Selective Downhole Flow Measurement of Water Phase Using Oxygen Activation Log in Gulf of Suez Wells
Authors: Minhas H.N. , Eissa M.A. 
Published: December 2007

Conventional production logs; spinner, density, capacitance, temperature, and pressure are routinely used in the Gulf of Suez (GOS) wells for reservoir monitoring and diagnosing production problems. These logs can adequately determine flow profile of fluids inside vertical or moderately deviated holes. However, these were found unsuitable for more complex problems including the following,–Identification of water entries or exits in horizontal wells.–Water flow behind casing in cement channels or behind tubing in dual completion systems.

Oxygen activation technique (or Water Flow Log, WFL), was successfully used to diagnose these problems. Subsequent workovers resulted in significant production increase, confirming the answers obtained from these surveys.

Unlike some other tools, oxygen activation log is immune to well deviation. It therefore works equally well in vertical or deviated holes. It further reacts to flowing water only. Thus standing water or any non-water fluids are simply neglected by this measurement. Being a nuclear technique its depth of investigation extends behind the pipe. This property allows us to monitor water movement in cement channels or behind tubing in dual completion injectors.

This paper explains the principal of oxygen activation log and shows its two important applications.

Introduction

In traditional production logging, spinner flow-meter measures the average fluid velocity of all phases. This is combined with a holdup measurement, such as density, to determine velocity of each phase. Frequently we come across situations where this procedure cannot be used,The measurement sensors in conventional measurements must be in direct contact with the fluid to be monitored. This is not possible when we want to monitor water movement behind casing, e.g., in cement channels (Fig. 1) or in dual completion wells (Fig. 7).In deviated wells, the spinner is often biased towards the fluid flowing on the low side of the hole. The fluid velocity of this heavy fluid is less than the average fluid velocity in the bore-hole. In extreme cases this fluid can be stagnant or flowing in opposite direction to normal flow. In most of the cases, the spinner will underestimate the fluid velocity in high deviation or horizontal wells.Since the slippage velocity curves cannot be used in horizontal wells, a direct measurement of phase velocity and holdup is needed to solve such multiphase flow problem. This has been the subject of a number of papers presented in the recent past (Ref. 1–4). A special set of tools have been developed to address these needs. This, however, is not the subject of this paper.

In GOS wells, we faced the first two limitations of conventional log that were adequately solved with WFL. While oxygen activation log was used standalone in the examples presented in this paper. It is also being used in conjunction with other measurements to solve multi-phase flow problems in horizontal wells.

Principle of oxygen activation log

The physics of oxygen activation measurement is described in Fig. 2. This measurement is made with a pulse neutron tool by making a series of 2 or 10 second neutron bursts. Some water in the vicinity of minitron is activated every time the neutron burst is made. The activated water is detected by -detectors placed at 1 ft, 2 ft or 15 ft depending on the velocity of water. The travel time of activated water from minitron to the detectors provides water velocity knowing the space between minitron and -detectors.

The oxygen in water is activated after absorbing a neutron emitted from the minitron of the pulsed neutron tool. The activated oxygen returns to its stable state by emitting a -ray. The half life of this reaction is about 7.1 seconds. Slow moving water may not be detected by -detectors at 15 ft as most of the water would deactivate before reaching there. The slow moving water is instead detected by -detectors present at 1 ft or 2 ft. Thus by using three different detectors a range of water velocities can be covered.

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