Using a small neutron tube, a new method for oil-well logging has been developed on a laboratory scale. Because of a pulsed fast neutron (14 MeV., pulse width of about 100 μs., repetition rate 1KHz.) which is injected into a formation, nuclear reaction takes place in the formation and two kinds of prompt gamma rays are emitted: one is the gamma ray resulting from inelastic scattering of neutrons and the other is from capture of neutrons. They are routed towards a gate which is synchronized with a neutron burst.
1) Both kinds of gamma rays are led to a pulse hights analyzer. The discrimination between oil and water-bearing sands is obtained by detecting the difference of counting rates on a gamma ray energy spectrum corresponding to carbon, oxygen and chlorine respectively.
2) Neutron-capture gamma rays are sent to a time analyzer simultaneously and the decay time constants of the thermal neutron in a formation are measured. This serves to reveal the kind of formation fluid and also the degree of porosity.
The first is effective for indicating oil sands when the invasion of a mudfiltrate is shallow. The second seems to give the true decay time constant of thermal neutrons in a formation.
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