NLL. The Neutron Lifetime logging technique employs a pulsed neutron source which is periodically actuated to produce short bursts of neutrons and is quiescent between bursts. During the interval between bursts, the neutrons (as well as the various types of radiation which always result from neutron interactions) die away. Their average lifetime can be measured by measuring the length of time required for the neutron population at a particular instant to die away to half value. 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.
