Method for obtaining near-wellbore true borehole sigma and true formation sigma by using a nuclear logging tool during oil and gas exploration

1. A method for obtaining near-wellbore true borehole sigma and true formation sigma by using a nuclear logging tool during oil and gas exploration having a pulsed neutron source coupled with at least three dual-function radiation detectors, high-voltage suppliers and an electronic instrument having non-transitory computer readable memory device for performing the operations of processing, generating, and computing, the method comprising:

initializing a wireline or logging-while-drilling system within a borehole in an earth formation having a nuclear logging tool for detecting and

processing thermal neutrons and neutron-induced gamma rays comprising, a pulsed neutron source from a neutron generator operated at a neuron pulse of at least 500 Hz and a pulse width of at least 20 microseconds, at least three dual-function radiation detectors wherein each dual-function radiation detector is pre-programmed through a non-transitory computer-readable memory device using pulsed shape discrimination technique;

capturing thermal neutrons and neutron-induced gamma rays, using the at least three dual-function radiation detectors;

processing the captured thermal neutrons and neutron-induced gamma rays, using the pre-programmed non-transitory computer-readable memory device for separating signals of thermal neutrons and neutron-induced gamma rays from each other;

pulsing the neutron source for at least two pulses;

generating two time-decay curves for each of the at least three dual-function radiation detectors by a pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed thermal neutrons and neutron-induced gamma rays immediately after the first pulse from the neutron source and before the second pulse from the neutron source, wherein said time-decay curves being one of thermal neutrons and one of neutron-induced thermal neutron capture gamma rays;

processing the generated time-decay curve of thermal neutrons by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, to obtain a neutron-induced borehole time-decay constant and a neutron-induced formation time-decay constant for each of the at least three dual-function radiation detectors;

generating an apparent borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curve of thermal neutrons for each of the at least three dual-function radiation detectors;

generating an apparent formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curve of thermal neutrons for each of the at least three dual-function radiation detectors;

computing the generated apparent borehole time-decay constant for each of the at least three dual-function radiation detectors with the generated apparent formation time-decay constant for each of the at least three dual-function radiation detectors, using the pre-programmed non-transitory computer-readable memory device on an above-surface processing system;

generating a neutron-induced borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors;

generating a neutron-induced formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors;

processing the generated time-decay curve of neutron-induced thermal neutron capture gamma rays using the pre-programmed non-transitory computer-readable memory device on an above-surface processing system to obtain a gamma-induced borehole time-decay constant and a gamma-induced formation time-decay constant for each of the at least three dual-function radiation detectors;

generating an apparent borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curve of neutron-induced thermal neutron capture gamma rays for each of the at least three dual-function radiation detectors;

generating an apparent formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curve of neutron-induced thermal neutron capture gamma rays for each of the at least three dual-function radiation detectors;

computing the generated apparent borehole time-decay constant for each of the at least three dual-function radiation detectors with the generated apparent formation time-decay constant for each of the at least three dual-function radiation detectors, using the pre-programmed non-transitory computer-readable memory device on an above-surface processing system;

generating a gamma-induced borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors;

generating a gamma-induced formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors;

processing the generated neutron-induced borehole time-decay constant with the gamma-induced borehole time-decay constant for each of the at least three dual-function radiation detectors, using the pre-programmed non-transitory computer-readable memory device on an above-surface processing system;

generating a true borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced borehole time-decay constant with the generated gamma-induced borehole time-decay constant;

processing the generated neutron-induced formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, with the generated gamma-induced formation time-decay constant for each of the at least three dual-function radiation detectors, using the pre-programmed non-transitory computer-readable memory device;

generating a true formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced formation time-decay constant with the generated gamma-induced formation time-decay constant;

computing a true borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated true borehole time-decay constant, using the pre-programmed non-transitory computer-readable memory device;

generating a true borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, from the computed true borehole sigma;

computing a true formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated true formation time-decay constant, using the pre-programmed non-transitory computer-readable memory device; and

generating a true formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, from the computed true formation sigma.

