Pulsed Oxygen-activation Technique For Measuring Water Flow Behind Pipe

A theoretical study was undertaken to determine whether a pulsed oxygen-activation technique might be a viable method for detecting and measuring water flow behind pipe. In principle, this approach has important advantages over the (essentially) continuous technique currently in use. For example, the pulse technique should provide a channel-flow signal clearly discernible from background, thus yielding a measurement that is easier to interpret. Further, reasonable signal-to-noise quality should be readily attained by sufficient signal averaging of repetitive pulses. To assess the feasibility of the pulse approach, count rates were estimated as a function of time after injection of a pulse of 14 MeV neutrons. For comparison, similar calculations were done for the continuous technique, using essentially the same tool and borehole parameters. The pulse technique had certain distinct advantages over the continuous technique, including the ability to clearly discern channel signal from background signal. Also, for reasonable signal-avenging times (e.g., 1 5 min.), source/detector spacings, channel cross-sectional areas, and (single-string) casing sizes, channel-flow velocities of 2–200 cdsec. should be measurable with the pulse technique. Both techniques have low count rates, which limits the regimes of borehole geometry and channel flow to which oxygen activation, either pulsed or continuous, may be applied. Yet, the inherent advantages of the pulse technique suggests that it may be the preferred approach.