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(11) | EP 1 088 152 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
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| (54) |
METHOD OF DETERMINING AZIMUTH OF A BOREHOLE VERFAHREN ZUR BOHRLOCHAZIMUTBESTIMMUNG PROCEDE DE DETERMINATION DE L'AZIMUT D'UN TROU DE FORAGE |
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| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[0001] 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. 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 measured local magnetic field includes, apart from the Earth magnetic field components, 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.
[0002] EP-A-0 193 230 discloses a method of determining azimuth of a borehole formed 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 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 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 very sensitive to inaccuracies in the Earth magnetic field data, especially in case of highly inclined boreholes extending substantially in east or west direction.
[0003] US 5 435 069 discloses 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;
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;
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 borehole azimuth, wherein step c) comprises solving a set of unknown parameters from a set of equations.
[0004] 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 boreholes extending substantially in east or west direction.
[0005] 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 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 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 borehole azimuth.
[0006] The contribution from the earth magnetic field to each measured component along the axis of selected orientation is different for 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 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 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 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 locations. An example of such determination is presented in the detailed description below.
[0007] 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. It is to be understood that the contribution from the 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 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).
[0008] 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.
[0009] For most applications it is sufficient to select two said locations of different borehole inclination.
[0010] For enhanced accuracy in applying the method of the invention, the borehole inclinations at at least two of said locations differ from each other by an angle of at least 40°.
[0011] In case the borehole inclination at a first one of the locations is less than 45° it is preferred to 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 locations, e.g. due to different Bottom Hole Assemblies, the borehole inclination angle at the second location is suitably between 80°-100°.
[0012] Conversely, in case the wellbore inclination at the first location exceeds 45° 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. If furthermore the drill string magnetisation at the first location is different than at the second location, the borehole inclination angle at the second location is suitably between 0° and +10°.
[0013] 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 (N,E,V) coordinate system;
Fig. 2 shows a 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 (x,y,z).
[0014] 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 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.
[0015] 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. Bv represents the vertical vector component of the earth magnetic field and Bn.cos A is the projection of the horizontal component of the earth magnetic field on line H.
[0016] 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 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 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 α 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 (x, y, z) co-ordinate system.
[0017] 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 the drill string is kept stationary. The tool-face angle α and the inclination angle I are determined from the equations:
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 α, 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, Cz) the local magnetic field vector isThe 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
The sum of the vertical components of BHSe and Bze is equal to the vertical component Bv of the magnetic field (BHSRe has no vertical component), thus yielding
and from eq. (5)
By operating the magnetometer tool at two borehole locations with different inclinations I1 and I2 two local magnetic field vectors (BHS1, BHSR1, Bz1) and (BHS2, BHSR2, Bz2) are obtained, and from eq. (6) it follows
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:
Equations (7), (8), (9) contain the unknowns Bv, Cz1 and Cz2. The inclinations I1 and I2 are known from measurements using one or more accelerometers included in the drill string. It is found that
from which Cz is determined.
The local magnetic field at each point can now be corrected for axial drill string magnetisation.
[0018] The above approach is preferred for low borehole inclinations, i.e. inclinations less than 45°, because Cz then is relatively insensitive to variations in borehole inclination.
[0020] 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
or
For two points with respective inclinations I1, I2 and azimuth A1, A2 it follows that
The HSR components of the local magnetic field, corrected for cross-axial drill string magnetisation as described above, for the two points are
From eqs. (13) - (16), and with Cz1 = Cz2 = Cz (e.g. for unchanged BHA), it follows that
[0021] 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 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.
[0022] If different BHA's are used during the measurements at the different survey points Cz1 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 90°, because then one of the components Cz1 cos I1 or Cz2 cos I2 in eqs. (7), (8) substantially vanishes.
