DYNAMIC BOREHOLE AZIMUTH MEASUREMENTS

• US 20130151157A1
• Filed: 12/12/2011
• Published: 06/13/2013
• Est. Priority Date: 12/12/2011
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Smith International Incorporated (Schlumberger NV)
• Inventors: Brooks, Andrew G., Junichi, Sugiura
• Granted Patent: US 9,273,547 B2
• Time in Patent Office: Days
• US Class Current: 702/9
• CPC Class Codes: E21B 47/022 of the borehole, e.g. using geomagnetism

Abstract

A method for making dynamic borehole azimuth measurements while drilling includes processing cross-axial magnetic field measurements in combination with accelerometer measurements to compute the dynamic borehole azimuth. In one or more embodiments, the cross-axial magnetic field measurements and the accelerometer measurements may be used to compute the magnitude of a cross-axial magnetic field component, a toolface offset, and a borehole inclination, which may in turn be used to compute the dynamic borehole azimuth. The disclosed methods may utilize near-bit sensor measurements obtained while drilling, thereby enabling a near-bit dynamic borehole azimuth to be computed while drilling.

Claims

1. A method for making a dynamic borehole azimuth measurement while rotating a downhole measurement tool in a borehole, the method comprising:

(a) rotating a downhole tool in the borehole, the downhole tool including a cross-axial magnetic field sensor and an axial accelerometer;

(b) obtaining a set of cross-axial magnetic field measurements and a set of axial accelerometer measurements while the downhole tool is rotating in (a);

(c) processing the set of cross-axial magnetic field measurements obtained in (b) to compute a magnitude of a cross-axial magnetic field component; and

(d) processing the magnitude of the cross axial magnetic field component computed in (c) and the set of axial accelerometer measurements obtained in (b) to compute the dynamic borehole azimuth.

2. The method of claim 1, wherein (c) further comprises:

processing the set of cross-axial magnetic field measurements to obtain a magnitude of a periodic variation; and

(ii) setting the magnitude of the cross-axial magnetic field component equal to the magnitude of the periodic variation obtained in (i).

3. The method of claim 1, wherein (c) further comprises:

(i) processing a first set of cross-axial magnetic field measurements with respect to a second set of cross-axial magnetic field measurements to obtain a radius of a circle or ellipse; and

(ii) setting the magnitude of the cross-axial magnetic field component equal to the radius determined in (i).

4. The method of claim 1, wherein the magnitude of the cross-axial magnetic field component is computed in (c) according to at least one of the following equations:

B_xy=√

{square root over (B_x²+B_y²)};

B_xy=√

{square root over (2·

σ

_Bx·

σ

_By)};

Σ

[√

{square root over (B_xc²+B_yc²)}−

B_xy]²

wherein B_xy represents the magnitude of the cross-axial magnetic field component, B_x and B_y represent first and second cross-axial magnetic field measurements made along x- and y-axes, σ

_Bx and σ

_By represent standard deviations of a first set of B_x measurements and a second set of B_y measurements made over several complete rotations of the downhole tool; and

B_xc and B_yc represent corrected B_x and B_y measurements after corrections have been applied.

5. The method of claim 1, wherein the dynamic borehole azimuth is computed in (d) according to the following equation:

6. A method for making a dynamic borehole azimuth measurement while rotating a downhole measurement tool in a borehole, the method comprising:

(a) rotating a downhole tool in the borehole, the downhole tool including a cross-axial magnetic field sensor, an axial accelerometer, and a cross-axial accelerometer;

(b) obtaining a set of cross-axial magnetic field measurements, a set of axial accelerometer measurements, and a set of cross-axial accelerometer measurements while the downhole tool rotates in (a);

(c) processing the set of cross-axial magnetic field measurements obtained in (b) to compute a magnitude of a cross-axial magnetic field component; and

(d) processing the magnitude of the cross axial magnetic field component computed in (c) and the set of axial accelerometer measurements and the set of cross-axial accelerometer measurements obtained in (b) to compute the dynamic borehole azimuth.

7. The method of claim 6, wherein the magnitude of the cross-axial magnetic field component is computed in (c) by evaluating at least one of the following equations:

B_xy=√

{square root over (B_x²+B_y²)};

B_xy=√

{square root over (2·

σ

_Bx·

σ

_By²)};

wherein B_xy represents the magnitude of the cross-axial magnetic field component, B_x and B_y represent first and second cross-axial magnetic field measurements made along x- and y-axes, and σ

_Bx and σ

_By represent standard deviations of a first set of B_x measurements and a second set of B_y measurements made over several complete rotations of the downhole tool or by minimizing the following function;

Σ

[√

{square root over (B_xc²+B_yc²−

B_xy)}]²

wherein B_xc and B_yc represent corrected B_x and B_y measurements after corrections have been applied.

8. The method of claim 6, wherein (c) further comprises processing the set of cross-axial magnetic field measurements and the set of cross-axial accelerometer measurements obtained in (b) to compute a toolface offset.

