Neutron porosity downhole tool with improved precision and reduced lithology effects

Patent metadata

Field	Value
Publication number	US-9372277-B2
Application number	US-76481410-A
Country	US
Kind code	B2
Filing date	Apr 21, 2010
Priority date	Apr 21, 2010
Publication date	Jun 21, 2016
Grant date	Jun 21, 2016

Abstract

Systems and methods for neutron porosity well logging with high precision and reduced lithology effects are provided. In accordance with an embodiment, a downhole neutron porosity tool may include a neutron source, a neutron monitor, a neutron detector, and data processing circuitry. The neutron source may emit neutrons into a subterranean formation while the neutron monitor detects a count of neutrons proportional to the neutrons emitted. The neutron detector may detect a count of neutrons that scatters off the subterranean formation. The data processing circuitry may determine an environmentally corrected porosity of the subterranean formation based at least in part on the count rate of neutrons scattered off the subterranean formation normalized to the count rate of neutrons proportional to the neutrons emitted by the neutron source.

First claim

What is claimed is:

1. A downhole neutron porosity tool comprising:
a neutron source configured to emit neutrons into a subterranean formation;
a neutron monitor configured to detect a count rate of neutrons proportional to the neutrons emitted by the neutron source;
a neutron detector configured to detect a count rate of neutrons scattered off the subterranean formation;
data processing circuitry configured to determine an environmentally corrected porosity configured to be output to an operator as representative of a true porosity of the subterranean formation using neutron measurements consisting of the count rate of neutrons scattered off the subterranean formation normalized to the count rate of neutrons proportional to the neutrons emitted by the neutron source to thereby determine the environmentally corrected porosity without using a ratio of count rates from any other neutron detector ; and
an output device configured to output an indication of the environmentally corrected porosity as representative of the true porosity of the subterranean formation.

2. The downhole tool of claim 1 , wherein the neutron source comprises a pulsed neutron source.

3. The downhole tool of claim 1 , wherein the neutron source comprises an electronic neutron generator configured to emit neutrons of approximately 14 MeV.

4. The downhole tool of claim 1 , wherein the neutron monitor is configured to detect fast neutrons.

5. The downhole tool of claim 1 , wherein the neutron monitor is configured to detect higher energy neutrons than the neutron detector.

6. The downhole tool of claim 1 , wherein the neutron monitor is configured to detect neutrons of greater than 1 MeV and the neutron detector is configured substantially not to detect neutrons of greater than 1 MeV.

7. The downhole tool of claim 1 , wherein the neutron monitor comprises a plastic scintillator.

8. The downhole tool of claim 1 , wherein the neutron monitor comprises a solid state neutron detector.

9. The downhole tool of claim 1 , wherein the neutron monitor comprises a He-4 gas counter, a Hydrogen proportional counter, or a liquid scintillator, or any combination thereof.

10. A method comprising:
emitting neutrons into a subterranean formation using a neutron source;
detecting a count rate of neutrons proportional to the neutrons that are emitted using a neutron monitor;
detecting a count rate of neutrons scattered off the subterranean formation using a neutron detector;
normalizing the count rate of neutrons scattered off the subterranean formation to the count rate of neutrons proportional to the neutrons that are emitted to obtain a normalized count rate of scattered neutrons using data processing circuitry;
determining an apparent porosity of the subterranean formation using neutron measurements consisting of the normalized count rate of scattered neutrons using the data processing circuitry and therefore without using a ratio of count rates from any two neutron detectors other than the neutron monitor;
applying an environmental correction to the apparent porosity of the subterranean formation to compensate for borehole effects and lithology effects using the data processing circuitry without using a ratio of count rates from any two neutron detectors other than the neutron monitor to obtain a corrected porosity of the subterranean formation, wherein the corrected porosity of the subterranean formation is configured to be output as representative of a true porosity of the subterranean formation; and
outputting the corrected porosity of the subterranean formation as representative of the true porosity of the subterranean formation in a report using a monitor or printing device coupled to the data processing circuitry.

11. The method of claim 10 , wherein the apparent porosity of the subterranean formation is determined from the normalized count of scattered neutrons based at least in part on a transform derived from modeled or experimental data, or a combination thereof, that relates the normalized count of scattered neutrons to the porosity of the subterranean formation under a plurality of possible lithologies to account for lithology effects.

12. The method of claim 10 , comprising detecting a count rate of neutrons that traverse through a borehole using a borehole-facing neutron detector disposed more closely to the neutron source than the neutron detector, wherein the environmental correction applied to the apparent porosity is based at least in part on the count rate of neutrons that traverse through the borehole.

13. The method of claim 10 , comprising determining a borehole apparent porosity based at least in part on the count rate of neutrons that traverse through the borehole using the data processing circuitry, wherein the environmental correction applied to the apparent porosity results in the corrected porosity that is determined based at least in part on the following relationship:

φ
corr

=

∑
ij

i
+
j

≤
n

⁢

a
ij

⁢

φ
near
i

⁢

φ
back
j

,

wherein φ corr represents the corrected porosity, φ near represents the apparent porosity, φ back represents the borehole apparent porosity, n represents the degree of the polynomial, and α ij represents coefficients chosen to minimize a difference between the apparent porosity and a true porosity of the subterranean formation, wherein the borehole apparent porosity is determined using a count rate from a borehole facing neutron detector spaced more closely to the neutron source than the neutron detector.

14. A method comprising:
emitting neutrons into a subterranean formation using a neutron source;
detecting the emitted neutrons using a neutron monitor;
detecting a first count rate of neutrons scattered off the subterranean formation using a first neutron detector;
detecting a second count rate of neutrons scattered off the subterranean formation using a second neutron detector, wherein the second neutron detector is disposed farther from the neutron source than the first neutron detector;
normalizing at least one of the first count and the second count rate to the count rate of emitted neutrons using data processing circuitry;
when the first neutron detector is expected to be less affected by a lithology effect of the subterranean formation than the second neutron detector, determining a porosity of the subterranean formation using neutron measurements consisting of the normalized first count rate, and therefore without using any ratio of count rates of the first neutron detector and the second neutron detector, using the data processing circuitry;
when the second neutron detector is expecte

Assignees

• Wraight Peter
• Thornton James
• Roscoe Bradley A
• Schlumberger Technology Corp

Inventors

• Wraight Peter
• Thornton James
• Roscoe Bradley A

Classifications

• G01V5/107 Primary

  and detecting reflected or back-scattered neutrons · CPC title