The radiation intensity is measured in each of two preselected intervals and, by intercomparing these measurements, determine thc rate of neutron die-away. This measured rate has been shown both by theory and experiment to be a measure of the thermal-neutron capture cross section of the medium in which the neutrons are captured. The thermal-neutron capture cross section per unit of volume of formation material is referred to as S . It is related to L, termed the lifetime of neutrons in a material, by the equation Thermal neutrons are captured mainly by the chlorine present. Hence the tool responds to the amount of salt in formation waters .
Hydrocarbons result in longer lifetimes than salt water.
Tool measurements are porosity -dependent and sensitive to clay content. Can be used in cased holes where resistivity logs cannot be run or to monitor reservoir changes to opt-imize production. Resembles a resistivity log with which it is generally correlatable. Neutron Lifetime Log is a Dresser Atlas registered trademark. A log of a response primarily related to hydrogen concentration but also affected by mineralogy and borehole effects.