Quantitative Monitoring of Water Flow Behind And in Wellbore Casing
Authors: Arnold D.M. , Paap H.J. 
Published: December 2007

A nuclear logging technique for measuring water flow in and behind casing was developed and found to determine accurately the linear velocity and volume rate of flow, regardless of flow geometry. This technique detects water channeling caused by faulty cement and allows accurate water production profiling in perforated intervals. production profiling in perforated intervals.

Introduction

A common problem encountered in production operations is the movement of water in channels within the cement annulus behind casing of oil and gas wells. Such channels provide undesirable paths of communication between formations of different pressures. The channeling of water into oil zones or brine into shallow freshwater formations, which are used to supply potable water for communities, is particularly undesirable.McKinley et al. describe a noise logging technique for locating certain types of fluid movement. This technique primarily measures acoustical noise generated by primarily measures acoustical noise generated by turbulence from high-energy expansion of fluids. Since the noise amplitude generated is a direct function of energy dissipated in the expansion process, this technique does not appear feasible for detecting water channeling when pressure differentials are too low to generate detectable pressure differentials are too low to generate detectable noise amplitudes.Wichmann et al. reported that water flowing outside a 17.78-cm (7-in.) casing was detected by activating the oxygen in the water with 14 million electron volt [MeV] (2243 fJ) neutrons and by detecting this oxygen activation as the water moved past a gamma ray detector. Wichmann et al. concluded that the ability to "tag" any fluid containing oxygen by making it radioactive (without its having been in close contact with the logging tool) is unique and is probably the only way possible to detect the flow of water outside the casing when the water cannot be tagged by conventional tracer techniques. Although the linear flow velocity for a specific interval could be obtained by measuring the time required for the activated water to travel from a point opposite the neutron source to one opposite the detector, they concluded that determination of volume flow rate generally would not be feasible by the activation method. except in very favorable cases.Texaco Inc. has developed a water-flow monitoring system that measures the direction, linear flow velocity, volume flow rate, and radial position of water flowing vertically behind or in wellbore casing. The system is based on a nuclear activation technique in which flowing water is irradiated with neutrons emitted by a logging sonde. These neutrons interact with oxygen nuclei in the water to produce the radioactive isotope nitrogen-16 through the 16 O(n, p) 16 N reaction. 16 N decays with a half-life of 7.13 s, emitting 6.13 and 7.12 MeV (982 to 1141 fJ) gamma radiation. The flow parameters of interest are computed from the energy and intensity response of two gamma ray detectors mounted in the logging sonde.The water-flow monitoring system is similar in some respects to various radioactive tracer techniques, but is unique in the sense that the tracer, 16 N, is "manufactured" in the water. This eliminates the need to perforate the casing and to inject tracer material from an external source. In addition, the technique yields a more quantitative description of the flow parameters than that possible when using conventional tracer techniques.

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