2. The method of claim 1 , wherein the wireline or logging-while-drilling system further comprises of a telemetry system for transmitting information to an above-surface processing system, and a mud channel filled with flowing drilling mud.

3. The method of claim 1 , wherein the by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system is further coupled to a computer system device, a non-transitory computer readable memory device, an above-surface telemetry system, and a memory device.

4. The method of claim 1 , wherein the pulsed neutron source further comprises of at least a deuterium-tritium (D-T) neutron generator or a deuterium-deuterium (D-D) neutron generator.

5. The method of claim 1 , wherein the at least three dual-function radiation detectors are disposed at same or different longitudinal directions inside the wireline or logging-while-drilling system having the pre-programmed non-transitory computer memory readable device to detect and separate characteristics of neutrons and gamma rays.

6. The method of claim 1 , wherein the pre-programmed non-transitory computer-readable memory device on an above-surface further stores a computer program comprising program code instructions which can be loaded in a programmable device to cause said programmable device to implement the instructions according to claim 1 , when said program is executed by an application server of said device, coupled through a communication bus to a memory resource.

7. The method of claim 1 , wherein generating a neutron-induced borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors further comprises the expression:

τ bn =ƒ 1 (τ bnn ,τ fnn ,τ bnm ,τ fnm ,τ bnf ,τ fnf ).

8. The method of claim 1 , wherein

generating a neutron-induced formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors further comprises the expression:

τ fn =ƒ 2 (τ bnn ,τ fnn ,τ bnm ,τ fnm ,τ bnf ,τ fnf ).

9. The method of claim 1 , wherein

generating a gamma-induced borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors further comprises the expression:

τ bg =ƒ 3 (τ bgn ,τ fgn ,τ bgm ,τ fgm ,τ bgf ,τ fgf ).

10. The method of claim 1 , wherein

generating a gamma-induced formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole time-decay constant and the computed apparent formation time-decay constant of each of the at least three dual-function radiation detectors further comprises the expression:

τ fg =ƒ 4 (τ bgn ,τ fgn ,τ bgm ,τ fgm ,τ bgf ,τ fg,f ).

11. The method of claim 1 , wherein

generating a true borehole time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced borehole time-decay constant with the generated gamma-induced borehole time-decay constant further comprises the expression:

τ b =ƒ 5 (τ bn ,τ bg ).

12. The method of claim 1 , wherein

generating a true formation time-decay constant by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced formation time-decay constant with the generated gamma-induced formation time-decay constant further comprises the expression:

τ f =ƒ 6 (τ fn ,τ fg ).

13. The method of claim 1 , wherein

generating a true borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, from the computed true borehole sigma further comprises the expression:

Σ

b

=

4

.

5

⁢

5

×

1

⁢

0

3

τ

b

.

14. The method of claim 1 ,

generating a true formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, from the computed true formation sigma further comprises the expression:

Σ

f

=

4

.

5

⁢

5

×

1

⁢

0

3

τ

f

.

15. The method of claim 1 , wherein

generating a neutron-induced borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors further comprises the expression:

Σ bn =g 1 (Σ bnn ,Σ fnn ,Σ bnm ,Σ fnm ,Σ bnf ,Σ fnf ).

16. The method of claim 1 , wherein

generating a neutron-induced formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors further comprises the expression:

Σ fn =g 2 (Σ bnn ,Σ fnn ,Σ bnm ,Σ fnm ,Σ bnf ,Σ fnf ).

17. The method of claim 1 ,

generating a gamma-induced borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors further comprises the expression:

Σ bg =g 3 (Σ bgn ,Σ fgn ,Σ bgm ,Σ fgm ,Σ bgf ,Σ fgf ).

18. The method of claim 1 , wherein

generating a gamma-induced formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, sigma using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors further comprises the expression:

Σ fg =g 4 (Σ bgn ,Σ fgn ,Σ bgm ,Σ fgm ,Σ bgf ,Σ fg,f ).

