The porosity response of four proposed generator-based neutron tool concepts is studied using Monte Carlo simulation of the radiation transport. The objective is to examine, at a fundamental level, the potential of these sources to replace americium-beryllium (Am-Be) sources primarily in openhole applications and, briefly, in a through-casing application of interest to a number of operators. The accelerator-based sources include a dense-plasma focus (DPF) alpha-particle accelerator and deuterium-tritium (DT), deuterium-deuterium (D-D), and deuterium-lithium (D-7Li) neutron generators. The DPF uses the (a-Be) reaction to generate a neutron spectrum that is nearly identical to that from an Am-Be source. D-T and D-D neutron generators use compact linear accelerators and produce, respectively, 14.1 and 2.45 MeV neutrons. The D-7Li neutron spectrum resembles the Am-Be spectrum at lower energies, and has a neutron peak at 13.3 MeV
Simple spherical-geometry models that do not include tool and borehole are used to explore the basic physics. An openhole tool-borehole-formation configuration is used to explore key observations from the simpler model. In both models, the responses at various detectors are examined to understand the behavior of the ratios constructed. Sensitivity to formation conditions, such as lithology, presence of gas, low porosity and presence of thermal absorbers, and operational conditions, such as tool standoff, are examined. A casedhole configuration is also analyzed where neutron counts are the only method for zonal correlation.
The state of neutron-generator technology is discussed in terms of neutron yield, target properties, power demands etc., which are important considerations for implementing such generators in nuclear logging tools.