Neutron porosity measurements are among the basic well logging services provided to operators. Most commonly, this measurement is performed by well logging tools equipped with chemical neutron sources, such as AmBe or Cf-252, using He-3 neutron detectors. The recent dramatic decrease in the He-3 gas supply and potential governmental restrictions on the use of chemical sources are likely to make the current form of neutron porosity measurements quite impractical. The potential solutions to these problems are to use a pulsed neutron generator (PNG) as replacement of chemical sources and Li-6 glass scintillation neutron detectors as an alternative to He-3 detectors. Well logging tools equipped with PNGs and Li-6 glass detectors have additional advantages, specifically being capable of performing formation Sigma (Σ) measurements because of Li-6 glass detector sensitivity to gamma rays, and controllable neutron output of the PNG. This paper discusses how to extract the neutron and gamma ray signal from the total spectra recorded with Li-6 glass detectors. It also presents results of the neutron porosity measurements obtained with a generic slim cased-hole test article fitted with two Li-6 glass detectors. Spectral decomposition parameters of the neutron capture spectrum, which are correlated to the thermal neutron flux, show stable and prominent dependence on porosity. Simultaneously measuring the time spectra for each record introduces the advantage of a combined formation Sigma and neutron porosity measurement. The possibility of using time spectra for determining diffusion corrections for measured Σ values is discussed as well. Finally, the example of an openhole neutron porosity log measured using this test article is presented. It will be compared with the neutron porosity values obtained from the measurements made by a chemical source compensated neutron porosity tool.
The reasons to move away from chemical neutron sources, such as the commonly used AmBe, are numerous.
