Direct Detection Of Water Flow Behind Pipe Using A Transient Oxygen Activation Technique
Authors: Uswak G. , Howes E. 
Published: March 2010

Abstract
An indirect approach to determine if a formation is hydraulically isolated employs the cement bond or other acoustic logs to analyze the cement sheath. An a/ternate route to evaluate hydraulic isolation is to detect fluid movement directly. A new cased hole wireline service, the Water Flow Log (WFL *) detects induced radioactive water in the flow-channel to determine the axial movement of fluids near the wellbore.
A solid state neutron source in a stationary logging tool is first turned on activating tile oxygen in the region near the wellbore, then turned off. If the oxygen is stationary, decay is exponential and predictable. However, if flow is present, axial fluid movement transports some of the temporarily activated atomic oxygen in the water past three detectors where gamma ray count rate transient responses are recorded. Analysis of the transients allows direct detection of the water flow in the annular space, a velocity measurement, and an estimate of the low magnitude.
The WFL log has similarities to other wireline methods of directly detecting flow behind casing. Comparisons to audio logging, temperature profile logging, and radioactive logging reveal overlapping ranges of application. The WFL measurement uniquely has a wide range of application and retains sufficient resolution in circumstances where the other direct techniques are ambiguous.
A constant flow interpretation model is introduced and tested with several field examples. The model allows a systematic evaluation of WFL data and has a format directly comparable to other logs.

Introduction
The Water Flow Log (WFL) directly detects water movement behind pipe by seeing activated oxygen in the moving water using the impulse activation technique(1). This allows the determination of water channeling in both producing and injection wells. Stationary measurements made with the tool in-between perforated zones and intervals of interest allow hydraulic isolation to be determined directly. Additionally, there are some special applications requiring a measurement of water flow between tubing and casing.
A unique feature of the impulse activation technique is that a zero flow measurement is not required. A gamma ray count rate transient profile from one detector is used to detect water flow and measure velocity. Three separate detectors at different spacings provide sensitivity for a wide rate of conditions. The tool may be configured for either downward or upward water flow detection.
Increased environmental sensitivity has placed a focus on containing disposed water in designated lithological formations, There are costs associated with unwanted co-production of water in oil and gas wells. Newer wells are drilled in reservoirs where hydraulic isolation over short intervals is a key requirement of the well design. Secondary and tertiary recovery schemes require hydraulic isolation between differing layers to function as designed.
There are several wireline methods that provide data for evaluating hydraulic isolation. Temperature logs and radioactive tracer surveys are carried out in very much the same fashion as when they were first used. Acoustic cement bond logging techniques have evolved significantly since first run, but the underlying principles of the measurement remains the same.