oxygen activation logging was successfully introduced in Alaska in 1990. It was immediately used to determine water injection profiles behind tubing in the annulus between the tubing and casing. In the past, this type of well completion was difficult, if not impossible, to evaluate with the techniques that were available before the introduction of the impulse oxygen activation logging method. The impulse oxygen activation technique uses the pulse neutron tool to activate the oxygen in the water so that the water flow profile can be determined. Since neutrons are used to activate the oxygen in the water, the metal in the tubing does not stop the activation of oxygen in the water when it is behind tubing. Impulse oxygen activation logging is also used to identify water producing zones in producing wells, and whether there is water flowing through channels in the cement behind casing.

This new method of using impulse oxygen activation provides the capability to trace water flow over a wide range of water injection and production rates. Using a series of station measurements, it is possible to measure water velocities from less that two ft/min (0.01 m/s) to more than 400 ft/min (2.0 m/s) with modified pulse neutron tools. The impulse oxygen activation method also has the ability to measure two different water velocities in the same direction at the same time. This is possible because only a small amount of water is activated by the impulse oxygen activation. The activated water then flows past three different detectors spaced at 2-ft, 6-ft, and 20- ft at its different velocities. Knowing the cross sectional areas of the different flow paths, water flow rates from 10 to over 10,000 BWPD are directly calculated. Since the pulse neutron tool has no moving parts, it cannot be plugged with scale or particles suspended in the injected water such as is the case with spinner type tools.

This measurement has significantly reduced the traditional uncertainties associated with the economics of attempting to shut-in high watercut intervals. With these advances the emphasis can now turn to assessing improved modification techniques. The final result will be increased oil recovery and reduced water cycling.

Introduction

The Kuparuk River Field is located on the North Slope of Alaska approximately 40 miles west of Prudhoe Bay Field, Figure 1. It is one of the United States' largest oil fields, producing approximately 320,000 STBD. Waterflooding started in 1983 and now covers 85 percent of the 115,000 acres currently developed. Evaluation of waterflood performance is essential for efficient management of the existing waterflood assessment of ultimate production, and determination of the potential for infill drilling and enhanced recovery processes in the field.

Production occurs from two horizons within the Kuparuk River Field, referred to as the A and C sands. For waterflood management purposes, "selective single" completions are used to isolate the prolific C Sands behind production tubing and to enable the lower A Sands to be fracture stimulated. Figure 2 shows a "selective single" completion for both an injection and production well. Since most current injection wells were initially completed for production service prior to conversion, all wells have hardened blastjoints across the C Sand perforations to minimize the potential for erosional cutting of the tubing.

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