U.S. patent number RE33,708 [Application Number 07/617,410] was granted by the patent office on 1991-10-08 for surveying of boreholes using shortened non-magnetic collars.
This patent grant is currently assigned to Baroid Technology, Inc.. Invention is credited to Richard F. Roesler.
United States Patent |
RE33,708 |
Roesler |
October 8, 1991 |
Surveying of boreholes using shortened non-magnetic collars
Abstract
.[.When surveying a borehole using an instrument responsive to
the earth's magnetic field, a length of non-magnetic drill collar
is necessary to house means for measuring the magnetic field in the
borehole perpendicular to the direction of the borehole axis. The
instrument determines the inclination angle and the highside angle
from the gravitation measurements, with these measurements and the
magnetic measurements, the azimuth angle is determined. Using the
method of this invention a minimum length of non-magnetic material
necessary for an accurate measurement may be calculated and
used..]. .Iadd.Disclosed are method and apparatus for surveying a
borehole including use of a survey instrument for making
gravitational measurements from which the inclination and highside
angles of the instrument may be determined. Measurements of two
components of the local magnetic field perpendicular to the
longitudinal axis of the instrument may be sensed with the
instrument, and may be used to determine the azimuth angle of the
instrument under the assumption that magnetic interference due to
the pipe string in which the instrument is located lies solely
along the longitudinal axis of the instrument. The accuracy of the
azimuth determination may be enhanced by an iteration process. To
the extent that the pipe string interference includes transverse
field components at the instrument, the sensors of the instrument
may be separated from such pipe string members by placing the
instrument in non-magnetic material whose minimum length may be
determined. .Iaddend.
Inventors: |
Roesler; Richard F. (Houston,
TX) |
Assignee: |
Baroid Technology, Inc.
(Houston, TX)
|
Family
ID: |
27414613 |
Appl.
No.: |
07/617,410 |
Filed: |
November 21, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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892502 |
Aug 1, 1986 |
|
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Reissue of: |
515716 |
Jul 20, 1983 |
04510696 |
Apr 16, 1985 |
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Current U.S.
Class: |
33/304;
33/313 |
Current CPC
Class: |
E21B
47/022 (20130101); E21B 17/16 (20130101) |
Current International
Class: |
E21B
17/16 (20060101); E21B 47/02 (20060101); E21B
47/022 (20060101); E21B 17/00 (20060101); E21B
047/022 () |
Field of
Search: |
;33/313,304,312,303,302 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Composite Catalog of Oil Field Equipment & Services", vol.
3--31st Revision, 1974-1975, p. 1825. .
Answer and Counterclaims by Drexel Instruments, Inc. and Sharewell,
Inc. to Plaintiff's Second Amended Complaint. .
Answer by Drexel Equipment (U.K.) Ltd. to Plaintiff's Second
Amended Complaint. .
Encyclopedia Britannica, vol. 10, Game Birds to Gun Metal, p. 171,
published 1960. .
"Magnetic Compasses and Magnetometers" by Alfred Hine; pp. 1-7 and
105-107, published 1968, London. .
"Magnetic Well Surveying Compasses" (six pages), Sperry-Sun Well
Surveying Company dated 12-15-56. .
Smith International, "Non-Magnetic Collar Length Selection Chart"
(two pages), dated 1976. .
"Determining Downhole Magnetic Interference on Directional Surveys"
by A. C. Scott et al., SPE 7748, 1979 (six pages). .
"Borehole Position Uncertainty-Analysis of Measuring Methods and
Derivation of Systematic Error Model" by Wolff et al, pp.
2339-2350; 1981, Society of Petroleum Engineers of AIME. .
"Magnetic Properties of Nonmagnetic Drill Collars and Their
Relation to Survey Compass Error" by Edward et al, pp. 229-241;
Geoexploration, 17 (1979). .
"Determining Nonmagnetic Survey Collar Requirements", World Oil,
May 1976, vol. 182, No. 6, pp. 79, 80. .
Sperry-Sun, "Guide for Selecting Non-Magnetic Drill Collars" (two
pages)..
|
Primary Examiner: Little; Willis
Parent Case Text
.Iadd.This is a continuation of co-pending application Ser. No.
