GB2241583A

GB2241583A - Determination of magnetic interference in a borehole

Info

Publication number: GB2241583A
Application number: GB9004812A
Authority: GB
Inventors: David Morgan; Anne Holmes
Original assignee: Baroid Technology Inc
Current assignee: Baroid Technology Inc
Priority date: 1990-03-03
Filing date: 1990-03-03
Publication date: 1991-09-04
Also published as: FR2659145A1; NL9100343A; NO910824D0; DE4106849A1; NO910824L; GB9004812D0; CA2037394A1

Abstract

To determine the magnetic interference within a non-magnetic drill collar 12 disposed in a drill string within a borehole due to the presence of magnetised sections of the drill string above and below the drill collar 12, two magnetic detectors 22 and 23 are spaced apart along the axis of the drill collar 12. Magnetic measurements of component of the magnetic field along the axis of the drill collar are then taken with the detectors 22 and 23, while the drill collar 12 is maintained at a common measurement location within the borehole. The magnetic interference due to the magnetised sections of the drill string above and below the drill collar 12 is then calculated from inter alia the measurements taken by the detectors 22 and 23 at the measurement location. Magnetic measurements of magnetic field components perpendicular to the axis and measurements of gravity may also be made. Both detectors are preferably fluxgate magnetometers and are located between the midpoint M and one end PL of the drill collar. <IMAGE>

Description

"Determination of Magnetic Interference in a Boreholell This invention

relates to the surveying of boreholes, and is more particularly concerned with methods of, and apparatus for, determining the magnetic interference within a non-magnetic drill collar disposed in a drill string within a borehole due to the presence of magnetised sections of the drill string above and below the drill collar.

It is well known to survey boreholes which are not cased with a steel lining by making measurements downhole using a three-axis magnetometer and a three-axis accelerometer package within a surveying instrument, so as to determine a series of parameters, such as the inclination angle and the azimuth angle, indicative of the directivity at locations along the borehole. However, the determination of the azimuth angle is based on measurements of the earth's magnetic field made by the magnetometer and these are rendered inaccurate by the presence of perturbing magnetic fields associated with magnetised sections of the drill string both above and below the surveying instrument. The effect of such magnetic interference can be minimised by disposing the instrument in a drill collar made of non-magnetic material extending for some distance both above and below the measuring location. However, the provision of sufficiently long non- magnetic drill collars to ensure high accuracy measurement is costly and is not practicable in some cases.

U.K. Patent Specification No. 1578053 describes a method enabling a high accuracy survey to be conducted using a surveying instrument disposed in only a relatively short non-magnetic drill collar in which a correction is applied to the measured azimuth angle to compensate for the effect of the perturbing magnetic fields. On the basis that the effect of the perturbing magnetic fields can be considered as an error vector in the direction of the axis of the borehole, the correction is determined as a function of the true earth's magnetic field, as ascertained from look- up tables for example, the local magnetic field, as measured by the surveying instrument, and values for the inclination angle and the apparent azimuth angle determined from the measured magnetic field and gravitational field values.

U.K. Patent Specification No. 2143644 describes of determining a correction to the azimuth angle to compensate for the magnetic interference. In this method the corrected azimuth angle is determined without directly measuring the component of the earth's magnetic field along the axis of the borehole. Furthermore U.S. Patent Specification No. 4709486 describes a further method in qhich the azimuth angle is determined directly from the magnetic and gravitational field measurements without correcting the magnetic field measurements. However all these methods can prove inaccurate under certain conditions. For instance, the last method cannot be used to survey a length of borehole another method 1 whose direction does not change significantly along its length. Also significant inaccuracies can result, due to factors such as misalignment, bias and scale factor errors, when a near horizontal length of borehole is being surveyed.

It is an object of the invention to provide a method which can be used to eliminate major inaccuracies under most conditions.

According to the present invention there is provided a method of determining the magnetic interference within a non- magnetic drill collar disposed in a drill string within a borehole due to the presence of magnetised sections of the drill string above and below the drill collar, the method comprising spacing apart two magnetic detectors along the axis of the drill collar so that each detector lies at an appropriate position on the axis of the drill collar, taking measurements with the detectors is maintained at a common measurement location within the borehole such that each measurement is indicative of the component of the magnetIc field along the axis of the drill collar at a respective position corresponding to the position of the associated detector in that measurement location, and calculating the magnetic interference due to the magnetised sections of the drill string from the measurements taken by the detectors at the measurement location.

