US 4,302,669 AGranted

Neutron thermalization time logging

AssigneeMobil Oil Corporation

InventorsLinus S. Allen, William R. Mills, Jr.

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Abstract

Method and apparatus for pulsed neutron logging employing detectors of different energy-dependent sensitivities to neutrons at energy levels less than the chemical binding energy of hydrogen in hydrogenous fluids found in subterranean formations. Fast neutrons from the pulsed neutron source enter the subterranean formation under investigation and are moderated therein to form populations of slow neutrons during a thermalization period occurring subsequent to the fast neutron burst and a thermal equilibrium diffusion period occurring subsequent to the thermalization period. The detectors are employed to obtain count rate measurements during at least one and preferably two time windows falling within the thermalization period and during a later time window occurring within the thermal equilibrium diffusion period. The count rate measurements may be employed in order to arrive at a parameter representative of the time constant of the thermalization period in order to characterize the formation with respect to its relative oil-water content.

Claims12

Dependent claims

In the logging of a well penetrating a subterranean formation containing hydrogenous fluid therein, the method comprising:

(a) irradiating said formation with a burst of fast neutrons whereby fast neutrons enter said formation and are moderated therein to form a population of slow neutrons comprised predominantly of epithermal neutrons during a thermalization period occurring subsequent to said fast neutron burst and comprised predominantly of thermal neutrons during a thermal equilibrium diffusion period occurring subsequent to said thermalization period,
(b) employing a first neutron detector to measure the count rate of slow neutrons within an energy range having a lower limit which is less than the chemical binding energy of hydrogen in said hydrogenous fluid during a first time window occurring within said thermalization period and during at least another time window occurring within said thermal equilibrium diffusion period, and
(c) employing a second neutron detector having a different energy-dependent sensitivity than said first detector to slow neutrons at energy levels less than the chemical binding energy of hydrogen in said hydrogenous fluid to measure the count rate of slow neutrons during said first time window occurring within said thermalization period and said at least another time window occurring within said thermal equilibrium diffusion period.

Dependencies are inferred from the claim text.