Benchmarking computer simulations of neutron-induced, gamma-ray spectroscopy for well logging

The spectroscopy of gamma rays created by thermal neutron capture and inelastic scattering of fast neutrons is commonly used in the study of wellbore geophysics. In this type of well logging, a sonde, consisting of a pulsed source of fast neutrons and gamma ray detectors, is pulled through the formations traversed by the borehole. The many geometries, lithologies, reservoir fluids, etc. require an extensive database to map the measured gamma-ray spectra into formation and borehole composition. This paper describes the design of computer models to simulate the logging sonde response. Once qualified, these models can be used to efficiently and accurately generate the necessary database. The Monte Carlo code MCNP was used for neutron and gamma-ray reactions and transport, and the code GAMRES was used to transform the incident flux spectra to detector response. The simulations are benchmmarked by comparison to the response of the Computalog Pulsed Neutron Decay sonde in known model environments. Details of the simulation models, computer efficiency, variance reduction, and available nuclear databases are discussed.