Large depth-of-investigation pulsed neutron measurements and enhanced reservoir saturation evaluation

1. A method, comprising:

emitting, from a source of ultrafast neutrons within a logging tool deployed in a borehole, a pulse of ultrafast neutrons into an irradiated portion of a formation surrounding the borehole, wherein the irradiated portion of the formation is a cylindrical volume with a greater external radius than a radius of a cylindrical volume irradiated by a neutron with a lower energy than the ultrafast neutrons;

detecting, with one or more gamma ray detectors located at increasing distances from the source of ultrafast neutrons, a flux of stimulated gamma rays generated within the irradiated portion of the formation by the pulse of ultrafast neutrons;

determining, from the detected flux of stimulated gamma rays, one or more petrophysical properties of the irradiated portion of the formation.

2. The method of claim 1 ,

wherein the ultrafast neutrons have an energy greater than 20 MeV.

3. The method of claim 1 , wherein the emitting of the pulse of ultrafast neutrons comprises:

accelerating, with a linear accelerator, a charged ion to a high energy; and

causing the charged ion to impact a target material, wherein the target material is a material that emits ultrafast neutrons when impacted by charged ions.

4. The method of claim 3 ,

wherein the charged ion is selected from the group consisting of a hydrogen ion and a deuterium ion.

5. The method of claim 3 ,

wherein the target material is a lithium isotope.

6. The method of claim 1 , wherein the detecting of the flux of stimulated gamma rays comprises:

detecting the flux of stimulated gamma rays arriving at the plurality of gamma ray detectors during a short time window after a start of the emitting of the pulse of ultrafast neutrons;

detecting the flux of stimulated gamma rays arriving at the plurality of gamma ray detectors during an intermediate time window after the start of the emitting of the pulse of ultrafast neutrons; and

detecting the flux of stimulated gamma rays arriving at the plurality of gamma ray detectors during a long time window after the start of the emitting of the pulse of ultrafast neutrons.

7. The method of claim 1 , wherein the determining of the one or more petrophysical properties of the irradiated portion of the formation, further comprises:

interpreting the flux of stimulated gamma rays arriving during a short time window after the emitting of the pulse of ultrafast neutrons as originating with an inelastic scattering of an ultrafast neutron by an atomic nuclei within the irradiated portion of the formation;

interpreting the flux of stimulated gamma rays arriving during an intermediate time window after the emitting of the pulse of ultrafast neutrons as originating with an elastic scattering of an ultrafast neutron by an atomic nuclei within the irradiated portion of the formation; and

interpreting the flux of stimulated gamma rays arriving during a long time window after the emitting of the pulse of ultrafast neutrons as originating with thermal capture of an ultrafast neutron by an atomic nuclei within the irradiated portion of the formation.

8. The method of claim 1 ,

wherein the external radius of the portion of the formation irradiated by the ultrafast neutrons is 30 percent or more larger than the radius of the portion of the formation irradiated by neutrons with a lower energy than the ultrafast neutrons.

9. The method of claim 1 ,

wherein the one or more petrophysical properties are selected from the group consisting of formation water saturation, and mineralogical composition.

10. A logging tool system, comprising:

a computer processor;

a wireline connected to the computer processor;

a logging tool housing;

a source of ultrafast neutrons mounted in the logging tool housing that emits a pulse of ultrafast neutrons into an irradiated portion of a formation surrounding the borehole, wherein the irradiated portion of the formation is a cylindrical volume with a greater external radius than a radius of a cylindrical volume irradiated by a neutron with a lower energy than the ultrafast neutrons;

one or more gamma ray detectors mounted in the logging tool housing at one or more distances from the source of ultrafast neutrons that detect and determine a characteristic of a flux of stimulated gamma rays stimulated in a formation by the pulse of ultrafast neutrons; and

a non-transitory computer memory that receives and stores a characteristic of the flux of stimulated gamma rays detected by the plurality of gamma ray detector.

11. The logging tool system of claim 10 , wherein the source of ultrafast neutrons comprises:

a linear accelerator to accelerate a charged ion to a high energy; and

a target material that emits ultrafast neutrons when struck by the high energy charged ion.

12. The logging tool system of claim 10 ,

wherein the ultrafast neutrons have an energy greater than 20 MeV.

13. The logging tool system of claim 10 ,

wherein the charged ion is selected from the group consisting of a hydrogen ion and a deuterium ion.

14. The logging tool system of claim 10 ,

wherein the target material is a lithium isotope.

15. The logging tool system of claim 10 ,

wherein the linear accelerator comprises a dielectric wall linear accelerator.

16. The logging tool system of claim 10 ,

wherein the one or more gamma ray detectors mounted in the logging tool housing at a one or more distances from the source of ultrafast neutrons are mounted at greater distances than one or more gamma ray detectors mounted in the logging tool housing at a one or more distances from the source of neutrons slower than ultrafast neutrons.

17. The logging tool system of claim 10 ,

wherein the external radius of the portion of the formation irradiated by the ultrafast neutrons is 30 percent or more larger than the radius of the portion of the formation irradiated by neutrons with a lower energy than the ultrafast neutrons.

18. A logging tool system, comprising:

a logging tool housing connected to the bottom-hole assembly of a drill-string;

a computer processor mounted in the logging tool housing;

a source of ultrafast neutrons mounted in the logging tool housing that emits a pulse of ultrafast neutrons into an irradiated portion of a formation surrounding the borehole, wherein the irradiated portion of the formation is a cylindrical volume with a greater external radius than a radius of a cylindrical volume irradiated by a neutron with a lower energy than the ultrafast neutrons;

one or more gamma ray detectors mounted in the logging tool housing at one or more distances from the source of ultrafast neutrons that detect and determine a characteristic of a flux of stimulated gamma rays stimulated in a formation by the pulse ultrafast neutrons; and

a non-transitory computer memory that receives and stores the characteristic of the flux of stimulated gamma rays detected by the gamma ray detector.

19. The logging tool system of claim 18 , wherein the ultrafast neutron source comprises:

a linear accelerator to accelerate a charged ion to a high energy; and

a target material that emits ultrafast neutrons when struck by the high energy charged ion.

20. The logging tool system of claim 19 ,

wherein the charged ion is selected from the group consisting of a hydrogen ion and a deuterium ion.

21. The logging tool system of claim 19 ,

wherein the target material is a lithium isotope.

22. The logging tool system of claim 19 :

wherein the linear accelerator comprises a dielectric wall linear accelerator.

23. The logging tool system of claim 18 ,

wherein the external radius of the portion of the formation irradiated by the ultrafast neutrons is 30 percent or more larger than the radius of the portion of the formation irradiated by neutrons with a lower energy than the ultrafast neutrons.