DEPTH POSITIONING USING GAMMA-RAY CORRELATION AND DOWNHOLE PARAMETER DIFFERENTIAL

1 . A method, comprising:

placing a tubular string having at least one depth measurement module into a wellbore, the wellbore emanating radiation at at least one location along the wellbore; and

determining the location of the depth measurement module in the wellbore based on a correlation between a wellbore property that is a function of depth and a radiation intensity at at least one location within the wellbore.

2 . The method of claim 1 , further comprising:

determining a length change, L Δ , of the tubular string in the wellbore utilized in order to obtain the wellbore property that is a function of depth and the radiation intensity at at least one location within the wellbore; and

wherein the correlation between the wellbore property and the radiation intensity for determining the location of the depth measurement module in the wellbore further includes the length change L Δ of the tubular string in the wellbore.

3 . The method of claim 1 , further comprising:

repositioning the depth measurement module to a desired wellbore location based on the determined location.

4 . The method of claim 1 , further comprising:

determining a plurality of wellbore properties that are a function of depth and plurality of radiation intensities at a plurality of locations along the wellbore.

5 . The method of claim 4 , wherein the at least one location along the wellbore emanating radiation is a known location in the wellbore.

6 . The method of claim 5 , wherein the wellbore has a plurality of known locations that emanate radiation and which form a known pattern of radiation intensity, thereby providing a radiation intensity signature along the wellbore.

7 . The method of claim 4 , wherein the plurality of locations comprise:

locations above a radioactive source along the wellbore, at the radioactive source, and below the radioactive source.

8 . The method of claim 1 , wherein the radiation emanating from the wellbore is caused by a radioactive source located along the wellbore, the radioactive source comprising at least one of an artificial radioactive source and a natural radioactive source.

9 . The method of claim 8 , wherein the artificial radioactive source comprises a pip-tag and the natural radioactive source comprises a natural background radiation emanating from a formation in which the wellbore is formed.

10 . The method of claim 9 , further comprising:

measuring the wellbore property at a first location above the pip-tag, DP start ;

measuring the wellbore property at a second location when the depth measurement module is at the radioactive pip-tag, DP pip ; and

measuring the wellbore property at a third location in the wellbore below the pip-tag DP end .

11 . The method of claim 10 , further comprising:

determining a length change, L Δ , of the tubular string in the wellbore utilized in order to obtain a plurality of wellbore properties that are a function of depth and a plurality of radiation intensities at a plurality of locations along the wellbore, wherein determining the length change L Δ of the tubular string in the wellbore further comprises:

measuring a first distance, h 1 , from a rig floor to a top of the tubular string when the depth measurement module is at the first location in the wellbore;

connecting one or more tubulars of known length L to the tubular string;

lowering the tubular string into the wellbore; and

measuring a second distance, h 2 , from the rig floor to the top of the tubular string when the tubular string is at the third location.

12 . The method of claim 11 , wherein determining the location of the depth measurement module in the wellbore further comprises:

determining a distance travelled by the tubular string based on a correlation of h 1 , h 2 , L, and the measured wellbore properties at the first, second, and third locations, DP start , DP pip , DP end .

13 . The method of claim 1 , further comprising:

transmitting signals representing at least one of a radiation intensity and the wellbore property from the depth measurement module to a wellbore surface system.

14 . The method of claim 1 , wherein the wellbore property comprises at least one of temperature, pressure, density, gravity, and acceleration.

15 . The method of claim 1 , wherein the tubular string includes two or more depth measurement modules placed at a known distance apart from each other along the tubular sting.

16 . A method of determining the position of a downhole tubular string in a wellbore, comprising:

placing a tubular string having a depth measurement module into a wellbore having a radioactive pip-tag;

measuring a first distance, h 1 , from a rig floor to a top of the tubular string when the depth measurement module is at a first location in the wellbore above the pip-tag;

measuring a wellbore property at the first location, DP start , using the depth measurement module;

connecting one or more tubulars of known length L to the tubular string;

lowering the tubular string into the wellbore;

measuring the wellbore property at a second location when the depth measurement module is at the radioactive pip-tag, DP pip ;

measuring the wellbore property at a third location in the wellbore below the pip-tag, DP end ;

measuring a second distance, h 2 , from the rig floor to the top of the tubular string when the tubular string is at the third location; and

determining the location of the depth measurement module in the wellbore based on a correlation of h 1 , h 2 , L, and the measured wellbore properties at the first, second, and third locations, DP start , DP pip , and DP end .

17 . The method of claim 16 , further comprising:

measuring a radiation intensity at the first, second, and third locations; and

repositioning the depth measurement module to a desired wellbore location based on the determined location.

18 . The method of claim 16 , further comprising:

transmitting signals representing at least one of a radiation sensor and the wellbore property from the depth measurement module to a wellbore surface system.

19 . An apparatus, comprising:

a tubular string having a depth measurement module, wherein the depth measurement module comprises:

a telemetry device;

a wellbore property sensor, wherein the sensed wellbore property is a function of depth; and

a radiation sensor.

20 . A system for determining the position of a downhole tubular string in a wellbore, comprising:

a tubular, string having a depth measurement module disposed in the wellbore, wherein the depth measurement module comprises:

a telemetry device;

a wellbore property sensor, wherein the sensed wellbore property is a function of depth; and

a radiation sensor;

a radioactive source disposed at a location along the wellbore; and

a telemetry system for communication between the depth measurement module and a wellbore surface system.