Basics of ratio-based porosity response of four proposed generator-based neutron tools are studied using Monte Carlo simulation of the radiation transport to examine, at a fundamental level, their potential to replace Americium-Beryllium (Am-Be) sources. Accelerator-based sources considered include a dense plasma focus (DPF) alpha-particle accelerator, Deuterium-Tritium (D-T), Deuterium-Deuterium (D-D), and Deuterium-Lithium (D-Li7) neutron generators. The DPF alpha-particle accelerator utilizes the (α-Be) reaction generating a neutron spectrum that is nearly identical to that from an Am-Be source. D-T and D-D neutron generators utilize compact linear accelerators and emit, respectively, 14.1 and 2.45 MeV neutrons. The D-Li7 neutron spectrum resembles the Am-Be spectrum at lower energies, and has a neutron peak at 13.3 MeV.
In the present work, simple spherical geometry models that do not include tool and borehole are first used to explore the basic physics. A tool-borehole-formation configuration is then utilized to briefly 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 low porosity and presence of thermal absorbers, and operational conditions, such as tool standoff are examined. The state of neutron generator technology is also discussed in terms of neutron yield, target properties, power demands, etc., which would be important considerations in actually utilizing generators in nuclear logging tools.
For over fifty years, down-hole devices using radioisotopes Cs-137 and Am-241, have been utilized, together with electrical resistivity/induction, to map the subsurface in open holes.1 [Ellis, 1987] The 662-keV gamma rays produced by Cs-137 are utilized in 2–3 detector tools to determine the formation bulk density which then provides the most accurate measure of porosity. In an Am-Be source, alpha particles (4He) emitted by Am-241 impinge on beryllium to produce a broad spectrum of source neutrons which can then be utilized to compute the neutron porosity. The neutron porosity, often in conjunction with the density, is used to determine lithology and locate gas. Recently, Am-Be (n-gamma) capture spectroscopy tools were developed to determine mineralogical information. [Herron and Herron 1996; Galford et al 2009] In addition, acoustic devices to measure porosity are often included in the suite of logging measurements. In special cases, nuclear magnetic resonance (NMR)-based techniques are also used to compute the porosity. [Ellis and Singer 2007] It should be noted that radionuclide tools are used for data acquisition both in the wireline mode where tools are inserted in the well-bore post-drilling and during logging-while-drilling (LWD).