19. The method of claim 1 ,

generating a true borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced borehole sigma with the generated gamma-induced borehole sigma further comprises the expression:

Σ b =g 5 (Σ bn ,Σ bg ).

20. The method of claim 1 , wherein

generating a true formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced formation sigma with the generated gamma-induced formation sigma further comprises the expression:

Σ f =g 6 (Σ fn ,Σ fg ).

21. A method for obtaining near-wellbore true borehole sigma and true formation sigma by using a nuclear logging tool during oil and gas exploration having a pulsed neutron source coupled with at least three dual-function radiation detectors, high-voltage suppliers and an electronic instrument having non-transitory computer readable memory device for performing the operations of processing, generating, and computing, the method comprising:

initializing a wireline or logging-while-drilling system within a borehole in an earth formation having a nuclear logging tool for detecting and

processing thermal neutrons and neutron-induced gamma rays comprising, a pulsed neutron source from a neutron generator operated at a neuron pulse of at least 500 Hz and a pulse width of at least 20 microseconds, at least three dual-function radiation detectors wherein each dual-function radiation detector is pre-programmed through a non-transitory computer-readable memory device using pulsed shape discrimination technique;

capturing thermal neutrons and neutron-induced gamma rays, using the at least three dual-function radiation detectors;

processing the captured thermal neutrons and neutron-induced gamma rays, using the pre-programmed non-transitory computer-readable memory device for separating signals of thermal neutrons and neutron-induced gamma rays from each other;

pulsing the neutron source for at least two pulses;

generating two time-decay curves for each of the at least three dual-function radiation detectors by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed thermal neutrons and neutron-induced gamma rays immediately after the first pulse from the neutron source and before the second pulse from the neutron source, wherein said time-decay curves being one of thermal neutrons and one of neutron-induced thermal neutron capture gamma rays;

processing the generated time-decay curve of thermal neutrons by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, to obtain a neutron-induced borehole time-decay constant and a neutron-induced formation time-decay constant for each of the at least three dual-function radiation detectors;

generating an apparent borehole sigma, by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curves of thermal neutrons for each of the at least three dual-function radiation detectors;

generating an apparent formation sigma, by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curves of thermal neutrons rays for each of the at least three dual-function radiation detectors;

computing the generated apparent borehole sigma for each of the at least three dual-function radiation detectors with the generated apparent formation sigma for each of the at least three dual-function radiation detectors, using by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system;

generating a neutron-induced borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors;

generating a neutron-induced formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors;

processing the generated time-decay curve of neutron-induced thermal neutron capture gamma rays using the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, to obtain a gamma-induced borehole sigma and a gamma-induced formation sigma for each of the at least three dual-function radiation detectors;

generating an apparent borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curve of neutron-induced thermal neutron capture gamma rays for each of the at least three dual-function radiation detectors;

generating an apparent formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the processed time-decay curve of neutron-induced thermal neutron capture gamma rays for each of the at least three dual-function radiation detectors;

computing the generated apparent borehole sigma for each of the at least three dual-function radiation detectors with the generated apparent formation sigma for each of the at least three dual-function radiation detectors, using by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system;

generating a gamma-induced borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors;

generating a gamma-induced formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, sigma using the computed apparent borehole sigma and the computed apparent formation sigma of each of the at least three dual-function radiation detectors;

processing the generated neutron-induced borehole sigma with the gamma-induced borehole sigma for each of the at least three dual-function radiation detectors, using the pre-programmed non-transitory computer-readable memory device on an above-surface processing system;

generating a true borehole sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced borehole sigma with the generated gamma-induced borehole sigma;

processing the generated neutron-induced formation sigma with the gamma-induced formation sigma for each of the at least three dual-function radiation detectors, using the pre-programmed non-transitory computer-readable memory device; and

generating a true formation sigma by the pre-programmed non-transitory computer-readable memory device on an above-surface processing system, using the generated neutron-induced formation sigma with the generated gamma-induced formation sigma.