The development of a neutron porosity tool using a pulsed source of 14-MeV neutrons and multiple detector spacings provides significant improvement in the determination of formation hydrogen index. Optimizing the source-to-detector spacing has substantially reduced the unwanted effects of increased formation atom density exhibited by clay minerals in shaly formations, and has also reduced the lithology effect The detector system has five neutron detectors: four epithermal and one thermal. By operating the source in a pulsed mode, the detector system can measure both neutron count rates and neutron arrival times. To reduce borehole effects, the detectors are backshielded and the tool is run eccentered in the borehole. The epithermal neutron porosity measurement is corrected in real time for tool standoff from the borehole wall by combining count rate ratios with the epithermal neutron slowing-down time measurement, The use of epithermal neutron detection removes the influence of thermal neutron absorbers commonly encountered in shaly formations. In addition, fluid salinity and temperature effects are significantly reduced. The improved vertical resolution of the measurement plus the lower sensitivity to clay makes it easier to identify and evaluate thin beds. The single thermal neutron detector is used to measure the capture cross section (sigma) of the formation close to the borehole. This extra measurement has good bed resolution and is valuable for formation evaluation and gas indication. The response of the tool to a wide range of formation and borehole parameters has been determined using a combination of laboratory measurements and Monte Carlo modeling. In carbonate formations, the responses to limestone and dolomite are almost identical. The use of an electronically controlled pulsed neutron source eliminates the need for the conventional radioactive AmBe neutron source for this type of measurement, improving radiation safety. This technology has been found particularly useful for gas exploration in shaly formations.

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