Neutron logging tool

Abstract

A well logging instrument for use in a cased well bore and containing a sealed source of fast neutrons and two identical thermal neutron detectors with a volume of four atmospheres of helium-3 gas. The formation surrounding the cased well bore is bombarded with high energy neutrons and the two thermal neutron detectors are spaced apart from one another and from the source to receive slowed down or thermal neutrons from the surrounding formations. The epithermal or fast neutrons striking the formation are slowed down by fluids containing great quantities of hydrogen or chlorine atoms, creating thermal or slow neutrons, which the detectors respond to logrithmically, but independently. The counting rate of the detectors are processed independently to count rate meters and to a recorder to present two outputs of information. The presence or absence of hydrogen and chlorine atoms in the formations as detected by each detector determines the amount of sweep or deflection of each recorder pen. By calibrating each output of the detectors to the other, in known formations, the long spaced curve will deflect to the right of the short spaced curve in water bearing zones and deflect to the left of the short spaced curve in oil bearing zones. In dense zones of very low porosity or non-fluid bearing or impermeable formations, the curves will track each other as the ratio of the two counting rates are equal to the ratio of the sensitivities in this situation or condition.