9. The method of claim 8, wherein (c) further comprises:

(i) processing the set of cross-axial magnetic field measurements and the set of cross-axial accelerometer measurements to obtain a magnitude of a periodic variation in the set of cross-axial magnetic field measurements and a phase difference between the periodic variation in the set of cross-axial magnetic field measurements and a periodic variation in the set of cross-axial accelerometer measurements;

(ii) setting the magnitude of the cross-axial magnetic field component equal to the magnitude of the periodic variation in the set of cross-axial magnetic field measurements obtained in (i); and

(iii) setting the toolface offset equal to the phase difference obtained in (i).

10. The method of claim 8, wherein the toolface offset is computed in (c) according to at least one of the following equations:

11. The method of claim 8, wherein (c) further comprises correcting the computed toolface offset to a zero-rpm equivalent value.

12. The method of claim 8, wherein the dynamic borehole azimuth is computed in (d) by solving the following equation:

P sin Azi+Q cos Azi+R sin Azi·

cos Azi=0

wherein Azi represent the dynamic borehole azimuth and P, Q, and R are coefficients that are mathematically related to at least one of the toolface offset, the magnitude of the cross-axial magnetic field component, and a borehole inclination.

13. The method of claim 12, wherein P, Q, and R are given as follows:

P=B sin D·

sin I·

cos I+B_xy cos I·

cos(T−

M)

Q=B_xy sin(T−

M); and

R=B cos D·

sin² I

wherein T−

M represents the toolface offset with T representing a gravity toolface and M representing a magnetic toolface, B_xy represents the magnitude of the cross-axial magnetic field component, I represents the borehole inclination, B represents the magnitude of the earth'"'"'s local magnetic field, and D represents the local magnetic dip angle.

14. The method of claim 12, wherein (d) further comprises:

(i) computing a plurality of possible dynamic borehole azimuth values;

(ii) computing a hypothetical earth'"'"'s magnetic field for each of the plurality of possible dynamic borehole azimuth values;

(iii) computing a difference between the hypothetical earth'"'"'s magnetic field and a reference magnetic field; and

(iv) selecting a dynamic borehole azimuth value that gives the smallest difference in (iii).

15. The method of claim 6, wherein:

the magnitude of the cross-axial magnetic field component is computed downhole in (c) using a downhole processor;

the computed magnitude of the cross-axial magnetic field component is then transmitted to the surface where it is used to process the dynamic borehole azimuth in (d).

16. A method for making a dynamic borehole azimuth measurement while rotating a downhole measurement tool in a borehole, the method comprising:

(a) rotating a downhole tool in the borehole, the downhole tool including an axial magnetic field sensor, a cross-axial magnetic field sensor, an axial accelerometer, and a cross-axial accelerometer;

(b) obtaining a set of axial magnetic field measurements, a set of cross-axial magnetic field measurements, a set of axial accelerometer measurements, and a set of cross-axial accelerometer measurements while the downhole tool rotates in (a);

(c) evaluating a magnetic model to obtain an induced axial magnetic field component and a remanent axial magnetic field component;

(d) correcting the set of axial magnetic field measurements by using the remanent axial magnetic field component as a bias and the induced axial magnetic field component as a scale factor to obtain a corrected axial magnetic field component; and

(e) processing the corrected axial magnetic field component to compute the dynamic borehole azimuth.

17. The method of claim 16, wherein the set of axial magnetic field measurements are corrected using the following equation:

B_z=Be_z(1+SBi_z)+Br_z

wherein B_z represents a measured axial magnetic field component, Be_z represents the corrected axial magnetic field component, SBi_z represents the scale factor due to the induced axial magnetic field component and Br_z represents the bias due to the remanent axial magnetic field.

18. The method of claim 16, wherein the scale factor is obtained using the following equation:

19. A method for making a dynamic borehole azimuth measurement while rotating a downhole measurement tool in a borehole, the method comprising:

(a) rotating a downhole tool in the borehole, the downhole tool including a cross-axial magnetic field sensor, an axial accelerometer, and a cross-axial accelerometer;

(b) obtaining a set of cross-axial magnetic field measurements, a set of axial accelerometer measurements, and a set of cross-axial accelerometer measurements while the downhole tool rotates in (a);

(c) causing a downhole processor to process the set of cross-axial magnetic field measurements, the set of axial accelerometer measurements, and the set of cross-axial accelerometer measurements to compute a magnitude of a cross-axial magnetic field component, a toolface offset, and a borehole inclination;

(d) transmitting the magnitude of a cross-axial magnetic field component, the toolface offset, and the borehole inclination to a surface location; and

(e) causing a surface processor to processing the magnitude of a cross-axial magnetic field component, the toolface offset, and the borehole inclination obtained in (c) to compute the dynamic borehole azimuth.

20. The method of claim 19, wherein:

(d) further comprises transmitting a rotation rate of the downhole tool to the surface location; and

(e) further comprises using the rotation rate to correct the toolface offset to a zero-rpm equivalent value prior to computing the dynamic borehole azimuth.