Abstract
Oil-well logging is of crucial importance for the oil and gas industry. Since the inception of the technology in the early 1900s, both long-lasting and newly emerging application challenges have pushed the technology advance in both breadth and depth. The primary problem to solve is optimizing hydrocarbon production with finite capital investment. Among all the petrophysical answers that logging tools provide, porosity is a key parameter that can be used to characterize reservoir storage capacity, make stimulation decisions, and optimize production returns. Neutron porosity tools were commercialized in the 1950s. Early tools used chemical neutron sources (PuBe, AmBe, etc.) and one neutron detector. Dual-neutron detector devices were soon considered to be necessary to compensate for borehole environments. The compensated design leads to much smaller borehole corrections. Many publications discuss the theory and applications of neutron porosity tools using chemical neutron sources. In recent years, the desire to use electrical sources instead of chemical sources for downhole applications has steadily grown. Traditional pulsed-neutron tools use a deuterium-tritium (D-T) pulsed-neutron generator (PNG) and two gamma-ray detectors. The capture count ratio between gamma-ray detectors is sensitive to formation porosity. However, the sensitivity is not as good as that of a chemical source, such as AmBe or Cf-252. Even more, the sensitivity declines above 35 porosity units (pu). This technical challenge is being solved by introducing three-detector pulsed-neutron tools. New algorithms have been developed to further enhance the measurement sensitivity. Laboratory data and field examples are presented to discuss this new capability in detail. In summary, new porosity measurements are being made available in the oil logging industry. The measurement is promising both to improve measurement sensitivity and to reduce the industry dependence on chemical sources, which can have health, safety, and environmental (HSE) concerns.
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