AU748917B2

AU748917B2 - Method of determining azimuth of a borehole

Info

Publication number: AU748917B2
Application number: AU45093/99A
Authority: AU
Inventors: Robin Adrianus Hartmann
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1998-06-18
Filing date: 1999-06-14
Publication date: 2002-06-13
Anticipated expiration: 2019-06-14
Also published as: CA2334920A1; EP1088152A1; NO20006450L; NO20006450D0; CN1305565A; CA2334920C; ES2237113T3; DE69923179D1; EP1088152B1; DE69923179T2; AU4509399A; NO320686B1; WO1999066173A1; BR9911307A

Description

P\OPER\Rd21(12\FebU2"65165 res do-12 Mr rh. 21)21 -1- METHOD OF DETERMINING AZIMUTH OF A BOREHOLE The present invention relates to a method of determining an azimuth angle of a borehole in an earth formation using magnetometer tool arranged in a drill string extending longitudinally in the borehole. During drilling of a borehole in an earth formation it is generally desirable to check the borehole course by measuring the inclination and azimuth of the borehole at regular intervals. The borehole inclination can be determined using accelerometer measurements in the borehole and the Earth gravity field as a reference. The borehole azimuth is determined using a package of magnetometers included in the Bottom Hole Assembly (BHA) of the drill string. The magnetometers are operated to measure the components of the local magnetic field from which the borehole azimuth is determined using the Earth magnetic field as a reference.

In many instances however the measure local magnetic field includes, apart from the Earth magnetic field components, 20 components attributable to drill string magnetisation. In order to obtain sufficiently accurate azimuth data it is :required that such drill string magnetisation effects are •taken into account.

EP-A-0 193 230 discloses a method of determining an 25 azimuth of a borehole in an earth formation using a magnetometer package included in a drill string extending into the borehole, wherein the effect of drill string magnetisation is taken into account by first eliminating the effect of cross-axial drill string magnetisation prior to eliminating the influence of axial drill string magnetisation. The cross-axial drill string magnetisation is eliminated by taking so-called P,'OPER\Rd\21)2\Fb\2365 165 res dec-12 M;lrch. 2002 -2rotational shots, i.e. by measuring the local magnetic field at different rotational locations of the magnetometer tool and determining the cross-axial drill string magnetisation from the magnetic field data thus obtained. The axial drill string magnetisation is computed from the measured magnetic field and from the Earth magnetic field. Once the measured magnetic field has been corrected for cross-axial and axial drill string magnetisation, the borehole azimuth is determined from the corrected field and from the Earth magnetic field which is generally known for most places on Earth. The computed azimuth however is ver sensitive to inaccuracies in the Earth magnetic field data, especially in case of highly inclined boreholes extending substantially in east or west direction.

Accordingly, the present invention seeks to provide a method of det2'mining an azimuth of a borehole, which is less sensitive to inaccuracies in the Earth magnetic field data even for highly inclined boreholes extending substantially in east or west direction.

In accordance with the invention there is provided a method of determining an azimuth angle of a borehole formed in an earth formation using a magnetometer tool arranged in a drill string extending in the borehole, the magnetometer tool having a selected orientation relative to the drill string, the method comprising a) selecting at least two locations along the borehole at which the borehole has selected different borehole inclinations; b) for each selected location, arranging the drill string in the borehole such that the magnetometer tool is positioned at the selected location and operating the magnetometer tool so as to measure a component of a local magnetic field along an axis having a selected P OPER\Rdl\1 52\Fecb2365 1(r5es doc-l12 Mrch. 20012 orientation relative to the magnetometer tool, the local magnetic field including the earth magnetic field and a drill string magnetisation field; c) determining from the measurements and from the selected borehole inclinations, a contribution from the cdrill string magnetisation field to the measured components; d) correcting the measurements for said contribution from the drill string magnetisation field; and e) determining from the corrected measurements, the borehole azimuth, wherein step c) comprises solving a set of equations linking said contribution from the drill string magnetisation to said measured components, said borehole inclination and a component of the earth magnetic field The contribution from the earth magnetic field to each measured component along the axis of selected orientation is different from the different borehole locations because the drill string, and therefore also said axis, is oriented differently relative to the earth magnetic field at the different locations. On the other hand, the contribution 20 from the drill string magnetisation field to the measured component is the same for the different borehole locations because the orientation of said axis relative to the drill S string magnetisation field does not change. Since the orientation of said axis is directly related to the S 25 orientation of the drill string and therefore to the eooe borehole inclination, the contribution from the drill string S.magnetisation field to the measured component can be determined from the difference between the measured components at the different locations and from the different borehole inclinations at the different locations. An example of such determination is presented in the detailed description below.

