Three frequency modulated thermal neutron lifetime log
AssigneeTexaco Inc.
InventorHans J. Paap
Life of this patent
Estimating the term…
Abstract
Methods are disclosed for measuring simultaneously the thermal neutron lifetime of the borehole fluid and earth formations in the vicinity of a well borehole. A harmonically intensity modulated source of fast neutrons is used to irradiate the earth formations with fast neutrons at three different intensity modulation frequencies. Intensity modulated clouds of thermal neutrons at each of the three modulation frequencies are detected by a single spaced detector and the relative phase shift of the thermal neutrons with respect to the fast neutrons is determined at each of the three modulation frequencies. These measurements are then combined to determine simultaneously the thermal neutron decay time of the borehole fluid and the surrounding earth formation media.
Claims20
A method for determining the thermal neutron decay time of earth formations in the vicinity of a well borehole comprising the steps of:
- continuously irradiating the earth formations in the vicinity of a well borehole with a phase coherent harmonically intensity modulated cloud of fast neutrons having at least three different frequency modulation components;
- detecting as a function of time the intensity modulated thermal neutron population resulting from said irradiation at said at least three different modulation frequencies;
- determining from said detected thermal neutron populations measured as a function of time, the relative phase shift between said fast neutron cloud and said thermal neutron population at said at least three different modulation frequencies;
- and
- combining said three different modulation frequency relative phase shift measurements according to a predetermined relationship to derive an indication of the thermal neutron decay time of the earth formations in the vicinity of a well borehole.
A method for logging earth formations traversed by a fluid filled well borehole to develop a measurement of thermal neutron interaction characteristics of such formations comprising the steps of:
- moving a well logging sonde sized and adapted for passage through a well borehole past earth formations penetrated by a well borehole;
- continuously generating fast neutrons in said sonde and bombarding the surrounding media with such neutrons to develop a thermal population in the borehole and surrounding media;
- harmonically modulating the intensity of said fast neutrons at, at least three different modulation frequencies;
- detecting the thermal neutron population in the vicinity of said sonde as a function of time and developing signals representative thereof;
- and
- combining said representative signals according to predetermined relationships to derive an indication of the thermal neutron decay time of the earth formations in the vicinity of the borehole.
Dependencies are inferred from the claim text.