a) selecting at least two locations (1, 2) along the borehole;
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 (B) along an axis having a selected orientation relative to the magnetometer tool, the local magnetic field including the earth magnetic field (Be) and a drill string magnetisation field (C);
c) determining from the measurements and from the selected borehole inclinations, a contribution from the drill string magnetisation field (C) to the measured components;
d) correcting the measurements for said contribution from the drill string magnetisation field (C); and
e) determining from the corrected measurements, the borehole azimuth (A), characterized in that the borehole has selected different borehole inclinations (I1, I2) at said at least two locations, wherein said component of the local magnetic field
(B) is the axial component of the local magnetic field, wherein for the borehole inclination
at a first one of said locations being less than 45°, step c) comprises determining
the contribution from axial component of the drill string magnetisation from the relationship:
and wherein for the borehole inclination angle at the first location exceeding 45°,
step c) comprises determining the axial component of the drill string magnetisation
from the relationship:
in which relationship the subscripts x, y, z indicate components in a tool fixed
coordinate system, and the subscripts HS, HSR, z indicate components in a borehole
fixed coordinate system.
a) Auswählen zumindest zweier Orte (1, 2) entlang des Bohrloches;
b) Anordnen des Bohrstranges im Bohrloch für jeden ausgewählten Ort, derart, daß das Magnetfeldstärke-Meßgerät an dem ausgewählten Ort angeordnet ist, und Betätigen des Magnetfeldstärke-Meßgerätes, um eine Komponente des örtlichen Magnetfeldes (B) entlang der Achse mit einer ausgewählten Orientierung bezüglich des Magnetfeldstärke-Meßgerätes zu messen, wobei das örtliche Magnetfeld das Erdmagnetfeld (Be) und ein Bohrstrangmagnetisierungsfeld (C) umfaßt;
c) Bestimmen eines Anteiles des Bohrstrangmagnetisierungsfeldes (C) an den gemessenen Komponenten aus den Messungen und aus den ausgewählten Bohrlochneigungen;
d) Korrigieren der Messungen für den Anteil des Bohrstrangmagnetisierungsfeldes (C); und
e) Bestimmen des Bohrlochazimutes (A) aus den korrigierten Messungen, dadurch gekennzeichnet, daß das Bohrloch an den zumindest zwei Orten ausgewählte unterschiedliche Bohrlochneigungen
(I, I2) aufweist, wobei die Komponente des örtlichen Magnetfeldes (B) die axiale Komponente
des örtlichen Magnetfeldes ist, wobei der Schritt c) für die Bohrlochneigung kleiner
als 45° an dem ersten der Orte das Bestimmen des Anteiles der axialen Komponente der
Bohrstrangmagnetisierung aus der Beziehung umfaßt:
wobei in der Beziehung die Indizes x, y, z Komponenten in einem gerätefesten Koordinatensystem angeben und die Indizes HS, HSR, z Komponenten in einem bohrlochfesten Koordinatensystem angeben.
a) sélectionner au moins deux emplacements (1, 2) le long du puits foré,
b) en chaque emplacement sélectionné, agencer l'ensemble de tubes de forage dans le puits foré de manière à positionner l'outil de magnétométrie à l'emplacement sélectionné et faire fonctionner l'outil de magnétométrie de façon à mesurer la composante du champ magnétique local (B) le long d'un axe qui présente une orientation sélectionnée par rapport à l'outil de magnétométrie, le champ magnétique local comprenant le champ magnétique terrestre (Be) et le champ magnétique (C) de l'ensemble de tubes de forage,
c) à partir des mesures et des inclinaisons sélectionnées du puits foré, déterminer la contribution du champ magnétique (C) de l'ensemble de tubes de forage aux composantes mesurées,
d) éliminer des mesures ladite contribution du champ magnétique (C) de l'ensemble de tubes de forage et
e) déterminer l'azimut (A) du puits foré à partir des mesures corrigées,
caractérisé en ce que le puits foré présente des inclinaisons (I1, I2) sélectionnées différentes en au moins deux emplacements, ladite composante du champ magnétique local (B) étant la composante axiale du champ magnétique local, l'inclinaison du puits foré en un premier desdits emplacements étant inférieure à 45°, l'étape c) comprenant la détermination de la contribution de la composante axiale de la magnétisation de l'ensemble de tubes de forage à partir de la relation :et en ce que, lorsque l'angle d'inclinaison du puits foré au premier emplacement dépasse 45°, l'étape c) comprend la détermination de la emplacement dépasse 45°, l'étape c) comprend la détermination de la composante axiale de la magnétisation de l'ensemble de tubes de forage à partir de la relation :
dans laquelle les indices x, y et z représentent les composantes dans un système de coordonnées fixé sur l'outil et les indices HS, HSR et z représentent les composantes dans un système de coordonnées fixé sur le puits foré.