Epithermal neutron die-away logging

• US 4,590,370 A
• Filed: 07/19/1984
• Issued: 05/20/1986
• Est. Priority Date: 04/22/1983
• Status: Expired due to Term

• Current Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Original Assignee: Mobil Oil Corporation (Exxon Mobil Corporation)
• Inventors: Mills, William R. Jr., Allen, Linus S.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Hannaher, Constantine
• Application Number:
  US06/632,354
• Time in Patent Office: 670 Days
• Field of Search: 250/261, 250/267, 250/269, 250/390 R
• US Class Current: 250/267
• CPC Class Codes: G01V 5/105 the neutron source being of the pulsed type
• CPC Class Codes: G01V 5/108 the neutron source being of the pulsed type

Abstract

A borehole logging tool includes a pulsed source of fast neutrons and a neutron sensitive counter. A neutron filter encases the counter and has a thickness to permit the longest possible delay time following each burst of fast neutrons before epithermal neutrons from the irradiated formation are detected by the neutron counter while at the same time minimizing thermal neutron effects.

Claims

1. A system for epithermal neutron die-away logging of subterranean formations surrounding a borehole comprising:

(a) a source of fast neutrons, for irradiating the formations surrounding a borehole,

(b) a neutron counter for counting epithermal neutrons returning to the borehole from said irradiated formations,

(c) a neutron filter surrounding said neutron counter having a thickness so as to permit the longest possible delay time following each burst of fast neutrons from said source before said epithermal neutrons from the irradiated formations are detected by said neutron counter while at the same time optimizing the epithermal neutron counting rates by minimizing thermal neutron effects.

2. The epithermal neutron die-away logging system of claim 1 wherein the thickness for said neutron filter is such that the following are satisfied in an optimum sense:

space="preserve" listing-type="equation">[τ

(t)-τ

.sub.epi (t)]/τ

.sub.epi (t)≦

η

, and

space="preserve" listing-type="equation">Counting rate (t)/Counting rate (0)≧

ρ

where;

τ

(t)=observed neutron lifetime at time delay t,

τ

_epi (t)=epithermal neutron lifetime at delay time t,

Counting rate (t)=counting rate at time delay t,

Counting rate (0)=counting rate at zero time delay,

η

=a dimensionless parameter chosen to permit a given admixture of thermal neutron response, and

ρ

=a dimensionless parameter chosen to permit a given relative statistical precision.

3. The epithermal neutron die-away logging system of claim 2 wherein optimization of the thickness of said neutron filter is satisfied by the following:

space="preserve" listing-type="equation">Maximize t.sub.min .tbd.Sm (t.sub.η

, t.sub.ρ

)

where;

t_min is defined to be equal to the smaller of times t.sub.η

and t.sub.ρ

, and

where;

t.sub.η

=delay time for which (τ

-τ

_epi)/τ

_epi =η

, and

t.sub.ρ

=delay time for which Counting rate (t)/Counting rate (0)=ρ

.

4. The method of claim 3 where η

is in the range of 0.02 to 0.10 and ρ

is in the range of 1/20 to 1/4.

5. The method of claim 3 wherein said neutron filter comprises gadolinium with a thickness of from 0.003 inch (0.0762 mm) to 0.012 inch (0.3048 mm).

6. The method of claim 3 wherein said neutron filter comprises cadmium with a thickness of from 0.003 inch (0.0762 mm) to 0.100 inch (2.5400 mm).

7. The method of claim 3 wherein said neutron filter comprises a combination of gadolinium with a thickness of from 0.003 inch (0.0762 mm) to 0.012 inch (0.3048 mm) and cadmium with a thickness of from 0.003 inch (0.0762 mm) to 0.100 inch (2.5400 mm).

8. An epithermal neutron detector, comprising:

(a) neutron-sensitive counter, and

(b) a neutron filter encasing said counter having an optimum thickness which provides for a counting rate delay time corresponding to a selected lower limit for statistical precision of epithermal die-away measurements, said delay time being equal to a counting rate delay time corresponding to a selected upper limit for thermal neutron effects.

9. The epithermal neutron detector of claim 8 wherein said neutron filter comprises gadolinium.

10. The epithermal neutron detector of claim 9 wherein the thickness of said gadolinium neutron filter is at least 0.003 inch (0.0762 mm).

11. The epithermal neutron detector of claim 9 wherein the thickness of said gadolinium neutron filter is no greater than 0.012 inch (0.3048 mm).

12. The epithermal neutron detector of claim 8 wherein said neutron filter comprises cadmium.

13. The epithermal neutron detector of claim 12 wherein the thickness of said cadmium neutron filter is at least 0.003 inch (0.0762 mm).

14. The epithermal neutron detector of claim 12 wherein the thickness of said cadmium neutron filter is no greater than 0.100 inch (2.5400 mm).

15. The epithermal neutron detector of claim 8 wherein said neutron filter comprises both gadolinium and cadmium.

16. The epithermal neutron detector of claim 15 wherein the thickness of said gadolinium is from 0.003 inch (0.0762 mm) to 0.012 inch (0.3048 mm) and the thickness of said cadmium is from 0.003 inch (0.0762 mm) to 0.100 inch (2.5400 mm).