Analysis of the spatial-energy distribution of neutrons in the vicinity of formation boundaries: Neutron log examples

We present cases of so-called artifacts observed in the response of neutron detectors in the vicinity of formation boundaries (e.g., in neutron logging for geophysical prospecting). The appearance of artifacts is related mainly to differences in the neutron transport properties of adjacent media: the fast neutron slowing-down length and the absorption cross section of thermal neutrons. The basis for examining the causes of the appearance of artifacts on the profiling curve was a numerical experiment consisting of our measuring the counts of the thermal neutron detector of the neutron probe under given measurement conditions in the well. The detector response was determined with a 1 cm step in the vicinity of the border of two geological formations, significantly different in terms of neutron parameters, which allowed accurate registration of artifacts on the logging curve. The numerical code MCNP6 with the ENDF B-VII.1 data library was used to solve neutron transport problems in complex systems and to explain the presence of artifacts on the basis of the physics of neutron transport and the spatial-energy distribution of neutron fields in the vicinity of formation boundaries.

Automated summary

This study presents cases of observed artifacts in neutron logging for geophysical prospecting, which are mainly related to differences in neutron transport properties of adjacent media. A numerical experiment was conducted to measure detector response near geological formation boundaries, accurately registering these artifacts on the logging curve. The MCNP6 code was used to solve neutron transport problems in complex systems and explain the presence of artifacts based on the physics of neutron transport.