Magnetic ranging while rotating
A method for magnetic ranging includes rotating a downhole tool in a drilling well in sensory range of magnetic flux emanating from a target well. The downhole tool includes a magnetometer set and an accelerometer set rotatably coupled to the tool. The magnetometer set and the accelerometer set acquire corresponding magnetometer measurements and accelerometer measurements while rotating. The magnetometer measurements are transformed to a rotation independent reference frame to obtain rotation invariant measurements which are in turn processed to compute at least one of a distance and a direction from the drilling well to the target well.
1. A method for magnetic ranging comprising:
(a) rotating a downhole drilling tool in a drilling well in sensory range of magnetic flux emanating from a target well, the downhole drilling tool including a magnetometer set and an accelerometer set rotatably coupled to the downhole drilling tool;
(b) causing the magnetometer set and the accelerometer set to acquire corresponding magnetometer measurements and accelerometer measurements while rotating in (a);
(c) transforming the magnetometer measurements and the accelerometer measurements acquired in (b) to a rotation independent reference frame to obtain rotation invariant measurements; and
(d) processing the rotation invariant measurements to compute at least one of a distance and a direction from the drilling well to the target well.
2. The method of claim 1 , wherein the magnetometer set comprises a tri-axial set of magnetometers and the accelerometer set comprises a tri-axial set of accelerometers.
3. The method of claim 1 , wherein the rotation independent reference frame comprises an axial component, a high side component BHS, and a right side component BRS of the magnetometer measurements acquired in (b).
4. The method of claim 3 , wherein BHS and BRS are obtained using the following equations:
BHS=B xy ·cos( T−M )
BRS=B xy ·sin( T−M )
wherein B xy represents a magnitude of a transverse component of the magnetometer measurements acquired in (b) and (T−M) represents a toolface offset of the transverse component of the magnetometer measurements acquired in (b).
5. The method of claim 1 , wherein the rotation independent reference frame comprises an axial component, a magnitude of a transverse component B xy , and a toolface offset of the transverse component (T−M) of the magnetometer measurements acquired in (b).
6. The method of claim 5 , wherein:
the magnitude of B xy is computed using at least one of the following equations:
B xy =√{square root over (( B x 2 +B y 2 ))}
B xy =√{square root over (2·σ( B x )·σ( B y ))}
and (T−M) is computed using the following equation:
(
T
-
M
)
=
tan
-
1
[
(
B
x
·
A
y
-
B
y
·
A
x
)
(
-
B
x
·
A
x
-
B
y
·
A
y
)
]
wherein A x and A y represent transverse accelerometer measurements acquired in (b), B x and B y represent transverse magnetometer measurements acquired in (b), and σ(B x ) and σ(B y ) represent standard deviations of corresponding sets of B x and B y measurements.
7. The method of claim 1 , wherein the rotation invariant measurements are transmitted to a surface location and the processing in (d) is performed at the surface location.
8. A method for magnetic ranging comprising:
(a) rotating a downhole drilling tool in a drilling well in sensory range of magnetic flux emanating from a target well having a magnetized casing string, the downhole drilling tool including a magnetometer set and an accelerometer set rotatably coupled to the downhole drilling tool;
(b) causing the magnetometer set and the accelerometer set to acquire corresponding magnetometer measurements and accelerometer measurements while rotating in (a);
(c) transforming the magnetometer measurements and the accelerometer measurements acquired in (b) to a rotation independent reference frame to obtain rotation invariant measurements; and
(d) processing the rotation invariant measurements to compute at least one of a distance and a direction from the drilling well to the target well.
9. The method of claim 8 , wherein the magnetized casing string comprises an alternating pattern of magnetically opposing north-north (NN) poles and magnetically opposing south-south (SS) poles spaced apart along an axis of the target well.
