Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relat~s to the surveying of
boreholes, and i5 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 down-
hole 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. ~lowever,
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.
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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
another method 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 which 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
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whose direction does not change significan~ly 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.
SUMMARY OF THE INVENTION
~ccording 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
while the drill collar is maintained at a common
measuremsnt 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 interfarence 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
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measurement apparatus for 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 supplying the measurements taken by the
detectors at the measurement location to permit the
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
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each instrument housing may also incorporate detectors for
taking magnetic measurements indicative of the components
o~ the magnetic field along two mutually transverse axes
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.
BRIEF DESCRIPTION OF ~HE DRAWINGS.
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.
DETAILED DESCRIPTION OF THE DRAWINGS.
Re~erring to Figure 1, a drill string 16
incorporates at one end a drilling ~it 10 and a non-
magnetic drill collar 12 and a set of magnetic collars 14
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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
1~.
As shown diagrammatically in Figure 2, the
surveying instrument 18 comprises a fluxgate section 22
consisting of three magnetometers for measuring the
components Bx, By and Bz of the magnetic field along three
mutually orthogonal axss, and an accelerometer section 24
consisting of three accelerometers for measuring the
components gx~ gy and gz of the gravitational field, the z
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
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 Bzl and Bz2 respectively.
The outputs Bx, Byl Bzl, Bz2, Gx, Gy and Gz are
in the form of proportional voltages which are applied to
a circuit processing unit 2~ comprising analogue to
digital converters. The circuit processing unit 26
provides outputs gx, gy and gz which are processed in a
computing unit 28 to yield values of inclination e and
2 ~ 3 1~ 3 ~ 4
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highside angle ~, 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 angle ~ . These computing operations may
be perfoxmed within the surveying instrument and the
computed values stored in a memory section 30.
Alternatively the outputs from the fluxgate and
acceleromater sections may be stored in the memory section
30 for processing in a separa~e computiny 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
measuraments are preferably made when the instrument is
stationary.
The azimuth angle ~ can be determined, for
example, using an expression basad on equation (14) of
U.K. Patent Spacification No. 1578053 where the value Bz
in that equation is repla~ed by a valua Hzt 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 ~ it is necessary to determine the
magnetic interference Bzi.
Figure 3 shows the positions of the magnetic
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detectors ~2 and 23 along the axis of the dr.ill collar 12
which has a predetermined length L. As is known from the
1983, IADC/SPE 11382 paper, "Calculation of NMDC Length
Required for Various Latitudes Developed from Field
Measurements of Drill String Magnetisation" by S. J.
Grindrod and J. M. Wol~f, 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 1~ 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 r1 and the detector 23 being at a distance r2
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
...... (1)
4~ r2
where B is expressed in Teslas and the pole strength is in
Webers.
30Thus the magnetic field or magnetic interference
BZli at the detector 22 due to the poles PU and PL is
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g
given by the expression:
Bzli _l U + i PL
4~ rl2 4~ ~L rl)2 ... (2~
and the magnetic intarference Bz2i at the detector 23 due
to the poles Pu and PL is given by the expression:
Bz2i = ¦ PU ¦ ¦ PL ¦
+
4~ r22 4~ (L-r2)2 ....~3)
It should be appreciated that the above expre.ssions 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 Bzl and Bz2 measured
by the detectors 22 and 23 by the following expressions:
¦ PU ¦ ¦ PL ¦
Bzt = Bzl
4~ rl2 4~ (L-r~ (4)
¦ PU ¦ ¦ PL ¦
25 Bzt = Bz2 ~ + ~ (5)
4~ r22 4~ (L-r2)2
Since the values of Ll, r1, r2 are known, the
pole strengths Pu and PL may be determined from the
measured values Bzl 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.
