AU4509399A

AU4509399A - Method of determining azimuth of a borehole

Info

Publication number: AU4509399A
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: 2000-01-05
Anticipated expiration: 2019-06-14
Also published as: NO320686B1; CA2334920C; EP1088152B1; NO20006450L; WO1999066173A1; ES2237113T3; CN1305565A; CA2334920A1; DE69923179T2; DE69923179D1; EP1088152A1; NO20006450D0; AU748917B2; BR9911307A

Description

WO 99/66173 PCT/EP99/03940 METHOD OF DETERMINING AZIMUTH OF A BOREHOLE The present invention relates to a method of determining an azimuth angle of a borehole formed in an earth formation using magnetometer tool arranged in a drill string extending longitudinally in the borehole. 5 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 10 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 15 which the borehole azimuth is determined using the Earth magnetic field as a reference. In many instances however the measured local magnetic field includes, apart from the Earth magnetic field components, components attributable to drill string magnetisation. In order to 20 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 azimuth of a borehole formed in an earth formation using 25 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 30 drill string magnetisation. The cross-axial drill string magnetisation is eliminated by taking so-called WO 99/66173 PCT/EP99/03940 2 rotational 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 5 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 10 the corrected field and from the Earth magnetic field which is generally known for most places on Earth. The computed azimuth however is very sensitive to inaccuracies in the Earth magnetic field data, especially in case of highly inclined boreholes extending 15 substantially in east or west direction. It is an object of the invention to provide an improved method of determining azimuth of a borehole, which method is less sensitive to inaccuracies in the Earth magnetic field data even for highly inclined 20 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 25 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 30 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 35 magnetic field along an axis having a selected WO 99/66173 PCT/EP99/03940 3 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 5 selected borehole inclinations, a contribution from the drill string magnetisation field tQ the measured components; d) correcting the measurements for said contribution from the drill string magnetisation field; and 10 e) determining from the corrected measurements, the borehole azimuth. The contribution from the earth magnetic field to each measured component along the axis of selected orientation is different for the different borehole 15 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 from the drill string magnetisation field to the measured component is the same 20 for the different borehole locations because the orientation of said axis relative to the drill string magnetisation field does not change. Since the orientation of said axis is directly related to the orientation of the drill string and therefore to the 25 borehole inclination, the contribution from the drill string 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 30 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 local magnetic field, which is the component in axial direction of the drill string. 35 It is to be understood that the contribution from the WO 99/66173 PCT/EP99/03940 4 drill string magnetisation field to the cross-axial component (if any at all) of magnetic field generally is an order of magnitude smaller than the axial contribution. Therefore, for most applications it is 5 sufficiently accurate to disregard such cross-axial contribution. Alternatively, the measured magnetic field can be corrected for a cross-axial contribution from the drill string magnetisation field prior to step c). The method can suitably be applied for a borehole of 10 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 15 invention, the borehole inclinations at at least two of said locations differ from each other by an angle of at least 400. In case the borehole inclination at a first one of the locations is less than 450 it is preferred to 20 determine the contribution to the axial components attributable to drill string magnetisation from the vertical component of the Earth magnetic field. If furthermore the drill string magnetisation at the first location is different than at a second one of the 25 locations, e.g. due to different Bottom Hole Assemblies, the borehole inclination angle at the second location is suitably between 80*-100*. Conversely, in case the wellbore inclination at the first location exceeds 450 it is preferred to determine 30 the contribution to the axial components attributable to drill string magnetisation from the horizontal component of the Earth magnetic field. If furthermore the drill string magnetisation at the first location is different than at the second location, the borehole inclination WO 99/66173 PCT/EP99/03940 5 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 5 accompanying drawings in which Fig. 1 shows a horizontal plane of the (N,E,V) coordinate system; Fig. 2 shows a vertical plane through line H of the coordinate system of Fig. 1; 10 Fig. 3 shows a borehole-fixed coordinate system (HS, HSR, z) and a tool-fixed coordinate system (x,y,z). In Fig. 1 is shown the horizontal N-E plane of the North (N) , East (E) , Vertical (V) coordinate system, wherein line H is a projection in the N-E plane of the 15 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. BN represents the horizontal vector component of the earth magnetic field. 20 In Fig. 2 is shown a vertical plane through line H. Line T represents the longitudinal axis of the borehole and angle I the borehole inclination which varies along the length of the borehole. By represents the vertical vector component of the earth magnetic field and Bn.cos A 25 is the projection of the horizontal component of the earth magnetic field on line H. In Fig. 3 is shown a cross-sectional view of the borehole 10, a co-ordinate system (HS, HSR, z) fixed to the borehole 10 and a co-ordinate system (x, y, z) fixed 30 to a magnetometer tool (not shown) for measuring the components of a local magnetic field B in the (x, y, z) co-ordinate system. The magnetometer tool is fixedly arranged in a drill string (not shown) extending through the borehole, therefore the (x, y, z) co-ordinate system WO 99/66173 6 PCT/EP99/03940 can be thought of as being fixed to the drill string. The HS-, HSR-, x-, and y-axes extend in the transverse plane of the borehole at point P whereby the x-, y-axes are rotated relative to the HS-, HSR-axes about an angle a 5 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 accelero meter tool (not shown) for measuring the components of the earth gravity field G in the (x, y, z) co-ordinate 10 system. During normal operation the magnetometer tool measures the components Bx, By, 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 15 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) 20 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 25 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 30 earth magnetic field vector by (BHSe, BHSRe, Bze) and the drill string magnetisation vector by (CHS, CHSR, Cz) 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 5 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 10 (BHS, BHSR, Bz) = (BHSe, BHSRe, Bze + CZ) (5) The sum of the vertical components of BHSe and Bze 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 15 and from eq. (5) By = -BHSe sin I + (Bz - Cz) 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, 20 BHSR2, Bz2) are obtained, and from eq. (6) it follows By = -BHSel sin Il + (Bzl - Czl) cos Il (7) By = -BHSe 2 sin 12 + (Bz2 - Cz2) cos 12 (8) Axial drill string magnetisation depends primarily on the magnetic properties of the BHA, not on borehole 25 inclination. Therefore it is considered that at least as long as the BHA is not changed: Czl = Cz2 = Cz (9) Equations (7), (8), (9) contain the unknowns By, Czl and Cz2. The inclinations Ii and 12 are known from 30 measurements using one or more accelerometer meters included in the drill string. It is found that WO 99/66173 8 PCT/EP99/03940 Cz(COS 12 - COS Il) = BHSel sin Il - Bzl Cos Il BHSe2 sin 12 + Bz2 COS 12 (10) from which Cz is determined. The local magnetic field at each point can now be 5 corrected for axial drill string magnetisation. The above approach is preferred for low borehole inclinations, i.e. inclinations less than 450, because Cz then is relatively insensitive to variations in borehole inclination. 10 For borehole inclinations beyond 45* 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 15 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 20 Bn cos Al = BHSel cos Il + (Bzl - 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 25 BHSRel = -Bn sin Al (15) BHSR 2 = -Bn sin A 2 (16) From eqs. (13) - (16), and with Czl = Cz2 = Cz (e.g. for unchanged BHA), it follows that (BHSRel) 2 + (BHSe1 cos I, + (Bzl - Cz) sin Il))2 _ 30 (BHSRe2) 2 + (BHSe2 cos 12 + (Bz2 - WO 99/66173 PCT/EP99/03940 9 Cz) sin 12))2 = 0 (17) Eq. (17) is a quadratic expression in Cz with generally two solutions for Cz. The solution which gives a horizontal magnetic field component closest to the 5 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 10 at the different survey points Czl is generally not equal to Cz2. Therefore it is preferred that for the low inclination mode, i.e. when using eq. (10), at least one survey point is at a borehole inclination between 80'-100*, preferably about 900, because then one of the 15 components Czl cos Ii or Cz2 cos 12 in eqs. (7), (8) substantially vanishes. Similarly, it is preferred that for the high inclination mode, i.e. when using eq. (17), at least one survey point is at a borehole inclination between 0* and 20 +10', preferably about 00, because then either Czl sin Il or Cz2 sin 12 in eq. (17) substantially vanishes. Instead of using two survey points as described above, more than two survey points can be used to correct for axial drill string magnetisation.