Preferably said component of the local magnetic field is the axial component of the location magnetic field, which is the component in axial direction of the drill string. It Ris to be understood that the contribution from the P:OPER\Rdi 2*2FcbX2365 165 rs doc-12 March. 2(X)2 -4drill string magnetisation field to the cross-axial component (if any at all) of magnetic field generE.lly is an order of magnitude smaller then the axial contribution.

Therefore, for most applications it is sufficiently accurate to disregard such cross-axial contribution. Alternatively, the measured magnetic field can be corrected for a crossaxial contribution from the drill sting magnetisation field prior to step c).

The method can suitably be applied for a borehole of which the longitudinal axis at the selected locations is substantially located in a vertical plane.

For most applications it is sufficient to select two said locations of different borehole inclination.

For enhanced accuracy in applying the method of the invention, the borehole inclinations of at least two of said locations preferably differ from each other by an angle of at least 400.

e Where the borehole inclination at a first one of the locations is less than 450, it is preferably to determine 20 the contribution to the axial components attributable to 0003 drill string magnetisation from the vertical components of the Earth magnetic field. Furthermore, if the drill string magnetisation at the first location is different than at a second one of the locations, e.g. due to different Bottom 25 Hole Assemblies, the borehole inclination angle at the second location is suitably between 800-100.

Conversely, in case the borehole inclination at the first location exceeds 450 it is preferred to determine the contribution to the axial components attributable to drill string magnetisation from the horizontal component of the Earth magnetic field. Furthermore, if the drill string magnetisation at the first location is different than at the second location, the borehole inclination P.OPER\Rd2(H12\Fcb\2365165 rcs.doc-12 March. 21l12 angle at the second location is suitably between 0° and +100.

The invention will be described further in more detail and by way of example with reference to the accompanying drawings in which Fig. 1 shows a horizontal plane of the E, V) coordinate system; Fig. 2 shows vertical plane through line H of the coordinate system of Fig. 1; Fig. 3 shows a borehole-fixed coordinate system (HS, HSR, z) and a tool-fixed coordinate system y, z).

In Fig. 1 the horizontal N-E plane of the North East Vertical coordinate system is shown, wherein line H is a projection in the N-E plane of the longitudinal axis of a borehole 10 (Fig. 3) and angle A indicates the ^borehole azimuth. It is to be understood that angle A may vary along the length of the borehole. B N represents the horizontal vector component of the earth magnetic field.

In Fig. 2 a vertical plane through line H is shown.

20 Line T represents the longitudinal axis of the borehole and angle I the borehole inclination which varies along the length of the borehole. B v represents the vertical vector component of the earth magnetic field and B n cos A is the projection of the horizontal components of the earth 25 magnetic field on line H.

In Fig. 3 a cross-sectional view of the borehole 10, a co-ordinate system (HS, HSR, z) fixed to the bore hole •and a co-ordinate system y, z) fixed to a magnetometer tool (not shown) for measuring the components of a local magnetic field B in the y, z) co-ordinate system is shown. The magnetometer tool is fixedly arranged in a drill string (not shown) extending through the borehole, therefore the y, z) co-ordinate system Wr OQQ/6I17 PCrT/F.P99/03940 6 can be thought of as being fixed to the drill string. The HS-, HSR-, and y-axes extend in the transverse plane of the borehole at point P whereby the y-axes are rotated relative to the HS-, HSR-axes about an angle a which is referred to as the tool-face angle. The z-axis extends in longitudinal direction of the borehole 10. The drill string is furthermore provided with an accelerometer tool (not shown) for measuring the components of the earth gravity field G in the y, z) co-ordinate system.