The invention also provides down-hole measurement apparatus for use in determining the magnetic while the drill collar the drill collar assembly adapted drill collar for interference within a non-magnetic drill collar disposed in a drill string within a borehole due to the presence of magnetised sections of the drill string above and below P the apparatus comprising a housing to be positioned coaxially within the the purpose of taking magnetic measurements at a measurement location within the borehole, two magnetic detectors within the housincl assembly spaced apart assembly so that each position on the axis measurements are to be and each detector is along the axis of the housing detector lies at an appropriate of the drill collar when magnetic taken at the measurement location arranged to take a respective magnetic measurement indicative of the component of the 15 magnetic field along the axis of the drill collar at a respective position corresponding to the position of the detector in the measurement location, and measurement supply means for supplying the measurements taken by the detectors at the measurement location to permit the 20 magnetic interference due to the magnetised sections of the drill string to be calculated from these measurements. The two magnetic detectors are preferably fluxgate magnetometers, and are -preferably disposed in a common instrument housing constituting the housing assembly. However, the magnetic detectors may alternatively be disposed in separate instrument housings which together constitute the housing assembly. The or each instrument housing may also incorporate detectors for i -5 taking magnetic measurements indicative of the components of the magnetic field along two mutually transverse axis extending transversely to the axis of the drill collar,' and the or each instrument housing may also incorporate detectors for taking gravitational measurements indicative of the components of the gravitational field along two or three mutually transverse axes.

Furthermore it is preferred that the magnetic detectors are spaced apart in a region which is between, and spaced from, one axial end of the drill collar and the midpoint between the two axial ends of the drill collar.

In order that the invention may be more fully understood, a preferred embodiment in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows part of the drill string incorporating the drill collar within which the surveying instrument is located; Figure 2 is a block diagram of the surveying instrument; and Figure 3 is a diagram illustrating the positions of the magnetic detectors within the drill collar.

Referring to Figure_ 1, a drill string 16 incorporates at one end a drilling bit 10 and a non- magnetic drill collar 12 and a set of magnetic collars 14 by means of which the drilling bit 10 is coupled to the main portion of the drill string 16. The non-magnetic drill collar 12 is of a predetermined length and contains a surveying instrument 18 (shown in broken lines in Figure As shown diagrammatically in Figure 2, the surveying instrument 18 comprises a fluxgate section 22 consisting of three magnetometers for measuring the components BXY B y and BZ of the magnetic field along three mutually orthogonal axes, and an accelerometer section 24 consisting of three accelerometers for measuring the components gx,, gy and gz of the gravitational field, the z 10 axis in each case being along the axis of the borehole. In addition a further fluxgate section 23 is provided spaced apart along the axis of the drill collar 12 from the fluxgate section 22. This fluxgate section 23 may consist simply of a magnetometer for measuring the BZ 15 component of the magnetic field, or alternatively it may consist of three magnetometers in a similar manner to the fluxgate section 22. In order to distinguish the BZ outputs from the fluxgate sections 22 and 23, these will be referred to hereafter as Bz, and Bz2 respectively. 20 The outputs Bx, By, Bzj, Bz2, Gx, GY and GZ are in the form of proportional voltages which are applied to a circuit processing unit 26 comprising analogue to digital converters. The circuit processing unit 26 provides outputs gx, gy and gz which are processed in a 25 computing unit 28 to yield values of inclination e and highside angle D, in accordance with the expressions given in U.K. Patent Specification No. 1578053. Furthermore the outputs from the circuit processing unit 26 may be processed in the computing unit 28 so as to yield a value of the azimuth angle4r. These computing operations may be performed within the surveying instrument and the computed values stored in a memory section 30.

Alternatively the outputs from the fluxgate and accelerometer sections may be stored in the memory section 30 for processing in a separate computing unit 28 at the surface. As a further alternative, the outputs from the fluxgate and accelerometer sections may be transmitted directly to the surface by telemetry. All the measurements are preferably made when the instrument is stationary.

The azimuth angle can be determined, for example, using an expression based on equation (14) of U.K. Patent Specification No. 1578053 where the value Bz in that equation is replaced by a value Bzt which corresponds to the true value of the z component of the earth's magnetic field taking into account an error in the measured value because of the magnetic interference Bzi due to the presence of the magnetised sections of the drill string both above and below the drill collar 12. Thus, in order to determine an accurate value for the azimuth angle n it is necessary to determine the magnetic interference Bzi- Figure 3 shows the positions of the magnetic detectors 22 and 23 along the axis of the drill collar 12 which has a predetermined length L. As is known from the 1983, IADC/SPE 11382 paper, "Calculation ofNMDC Length Required for Various Latitudes Developed from Field Measurements of Drill String Magnetisation" by S. J. Grindrod and J. M. Wolff, the effect of the magnetic drill string sections above and below the drill collar 12 may be considered to be represented by an upper magnetic pole PU at the upper end of the drill collar 12 and a lower magnetic pole PL at the lower end of the drill collar 12. The magnetic detectors 22 and 23 are spaced apart along the drill collar axis in the region which is between, and spaced from, the pole PL at the lower end of the drill collar 12 and the mid-point M between the two axial ends of the drill collar 12, the detector 22 being at a distance r, and the detector 23 being at a distance r.) from the pole PU.

