Neutron-gamma density through normalized inelastic ratio

Patent metadata

Field	Value
Publication number	US-9671519-B2
Application number	US-201113884863-A
Country	US
Kind code	B2
Filing date	Nov 9, 2011
Priority date	Nov 11, 2010
Publication date	Jun 6, 2017
Grant date	Jun 6, 2017

Abstract

Systems, methods, and devices for determining neutron-gamma density (NGD) measurement of a subterranean formation that is accurate in both liquid- and gas-filled formations are provided. For example, a downhole tool for obtaining such an NGD measurement may include a neutron generator, neutron detector, two gamma-ray detectors, and data processing circuitry. Neutron generator may emit neutrons into a formation, causing a fast neutron cloud to form. Neutron detector may detect a count of neutrons representing the extent of the neutron cloud. Gamma-ray detectors may detect counts of inelastic gamma-rays caused by neutrons that inelastically scatter off the formation. Since the extent of the fast neutron cloud may vary depending on whether the formation is liquid- or gas-filled, data processing circuitry may determine the density of the formation based at least in part on the counts of inelastic gamma-rays normalized to the count of neutrons.

First claim

What is claimed is:

1. A method comprising:
lowering a downhole tool into a subterranean formation through a borehole;
emitting a burst of neutrons of at least 2 MeV into the subterranean formation using a neutron generator in the downhole tool during a burst gate;
detecting, during the burst gate, two first counts of gamma-rays that include inelastic gamma-rays formed when the emitted neutrons inelastically scatter off the subterranean formation using two gamma-ray detectors spaced two respective distances from the neutron generator in the downhole tool;
detecting, during the burst gate, a first count of neutrons that return to the downhole tool after interacting with the subterranean formation using a neutron detector in the downhole tool, said first count of neutrons substantially free of neutrons from the neutron generator passing internally through the downhole tool;
determining a density measurement of the subterranean formation based at least in part on a ratio of the two first counts of gamma-rays normalized to the first count of neutrons using data processing circuitry associated with the downhole tool; and
wherein the density measurement is determined based at least in part on the following relationship:
IRAT−c*log( 3 He−b),

where IRAT represents the ratio of the two first counts of gamma-rays, c and b represent normalization constants, and 3 He represents the first count of neutrons.

2. The method of claim 1 , wherein the burst gate is a period of time sufficient to allow the inelastic gamma-rays to be created through inelastic scattering off the subterranean formation but not to allow thermal neutrons to outnumber epithermal neutrons detected by the neutron detector during the burst gate.

3. The method of claim 1 , wherein the burst gate is a period of time between approximately 10 μs and 20 μs.

4. The method of claim 1 , wherein the density measurement is determined using the data processing circuitry, wherein the data processing circuitry is disposed within the downhole tool or at a remote location, or partially within the downhole tool and partially at the remote location.

5. The method of claim 1 , comprising detecting, after the burst gate, two second counts of gamma-rays that include a background of neutron capture gamma-rays and subtracting the two second counts of gamma-rays from the two first counts of gamma-rays to remove the background from the two first counts of gamma-rays.

6. A system comprising:
a downhole tool configured to emit a burst of neutrons into a subterranean formation during a burst gate, detect two counts of inelastic gamma-rays resulting from the burst of neutrons during the burst gate, and detect a count of epithermal neutrons that return to the downhole tool during the burst gate, said count of epithermal neutrons substantially free of neutrons from the neutron generator passing internally through the downhole tool;
data processing circuitry associated with the downhole tool, wherein the data processing circuitry is configured to determine a density of the subterranean formation based at least in part on a ratio of the two counts of inelastic gamma-rays normalized to the count of epithermal neutrons, wherein the ratio of the two counts of inelastic gamma-rays is normalized to the count of epithermal neutrons such that the density of the subterranean formation determined by the data processing circuitry is independent of whether the subterranean formation is liquid-filled or gas-filled; and
wherein the ratio of the two counts of inelastic gamma-rays is normalized to the count of epithermal neutrons according to the following relationship:
IRAT−c *log( 3 He−b)

where IRAT represents the ratio of the two counts of inelastic gamma-rays, 3 He represents the count of epithermal neutrons, and c and b represent constants selected to cause the normalized ratio of inelastic gamma-rays to vary linearly with density regardless of whether the subterranean formation is liquid-filled or gas-filled.

7. The system of claim 6 , wherein the ratio of the two counts of inelastic gamma-rays is normalized to the count of epithermal neutrons such that the normalized ratio of inelastic gamma-rays varies linearly with density regardless of whether the subterranean formation is liquid-filled or gas-filled.

Assignees

• Tkabladze Avtandil
• Evans Michael
• Stephenson Kenneth
• Schlumberger Technology Corp

Inventors

• Tkabladze Avtandil
• Evans Michael
• Stephenson Kenneth

Classifications

• G01V5/107 Primary

  and detecting reflected or back-scattered neutrons · CPC title

• G01V5/104 Primary

  and detecting secondary Y-rays as well as reflected or back-scattered neutrons · CPC title

• G01V5/108 Primary

  the neutron source being of the pulsed type · CPC title

• G01V5/105

  the neutron source being of the pulsed type · CPC title