Claims

1. A method of determining an azimuth angle of a borehole formed in an earth formation using a magneto meter tool arranged in a drill string extending in the borehole, the magnetometer tool having a selected 5 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; 10 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 15 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 20 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.

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

3. The method of claim 1 or 2, wherein the borehole 30 inclinations at at least two of said locations differ from each other by an angle of at least 40'. WO99/66173 PCT/EP99/03940

4. The method of any one of claims 1-3, wherein said component of the local magnetic field is the axial component of the local magnetic field.

5. The method of claim 4, wherein the borehole 5 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 magnetisation from the relationship: Cz (cos 12 - cos Il) = BHSel sin Il - Bzl cos Il 10 BHSe2 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 15 the second location, and wherein the borehole inclination angle at the second location is between 80*-100'.

7. The method of claim 4, wherein the borehole inclination angle at a first one of said locations exceeds 45* and wherein step c) comprises determining the 20 axial component of the drill string magnetisation from the relationship: (BHSRel) 2 + (BHSel cos I, + (Bzl - C zl) sin Il)) 2 _ (BHSRe2) 2 + (BHSe2 cos 12 + (Bz2 - Cz2) sin I 2 )) 2 = 0 as defined hereinbefore. 25

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 the second location, and wherein the borehole inclination angle at the second location is between 0* and +10'. 30

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 WO 99/66173 12 PCT/EP99/03940 secondary axis substantially in high-side direction of the borehole, and a tertiary axis substantially in high side right direction of the borehole.

10. The method of any one of claims 1-9, wherein two said 5 locations are selected along the borehole.

11. The method substantially as described hereinbefore with reference to the drawings.