Epithermal neutron decay logging
AssigneeMobil Oil Corporation
InventorWilliam R. Mills, Jr.
Life of this patent
Estimating the term…
Abstract
Method and apparatus for epithermal neutron decay logging wherein the formation under investigation is irradiated with bursts of fast neutrons which are moderated therein to form a population of epithermal neutrons. The decay rate of epithermal neutrons within the formation is measured within an energy range having lower limit which is less than the chemical binding energy of bound hydrogen in the formation. In addition, the decay rate of epithermal neutrons within the formation is measured within a second energy range having a lower limit which is greater than the lower limit of the first energy range.
Claims12
In the logging of a well penetrating a subterranean formation containing free hydrogen which is mobile in said formation and bound hydrogen which is immobile in said formation, the method comprising:
- (a) irradiating said formation with a burst of fast neutrons whereby said fast neutrons enter said formation and are moderated therein to form a population of epithermal neutrons,
- (b) measuring the decay rate of epithermal neutrons in said formation within an energy range having a lower limit which is less than the chemical binding energy of bound hydrogen in said formation, and
- (c) recording the decay rate measurement obtained in step (b) in correlation with depth.
In the logging of a well penetrating a subterranean formation containing free hydrogen which is mobile in said formation and bound hydrogen which is immobile in said formation, the method comprising:
- (a) irradiating said formation with a burst of fast neutrons whereby said fast neutrons enter said formation and are moderated therein to form a population of epithermal neutrons,
- (b) detecting epithermal neutrons within an energy range having a lower limit which is less than the chemical binding energy of bound hydrogen in said formation,
- (c) selectively measuring the count rate of said detected epithermal neutrons over each of a plurality of time windows occurring subsequent to said fast neutron burst, and
- (d) recording a function representative of the measurements obtained during step (c) in correlation with depth.
In a well logging system, the combination comprising:
- (a) a logging tool adapted for insertion into a wellbore,
- (b) a neutron source in said tool for emitting repetitive time-spaced bursts of fast neutrons,
- (c) detector means in said tool for detecting epithermal neutrons within an energy range having a lower limit within the range of 0.1 to 1 electron volt and producing an output signal in response to said detected epithermal neutrons, and
- (d) means for measuring the rate of decline of said output signal over a time interval between said fast neutron bursts.
Dependencies are inferred from the claim text.