10. The method of claim 9 , wherein (d) comprises (i) acquiring a model that relates the rotation invariant measurements to a distance from the drilling well to the target well and (ii) processing the rotation invariant measurements in combination with the model acquired in (i) to determine the distance from the drilling well to the target well.
11. The method of claim 9 , wherein the processing in (d) comprises (i) subtracting a magnetic field of the Earth from the rotation invariant measurements obtained in (c) to obtain a target magnetic field and (ii) processing target magnetic field to compute at least one of the distance and the direction from the drilling well to the target well.
12. The method of claim 11 , wherein (ii) comprises processing high side and right side components of the target magnetic field to compute the distance and the direction from the drilling well to the target well.
13. The method of claim 11 , wherein:
a transverse component of the magnetic field of the Earth is obtained at a location of maximum or minimum axial magnetic field; and
an axial component of the magnetic field of the Earth is obtained at a location of maximum or minimum transverse magnetic field.
14. The method of claim 9 , wherein the processing in (d) comprises (i) processing a high side component and a right side component of the rotation invariant measurements to compute a slope and (ii) relating the slope to the direction from the drilling well to the target well.
15. The method of claim 8 , wherein the target well comprises ferromagnetic casing having residual remanent magnetism at casing joints therein.
16. The method of claim 15 , wherein the processing in (d) comprises (i) processing the magnetometer measurements acquired in (b) to obtain an axial separation between an adjacent maximum and minimum of an axial component of the magnetometer measurements and (ii) processing the axial separation to compute the distance from the drilling well to the target well.
17. The method of claim 16 , wherein the distance from the drilling well to the target well is computed using the following equation:
d=δz/ √{square root over (2)}
wherein d represents the distance from the drilling well to the target well and δz represents the axial separation.
18. The method of claim 15 , wherein the processing in (d) comprises (i) processing a high side component and a right side component of the rotation invariant measurements to compute a slope and (ii) relating the slope to the direction from the drilling well to the target well.
19. The method of claim 15 , wherein the processing in (d) comprises (i) subtracting a magnetic field of the Earth from the rotation invariant measurements obtained in (c) to obtain a target magnetic field and (ii) processing target magnetic field to compute at least one of the distance and the direction from the drilling well to the target well.
20. A method for magnetic ranging comprising:
(a) rotating a downhole drilling tool in a drilling well in sensory range of magnetic flux emanating from a target well having an electromagnet deployed therein, the downhole drilling tool including a magnetometer set and an accelerometer set rotatably coupled to the downhole drilling tool;
(b) causing the magnetometer set and the accelerometer set to acquire corresponding magnetometer measurements and accelerometer measurements while rotating in (a);
(c) transforming the magnetometer measurements and the accelerometer measurements acquired in (b) to a rotation independent reference frame to obtain rotation invariant measurements; and
(d) processing the rotation invariant measurements to compute at least one of a distance and a direction from the drilling well to the target well.
21. The method of claim 20 , wherein (d) comprises (i) acquiring a model that relates the rotation invariant measurements to a distance from the drilling well to the target well and (ii) processing the rotation invariant measurements in combination with the model acquired in (i) to determine the distance from the drilling well to the target well.
22. The method of claim 20 , wherein the processing in (d) comprises (i) subtracting a magnetic field of the Earth from the rotation invariant measurements obtained in (c) to obtain a target magnetic field and (ii) processing the target magnetic field to compute at least one of the distance and the direction from the drilling well to the target well.
23. The method of claim 22 , wherein the magnetic field of the Earth is measured in (b) when the electromagnet is de-energized.
24. The method of claim 20 , wherein:
(b) and (c) in combination further comprise obtaining a first set of rotation invariant measurements when the electromagnet is energized with a first polarity and a second set of rotation invariant measurements when the electromagnet is energized with a second opposing polarity; and
(d) comprises (i) processing a difference between the first and second sets of rotation invariant measurements to compute a target magnetic field and (ii) processing the target magnetic field to compute at least one of the distance and the direction from the drilling well to the target well.