Neutron generator tool design to replace Am-Be neutron sources in oil and gas applications

There are several concerns with the current widespread use of high-activity 241Am-Be (a,n) neutron sources in the oil and gas well-logging field. These include societal concerns of accidental or intentional diversion that can result in ransom or actual use of them in radiological dispersion devices but there is also growing concern within the industry due to the liability exposure and rising purchase and logistical costs. Lower-activity 252Cf spontaneous fission neutron sources or D-T neutron generators that are already in use have been proposed as Am-Be replacements and do not pose nearly as much of a risk as Am-Be, but have challenges of their own. As non-nuclear replacement options cannot provide the necessary measurements, alternative neutron sources are needed and development of neutron sources without use of radioactive materials is ongoing. With neutron-based logging tools, the fundamental parameter that needs to be addressed is energy spectrum of the neutron source, as it dictates the tool response and sensitivity. The use of high D-D neutron generators is a good alternative to Am-Be for several applications and can even relax source rate requirements because of more favorable detection geometry. D-D however may not be as suitable for spectroscopic tools for minerology because the higher energy portions of the Am-Be spectrum produce additional gamma rays from inelastic neutron scattering provide additional key information that D-D cannot. A neutron generator based on the D-7Li reaction produces neutrons with an energy spectrum similar to Am-Be, but with an additional highenergy tail similar to D-T and can generate the inelastic gammas needed. Recent measurements confirmed the viability of using this reaction, however, acceleration voltages need to be higher than for typical borehole tooling. This paper describes the scaling of these measurements with voltage and practical implications including source-detector spacing changes that may benefit use of lower-energy D-D units over D-T units for neutron porosity tooling and other system requirements.