Larry A. Jacobson

A New Water-Flow Velocity Log for Use in Conformance Evaluation

The conformance of unwanted water in producing wells is becoming increasingly important because of costs associated with separation and disposal. A new water-velocity log based on oxygen-activation measurements was recently introduced. The measurements are made with a new small-diameter (1-11/16-in., 43 mm) tool whose design was directed towards enhanced evaluation of water conformance. This paper discusses the laboratory measurements that characterize the logging technique. Field logs from the Gulf of Mexico region illustrate various water-flow regimes that can be identified with the technique. An interesting example of simultaneous upward and downward flow demonstrates the utility of the tool. The tool uses two spectral gamma ray detectors and a pulsed neutron generator with a special timing sequence designed to emphasize activati n measurements. Spectral measurements enhance velocity estimates by allowing gamma rays from oxygen activation to be distinguished from those arising from iron activation, silicon activation, and natural activity. Furthermore, spectral measurements permit analysis of Compton scattering to indicate whether water is flowing inside or outside the casing. Water velocity is determined by two methods: ○ Use of a count-rate ratio from the two spectral gamma detectors, which provides a continuous log but requires well-calibrated detectors. ○ Use of an impulse/shutdown sequence, which is performed while the tool is stationary and which is calibration independent. The latter method is preferred. Velocity measurements can be performed at water flow rates from 3 to 50 ft/min when detectors are spaced 3 and 4 ft from the neutron generator. With additional longer-spaced natural gamma ray measurements, the range can be expanded to approximately 200 ft/min with the impulse/shutdown method. Modular design allows the tool to be configured with additional gamma detectors placed both uphole and downhole relative to the neutron generator. This configuration allows the tool to measure simultaneous upward and downward water flow with the impulse/shutdown method. Additionally, modular design permits the tool to be combined with production logging sensors, such as spinner, temperature, density, and hold-up tools, for more comprehensive flow analysis.