The Neutron Lifetime Log

The Neutron Lifetime Log is a new nuclear log which is based on a measurement of the length of time slow neutrons survive in rocks and fluids before they are captured. The logging instrument employs a cyclically pulsed neutron generator and a gated scintillation counter synchronized with the source. Neutrons are emitted by the source in short bursts at 1000 microsecond intervals. The neutrons are thermalized by formation materials and most of them are captured during the quiescent interval between bursts. Gamma radiation due to thermal neutron capture is detected by a scintillation counter which is electronically gated to respond during two independent preselected intervals. The counting rates observed during these intervals are separately recorded in a continuous logging operation. The ratio of the two curves can also be continuously recorded. Formulas have been derived which give the relationship between log response and formation porosity and fluid saturation. The log is particularly sensitive to formation fluid salinity, so that salt water saturation can be measured accurately under conditions which are unfavorable for other nuclear logs. Many Neutron Lifetime logs have been run during the past year which demonstrate the accuracy and sensitivity of the method for formation evaluation. Example logs and interpretation data are shown.