Response Of A Multidetector Pulsed Neutron Porosity Tool

Abstract

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. INTRODUCTION Neutron porosity logging plays an important role in the evaluation of newly drilled wells when used in combination with resistivity and density logging. For many years, this triple combination has been the industry standard for quantifying new oil and gas reserves. Evaluations have been improved recently as a result of better resistivity tool designs, but the basic design of commercial neutron tools used for openhole logging has remained unchanged for many years. Conventional neutron tools of this type typically have a continuously emitting source of neutrons and use either one or two neutron detectors. The performance of such devices has been summarized by several authors (Alger et al., 1972; Arnold and Smith Jr, 1981; Smith, 1986; Tittman, 1986; Ellis, 1987; Galford et al., 1988; Mickael and Gilchrist, 1993). An improved neutron porosity measurement has been designed using an electronically controlled miniature (minitron) neutron source which reduces radiation hazards. The basic tool design, known as the Accelerator Porosity Sonde (APS), was described in an earlier paper (Flanagan et al., 1991). Since that time, several innovations have been implemented. The APS measurement has the following advantages over conventional compensated neutron porosity logs: • The porosity response is affected primarily by the hydrogen index of the formation and is relatively insensitive to changes in formation atom density. • The vertical resolution of the measurement is improved. • In carbonates, the response to limestone and dolomite is almost identical. • Combining appropriately spaced measurements allows gas detection without the use of other logs. • Environmental effects are reduced. • An epithermal neutron slowing-down time measurement is provided. • A thermal neutron capture cross-section measurement (sigma) is also provided. This paper gives a detailed description of the tool, results of Monte Carlo modeling of the tool response to various conditions, results of laboratory measurements,