LOGGING TOOL FOR DETERMINATION OF FORMATION DENSITY AND METHODS OF USE

• US 20130261974A1
• Filed: 03/28/2013
• Published: 10/03/2013
• Est. Priority Date: 04/03/2012
• Status: Active Grant

• Current Assignee: J.M. Wood Investments Ltd.
• Original Assignee: Quantum Petrophysics, Inc.
• Inventors: STEWART, Kirk, RANDALL, Russel R.
• Granted Patent: US 10,197,701 B2
• Time in Patent Office: Days
• US Class Current: 702/8
• CPC Class Codes: G01V 5/045 Transmitting data to recording or processing apparatus; Recording data
• CPC Class Codes: G01V 5/125 and detecting the secondary gamma- or X-rays in different places along the bore hole

Abstract

Apparatus for wellbore logging and method of use are provided for measurement of the annular materials interposed between the logging tool and formation and the apparent formation bulk density in open-hole, cased-hole or other conduit. The logging tool can be centralized in the borehole. Non-collimated gamma radiation is emitted into the surrounding media and the amount of radiation which returns to the tool is measured by multiple omni-directional detectors spaced a pre-set distances from the source to provide multiple radial depths of investigation. The annular thickness and density of materials interposed between the logging tool and the formation may be determined from the detector responses and be used to derive the apparent formation bulk density.

Claims

1. A method for downhole logging for obtaining information indicative of the bulk formation density of a subterranean earth formation penetrated by a borehole comprising:

irradiating the formation with a non-collimated emission of gamma ray radiation produced by a gamma ray source generally centralized in the borehole;

detecting non-collimated scattered gamma rays at

a first detector spaced from the source a first pre-set distance optimized to receive the scattered gamma rays from about the periphery of the borehole adjacent the source for establishing a first detector response;

a second detector spaced from the source a second pre-set distance optimized to receive scattered gamma rays from outside the periphery of the borehole for establishing a second detector response; and

at least a third detector spaced from the source a third pre-set distance optimized to receive scattered gamma rays from the formation outside the periphery of the borehole for establishing a third detector response; and

establishing measures of the formation density by solving the response of the first, second and third detectors.

2. The method of claim 1 where the borehole is cased, the casing forming the periphery of the borehole, further comprising:

establishing a relationship for the first detector for casing thickness as a function of the first response for at least one casing diameter,

determining the casing diameter; and

establishing measures of the casing thickness from the substantially linear function of the response of the first detector.

3. The method of claim 2 wherein casing thickness is substantially linear function of the first response.

4. The method of claim 2 further comprising:

establishing a relationship for the second detector and at least a third detector for formation density as a function of the second and third responses respectively for a known casing diameter and the measured casing thickness; and

establishing measures of the cement sheath thickness, cement density and formation density from the response of the second and third detectors.

5. The method of claim 2 further comprising:

detecting non-collimated scattered gamma rays at

a fourth detector spaced from the source a fourth pre-set distance optimized to receive the scattered gamma rays from outside the periphery of the borehole for establishing a fourth detector response; and

establishing a relationship for formation density as a function of the second, third responses and fourth respectively for known casing diameter and the measured casing thickness; and

establishing measures of the cement sheath thickness, cement density and formation density from the response of the second, third and fourth detectors.

6. The method of claim 2 further comprising:

determining a low energy response and a higher energy response from one of the second or third detector; and

establishing measures of a fluid density of fluid in the borehole from a ratio of the low energy and higher energy responses for establishing measures of the cement sheath thickness, cement density and formation density from the response of the second and third detectors.

7. The method of claim 5 further comprising:

determining a low energy response and a higher energy response from one of the second or third detector; and

establishing measures of a fluid density of fluid in the borehole from a ratio of the low energy and higher energy responses for establishing measures of the cement sheath thickness, cement density and formation density from the response of the second, third and fourth detectors.

8. The method of claim 5 further comprising:

establishing relationships for the response of the second, third responses and fourth detectors respectively using a general-purpose Monte Carlo N-Particle model of the borehole.

9. The method of claim 8 further comprising:

establishing measures of the cement sheath thickness, cement density and formation density from the response of the second, third and fourth detectors respectively for a known casing diameter and the measured casing thickness.

10. A downhole formation density logging tool comprising:

a non-collimated gamma ray source operative to irradiate a subterranean formation with a non-collimated emission of gamma ray radiation;

a plurality of detectors for at least establishing measures of the cement sheath thickness, cement density and formation density, the plurality of detectors comprising,

a first non-collimated gamma ray detector spaced a first pre-set distance axially from the source and shielded therefrom, the first detector operative to measure at least the count-rates of non-collimated scattered gamma rays from a near wellbore area;

a second non-collimated gamma ray detector spaced a second pre-set distance axially from the source and shielded therefrom, the second detector operative to measure at least the count-rates of non-collimated scattered gamma rays adjacent the near wellbore area; and

at least a third non-collimated gamma ray detector spaced a third pre-set distance axially from the source and shielded therefrom, the third detector operative to measure at least the count-rates of non-collimated scattered gamma rays from the formation outside the near wellbore area.

11. The logging tool of claim 10 wherein the at least a third detector further comprises the third detector and

at least a fourth non-collimated gamma ray detector spaced a fourth pre-set distance axially from the source and shielded therefrom, the third detector operative to measure at least the count-rates of non-collimated scattered gamma rays from the formation outside the near wellbore area.

12. The logging tool of claim 10 wherein each of the source and detectors are substantially non-shielded.

13. The logging tool of claim 10 wherein the first and at least a third detector are positioned at different distances from the source, said distances being selected to ensure the detectors are able to detect and establish scattered gamma ray measurements at multiple radial depths of investigation.

14. The logging tool of claim 10 wherein the source and detectors are centrally located within the wellbore.

15. The logging tool of claim 10 wherein the borehole is a cased-hole and the first detector establishes measures of casing thickness.

16. The logging tool of claim 10 wherein the wellbore is an open hole.

17. The logging tool of claim 10 wherein the source radioactivity is less than 1.5 Curies and selected to be compatible with the detectors.

18. The logging tool of claim 17 wherein the source radioactivity is cobalt-60.

19. The logging tool of claim 10 wherein one or more of the first, second and at least a third detector is a circumferential array of detectors for receiving scattered gamma radiation from about a circumference of he borehole.

20. The logging tool of claim 19 wherein the circumferential array of detectors is an array of four detectors or six detectors.