During normal operation the magnetometer tool measures the components B, B Bz of the local magnetic field vector B and the accelerometer tool measures the components Gx, Gy, Gz of the gravity field vector G while the drill string is kept stationary. The tool-face angle a and the inclination angle I are determined from the equations: GHS Gxcos a Gysin a (1) Gv Gzcos I GHSsin I (2) Gzsin I +GHScos I o (3) wherein GHS is the component of G in HS-direction; Gv is the (known) component of G in V-direction.

From the measured magnitudes of Bx, By, Bz and from the tool face angle a, the components of B in the (HS, HSR, z) co-ordinate system are determined thus yielding the local magnetic field vector (BHS, BHSR, Bz). These components include contributions from the earth magnetic field and from drill string magnetisation. Denoting the earth magnetic field vector by (BHSe, BHSRe, Bze) and the drill string magnetisation vector by (CHS, CHSR, C z the local magnetic field vector is WO 99/66173 PCT/EP99/03940 7 (BHS, BHSR, Bz) (BHSe, BHSRe, Bze) (CHS, CHSR, Cz) (4) The cross-axial contributions from drill string magnetisation are then determined and eliminated from the magnetic field vector, for example by means of a "rotational shot" whereby a number of surveys are taken at various rotational angles of the magnetometer tool in the borehole as described in EP-A-0 193 230. After such elimination the local magnetic field vector is (BHS, BHSR, Bz) (BHSe, BHSRe, Bze Cz) The sum of the vertical components of BHSe and Bz e is equal to the vertical component By of the magnetic field (BHSRe has no vertical component), thus yielding By -BHSe sin I Bze cos I and from eq. By -BHSe sin I (B z

C

z cos I (6) By operating the magnetometer tool at two borehole locations with different inclinations Il and 12 two local magnetic field vectors (BHS1, BHSR1, Bzl) and (BHS2, BHSR2, Bz2) are obtained, and from eq. it follows By -BHSel sin Il (Bzl Czj) cos Il (7) By -BHSe2 sin 12 (Bz 2 Cz2) cos 12 (8) Axial drill string magnetisation depends primarily on the magnetic properties of the BHA, not on borehole inclination. Therefore it is considered that at least as long as the BHA is not changed: Cz 1 Cz2 C z (9) Equations contain the unknowns By, Czl and Cz 2 The inclinations I 1 and 12 are known from measurements using one or more accelerometer meters included in the drill string. It is found that wn OQ/;fi173 PrT/P9PO/no40 8 Cz(cos 12 cos Il) BHSel sin Il Bz1 cos I1 BHSe2 sin 12 Bz2 cos 12 from which C z is determined.

The local magnetic field at each point can now be corrected for axial,drill string magnetisation.

The above approach is preferred for low borehole inclinations, i.e. inclinations less than 450, because C z then is relatively insensitive to variations in borehole inclination.

For borehole inclinations beyond 450 the following approach is preferred.

The sum of the components of BHSe and Bze in direction H is equal to the component of the earth magnetic field in direction H, thus yielding Bn cos A BHSe cos I Bze sin I (11) or Bn cos A BHSe cos I (Bz Cz) sin I (12) For two points with respective inclinations I, 12 and azimuth A 1

A

2 it follows that Bn cos Al BHSel cos Il (Bz1 Czl) sin Il (13) Bn cos A 2 BHSe2 cos 12 (Bz2 Cz2) sin 12 (14) The HSR components of the local magnetic field, corrected for cross-axial drill string magnetisation as described above, for the two points are BHSRel -Bn sin Al BHSRe2 -Bn sin A 2 (16) From eqs. (13) and with Cz1 Cz2 C z for unchanged BHA), it follows that (BHSRel) 2 (BHSel cos I 1 (Bz1 C z sin I1))2 (BHSRe2) 2 (BHSe2 cos 12 (Bz2 v P:\OPER\Rd\2l6L HFeb\2365165 res do-12 March. 2(0o2 -9- C. sin I2)) 2 0 Eq. (17) is a quadratic expression in C, with generally two solutions for The solution which gives a horizontal magnetic field component closest to the expected horizontal magnetic field component is to be selected from the two.

