Borehole compensation during pulsed-neutron porosity logging

Patent metadata

Field	Value
Publication number	US-11048015-B2
Application number	US-201916584318-A
Country	US
Kind code	B2
Filing date	Sep 26, 2019
Priority date	Apr 10, 2019
Publication date	Jun 29, 2021
Grant date	Jun 29, 2021

Abstract

Methods, tools, and systems for determining porosity in an earth formation are disclosed. Neutrons are emitted into the formation to induce inelastic scattering gamma rays and thermal capture gamma rays in the formation. The induced gamma rays are detected at a proximal gamma detector and a far gamma detector, which are spaced at different axial distances from the neutron source. A measured proximal-to-far inelastic ratio (a ratio of inelastic scattering gammas detected at the proximal and far detector) and a proximal-to-far thermal capture ratio (a ratio of thermal capture gammas detected at the proximal and far detector) are determined and used to calculate the formation porosity. Techniques are disclosed for removing borehole and casing configuration effects from the measured proximal-to-far thermal capture ratio, leaving only porosity dependence.

First claim

What is claimed is:

1. A method of measuring a porosity of an earth formation traversed by a wellbore, the method comprising:
receiving data generated by a logging tool, wherein the logging tool comprises:
a neutron source configured to emit neutrons into the formation at an energy sufficient to induce inelastic scattering gamma rays and thermal capture gamma rays in the formation,
a proximal gamma detector spaced a first axial distance from the neutron source, and
a far gamma detector spaced a second axial distance from the neutron source, and wherein
the data indicates total gamma rays detected at the proximal gamma detector and total gamma rays detected at the far gamma detector,

from the data, determining a count of thermal capture gamma rays detected at the proximal gamma detector and a count of thermal capture gamma rays detected at the far gamma detector,
from the data, determining a count of inelastic scattering gamma rays detected at the proximal gamma detector and a count of inelastic scattering gamma rays detected at the far gamma detector,
determining a proximal-to-far thermal capture ratio as a ratio of the count of thermal capture gamma rays detected at the proximal gamma detector to the count of thermal capture gamma rays detected at the far gamma detector,
determining a proximal-to-far inelastic ratio as a ratio of the count of inelastic scattering gamma rays detected at the proximal gamma detector to the count of inelastic scattering gamma rays detected at the far gamma detector, and
using the proximal-to-far thermal capture ratio and the proximal-to-far inelastic ratio to determine the porosity.

2. The method of claim 1 , wherein the data indicating the total gamma rays detected at the proximal gamma detector and the total gamma rays detected at the far gamma detector comprises, for each detector, a time spectrum indicating gamma ray counts detected at the detector as a function of time, wherein each time spectrum comprises a burst interval indicating gamma ray counts detected while the neutron source is emitting neutrons and a decay interval indicating gamma ray counts detected while the neutron source is not emitting neutrons.

3. The method of claim 2 , wherein determining the count of thermal capture gamma rays detected at the proximal gamma detector and the count of thermal capture gamma rays detected at the far gamma detector comprises, for each of the proximal gamma detector and the far gamma detector:
determining a count of thermal capture gamma rays detected during the decay interval at that detector, and
determining a count of thermal capture gamma rays detected during the burst interval at that detector.

4. The method of claim 3 , wherein determining a count of thermal capture gamma rays detected during the decay interval comprises integrating the time spectrum over the decay interval.

5. The method of claim 3 , wherein determining a count of thermal capture gamma rays detected during the burst interval comprises:
fitting a decay function to the decay interval of the time spectrum,
determining a borehole component and a formation component of the decay function over the decay interval,
convolving the borehole component and the formation component of the decay function over the burst interval, and
summing the convolved borehole component and the convolved formation component over the burst interval to determine the count of thermal capture gamma rays detected during the burst interval.

6. The method of claim 5 , wherein the decay function is a dual exponential function.

7. The method of claim 5 , wherein determining a count of inelastic scattering gamma rays detected at the proximal gamma detector and a count of inelastic scattering gamma rays detected at the far gamma detector comprises, for each detector, subtracting the count of thermal capture gamma rays detected during the burst interval from the total gamma rays detected during the burst interval.

8. The method of claim 1 , wherein using the proximal-to-far thermal capture ratio and the proximal-to-far inelastic ratio to determine the porosity comprises applying a correction function to the proximal-to-far thermal capture ratio to determine a corrected proximal-to-far capture ratio that is independent of borehole and casing configuration effects, wherein the correction function is a function of the proximal-to-far thermal capture ratio and the proximal-to-far inelastic ratio.

9. The method of claim 8 , wherein the correction function is determined based on a plurality of calibration proximal-to-far ratios determined by modeling responses of the logging tool response under a plurality of modeled formation conditions.

10. The method of claim 8 , wherein the correction function is determined based on a plurality of calibration proximal-to-far ratios determined by measuring responses of the logging tool to a plurality of calibration formation conditions.

11. The method of claim 1 , further comprising deploying the logging tool in the borehole and acquiring the data.

12. A system for measuring a porosity of an earth formation traversed by a wellbore, the system comprising:
a logging tool comprising:
a neutron source configured to emit neutrons into the formation at an energy sufficient to induce inelastic scattering gamma rays and thermal capture gamma rays in the formation,
a proximal gamma detector spaced a first axial distance from the neutron source, and
a far gamma detector spaced a second axial distance from the neutron source, and

a computer configured to:
receive data generated by the logging tool, wherein the data indicates total gamma rays detected at the proximal gamma detector and total gamma rays detected at the far gamma detector,
from the data, determine a count of thermal capture gamma rays detected at the proximal gamma detector and a count of thermal capture gamma rays detected at the far gamma detector,
from the data, determine a count of inelastic scattering gamma rays detected at the proximal gamma detector and a count of inelastic scattering gamma rays detected at the far gamma detector,
determine a proximal-to-far thermal capture ratio as a ratio of the count of thermal capture gamma rays detected at the proximal gamma detector to the count of thermal capture gamma rays detected at the far gamma detector,
determine a proximal-to-far inelastic ratio as a ratio of the count of inelastic scattering gamma rays detected at the proximal gamma detector to the count of inelastic scattering gamma rays detected at the far gamma detector, and
determine the porosity from the proximal-to-far thermal capture ratio and the proximal-to-far inelastic ratio.

13. The system of claim 12 , wherein the data indicating the total gamma rays detected at the proximal gamma detector and the total gamma rays detected at the far gamma detector comprises, for each detector, a time spectrum indicating gamma ray counts detected at the detector as a function of time, wherein each time spectrum comprises a burst interval indicating gamma ray counts detected while the neutron source is emitting neutrons and a decay interval indicating gamma ray counts detected while the neutron source is not emitting neutrons.

14. The system of claim 13 , wherein determining the count of thermal capture gamma rays detected at the proximal gamma detector and the count of thermal capture gamma rays detected at the far gamma detector comprises, for each of the proximal gamma detector and the far gamma detector:
determining a count of thermal capture gamma rays detected during the decay interval at that detector, and
determining a count of thermal capture gamm

Assignees

• Weatherford Tech Holdings Llc

Inventors

• Schmid Gregory
• Pemper Richard
• Dolliver Darrell

Classifications

• G01V5/101 Primary

  and detecting the secondary Y-rays produced in the surrounding layers of the bore hole · CPC title

• G01V5/102 Primary

  the neutron source being of the pulsed type · CPC title

• G01V5/045

  Transmitting data to recording or processing apparatus; Recording data · CPC title