As is known from the above paper, the magnetic field strength (flux density) at a distance r from a magnetic pole having pole strength P is given by the expression:

B = P 4n r2 where B is expressed in Teslas and the pole strength is in Webers.

Thus the magnetic field or magnetic interference

Bzii at the detector 22 due to the poles PU and PL is given by the expression:

Bzli PU PL 4n r,2 1 4n (L-rj)2... (2) -g- and the magnetic interference Bz2i at the detector 23 to the poles PU and PL is given by the expression:

Bz2i 1 Pu 1 1 PL 1 4n r2 2 4n (L-r2)".... (3) It should be appreciated that the above expressions are simplified expressions obtained by simplifying a three- pole system to a two-pole system as described in the above paper.

Thus, the true earth's magnetic field component Bzt may be related to the components Bz, and Bz2 measured by the detectors 22 and 23 by the following expressions:

PU PL Bzt = BZ1 - 2 + 4n ri 4n (L-ri)l (4) Bzt = Bz2 - 1 Pu + 1 PL 1)2 4n r2 2 4n (L-r2 Since the values of Ll, rj, r2 are known, the pole strengths PU and PL may be determined from the measured values BZ1 and BZ2 using these expressions, and an iterative technique may be used to determine the magnetic gradient between the detectors 22 and 23 due to the poles PU and PL. The magnetic interference may be determined from this magnetic gradient.

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Claims

A method of determining the magnetic interference within a non-magnetic drill collar disposed in a drill string within a borehole due to the presence of magnetised sections of the drill string above and below the drill collar, the method comprising spacing apart two magnetic detectors along the axis of the drill collar so that each detector lies at an appropriate position on the axis of the drill collar, taking measurements with the detectors while the drill collar is maintained at a common measurement location within the borehole such that each measurement is indicative of the component of the magnetic field along the axis of the drill collar at a respective position corresponding to the position of the associated detector in that measurement location, and calculating the magnetic interference due to the magnetised sections of the drill string from the measurements taken by the detectors at the measurement location.
2. A method according to claim 1, wherein the two 20 magnetic detectors are disposed in a common instrument housing.
3. A method according to claim1 or 2, further comprising taking magnetic measur-o-ments indicative of the components of the magnetic field along two mutually transverse axes extending transversely to the axis of the drill collar.
4. A method according to claim 1, 2 or 3, further comprising taking gravitational measurements indicative of 1 1 the components of the gravitational field along two or three mutually transverse axes.
5. Down-hole measurement apparatus f or use in determining the magnetic interference within a non- magnetic drill collar disposed in a drill string within a borehole due to the presence of magnetised sections of the drill string above and below the drill collar, the apparatus comprising a housing assembly adapted to be positioned coaxially within the drill collar for the purpose of taking magnetic measurements at a measurement location within the borehole, two magnetic detectors within the housing assembly spaced apart along the axis of the housing assembly so that each detector lies at an appropriate position on the axis of the drill collar when magnetic measurements are to be taken at the measurement location and each detector is arranged to take a respective magnetic measurement indicative of the component of the magnetic field along the axis of the drill collar at a respective position corresponding to the position of the detector in the measurement location, and measurement supply means for su pplying the measurements taken by the detectors at the measurement location to permit the magnetic interference7 due to the magnetised sections.of the drill string to be calculated from these measurements.
6. Apparatus according to claim 5, wherein the two magnetic detectors are fluxgate magnetometers.
7. Apparatus according to claim 5 or 6, wherein the two magnetic detectors are disposed in a common instrument housing constituting the housing assembly.
8. Apparatus according to claim 5, 6 or 7, wherein, the housing assembly also incorporates detectors for taking magnetic measurements indicative of the components of the magnetic field along two mutually transverse axes extending transversely to the axis of the drill collar.
9. Apparatus according to claim 5, 6, 7 or 8, wherein the housing assembly also incorporates detectors for taking gravitational measurements indicative of the components of the gravitational field along two or three mutually transverse axes.
10. Apparatus according to any one of claims 5 to 9, wherein the magnetic detectors are spaced apart in a region which is between, and spaced from, one axial end of the drill collar and the midpoint between the two axial ends of the drill 'collar.
11. A method of determining the magnetic interference within a non-magnetic drill collar disposed in a drill string within a borehole due to the presence of magnetised sections of the drill string above and below the drill collar, the method being substantially as hereinbefore described with reference to the drawings.
12. Down-hole measurement apparatus for use in 25 determining the magnetic interference within a nonmagnetic drill collar disposed in a drill string within a borehole due to the present of magnetised sections of the drill string above and below the drill collar, the z 4 1 1 apparatus being substantially as hereinbefore described with reference to the drawings.

Published 1991 at The Patent Office. Concept House. Cardiff Road. Neuport. Gwent NP9 I RH. Further copies maybe obtained from Puhlished 199 1 at The Patent Ullice. eonrept tiouse. earain d. NCATOrt. (went NP9 1 RH. Further c6ptis ma be-66tiLtnecrtroin""