The local magnetic field at each point can then be corrected for axial drill string magnetisation.

If different BHA's are used during the measurements at the different survey points Cz is generally not equal to C,.

Therefore it is preferred that for the low inclination mode, i.e. when using eq. at least one survey point is at a borehole inclination between 800-1000, preferably about 900°, because then one of the components Cz cos I, or Cz2 cos 12 in eqs. substantially vanishes.

Similarly, it is preferred that for the high tinclination mode, i.e. when using eq. at least one survey point is at a borehole inclination between 00 and +100, preferably about 00, because then either sin I, or

SC

2 sin 12 is eq. (17) substantially vanishes.

20 Instead of using two survey points as described above, more than two survey points can be used to correct the axial drill string magnetisation.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word S. 25 "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

Claims

1. A method of determining an azimuth angle of a borehole in an earth formation using a magnetometer tool arranged in a drill string extending in the borehole, the magnetometer tool having a selected orientation relative to the drill string, the method comprising: a) selecting at least two locations along the borehole at which the borehole has selected different borehole inclinations; b) for each selected location, arranging the drill string in the borehole such that the magnetometer tool is positioned at the selected location and operating the magnetometer tool so as to measure a component of a local magnetic field along an axis having a selected orientation relative to the magnetometer tool, the local magnetic field including the earth magnetic field and a drill string magnetisation field to the measured components; c) determining from the measurements and from the selected borehole inclinations, a contribution from the drill string magnetisation field to the measured components; d) correcting the measurements for said contribution from the drill string magnetisation field; and e) determining from the corrected measurements, the 25 borehole azimuth, wherein step c) comprises solving a set of equations linking said contribution from the drill string magnetisation to said measured components, said borehole .".inclination and a component of the earth magnetic field.

2. The method of claim 1, wherein the longitudinal axis of S 30 the borehole at the selected locations is substantially located in a vertical plane.

3. The method of claim 1 or 2, wherein the borehole inclinations of at least two of said locations differ from each other by an angle of at least 400

4. The method of any one of claims 1-3, wherein said component of the local magnetic field is the axial component P:\OPER\Rd\2(I)2\Fcb\2365165 rcs.doc-12 March. 21KI2 -11- of the local magnetic field.

The method of claim 4, wherein the borehole inclination at a first one of said locations is less than 450 and wherein step c) comprises determining the contribution from axial component of the drill string magnetisaticn from the relationship: C z (cos 12 cos BHse sin I, Bz 1 cos I, BHSe 2 sin 12 Bz2 cos 12 as defined hereinbefore.

6. The method of claim 5, wherein the drill string magnetisation for the magnetometer tool at the first location is different than for the magnetometer tool at a second location, and wherein the borehole inclination angle at the second location is between 800-1000

7. The method of claim 4, wherein the borehole inclination angle at a first one of said locations exceeds 450 and wherein step c) comprises determining the axial component of the drill string magnetisation from the relationship: (BHSRel)2 (BHSel COS Il (Bzl Czl) sin (BHSRe 2 )2 20 (BHe 2 COS 12 (B C 2 sin 2)) 2 0 as defined hereinbefore.

8. The method of claim 7, wherein the drill string magnetisation for the magnetometer tool at the first location is different than for the magnetometer tool at a 25 second location, and wherein the borehole inclination angle. at the second location is between 00 and +100

9. The method of any one of claims 1-8, wherein the magnetometer tool determines the components of the local •magnetic field in a co-ordinate-system having a primary axis substantially in axial direction of the borehole, a secondary axis substantially in HS direction of the borehole, and a tertiary axis substantially in HSR direction of the borehole. R 5

10. The method of any one of claims 1-9, wherein two said locations are selected along the borehole. P:AOPERMOd 2TW,2-c1165 163 -0d-12 K-hc. 24X)2 12

11. A method of determining an azimuth angle of E. borehole in an earth formation as claimed in claim 1 substantially as hereinbefore described. DATED this 13th day of March, 2002 SHELL INTERNATIONALE RESEARCH MA&ATSCHAPPIJ B.V. by its Patent Attorneys DAVIES COLLISON CAVE .0.0 0