06/892,502 filed on Aug. 1, 1986, now abandoned; which is a reissue
of Ser. No. 06/516,716 filed 7/20/83, now U.S. Pat. No. 4,510,696.
.Iaddend.
Claims
What is claimed is: .[.1. A system for determining the orientation
of a downhole instrument positioned in a drill collar in a borehole
comprising: a means for determining inclination angle of the
instrument at a location thereof in said borehole; a means for
determining the highside angle of said instrument at said location;
a means for determining the true horizontal and vertical components
of the earth's magnetic field at the location of the borehole; a
means for determining components of the local magnetic field
perpendicular to the direction of a primary axis of the instrument
aligned with the borehole at said location, said drill collar being
constructed of non-magnetic material, and having a minimum length,
L, determined from the equation: ##EQU13## where P.sub.u is the
magnetic pole created by the magnetic material above the sensor,
P.sub.L is the magnetic pole created by the magnetic material below
the sensor, d is the displacement of the poles P.sub.u and P.sub.L
from the axis of the instrument, B.sub.n is the North component of
the earth's magnetic field at the tinstrument, and .delta..psi. is
the error
in the azimuth angle..]. .[.2. The orientation system of claim 1
wherein said means for determining the components of local magnetic
field comprises a means for sensing measured components of said
local magnetic field, said sensing means being located at least one
third of said length of said drill collar from an end of said drill
collar..]. .[.3. The orientation system of claim 1 wherein said
instrument is located in a drill string extending in said borehole,
said system being located between the lower drill string end
connecting to the drill bit and an upper drill string end
connecting to the surface..]. .[.4. The orientation system of claim
3 wherein said drill string is comprised of magnetic
material..].
.Iadd.5. A method of determining the orientation of a surveying
instrument in a borehole comprising the steps of:
a. determining the inclination angle of the instrument in the
borehole;
b. determining highside angle of the instrument in the
borehole;
c. determining two transverse components of the local magnetic
field perpendicular to the longitudinal axis of said instrument in
the borehole;
d. determining, without directly measuring, a value for the
component of the local magnetic field along the longitudinal axis
of the instrument in the borehole utilizing the inclination angle;
and
e. determining a value of azimuth angle of the instrument utilizing
the local magnetic field components, the inclination angle and the
highside angle, and without utilizing a directly measured value for
the local magnetic field component along the longitudinal axis of
said instrument.
.Iaddend. .Iadd.6. A method as defined in claim 5 further
comprising:
a. providing data indicative of the earth's magnetic field at said
borehole; and
b. using said earth's magnetic field data in the step of
determining a value for the component of the local magnetic field
along the longitudinal axis of the instrument in the borehole.
.Iaddend. .Iadd.7. A method as defined in claim 6 further
comprising:
a. utilizing the inclination angle, the highside angle, earth's
magnetic field data and said transverse components of the local
magnetic field perpendicular to the longitudinal axis of the
instrument to obtain an approximate value of azimuth angle;
b. using the approximate value of the azimuth angle also in
determining the value for the component of the local magnetic field
along the longitudinal axis of the instrument in the borehole;
and
c. so determining a more accurate value for azimuth angle.
.Iaddend.
.Iadd. . A method as defined in claim 7 comprising the additional
steps of using such more accurate value of azimuth angle as an
approximate value of azimuth angle to determine a further value for
the component of the local magnetic field along the longitudinal
axis of the instrument in the borehole, and determining a new more
accurate value of azimuth angle using said further value for the
component of the local magnetic field along the longitudinal axis
of the instrument as in claim 7. .Iaddend. .Iadd.9. A method as
defined in claim 8 comprising the additional steps of repeating the
steps of claim 8 until the obtained values of azimuth angle
converge to within an acceptable error. .Iaddend. .Iadd.10. A
method as defined in claim 6 wherein the earth's magnetic field
data is determined by utilizing sensing means included in such a
surveying instrument. .Iaddend. .Iadd.11. A method as defined in
claim 6 wherein the earth's magnetic field data is determined at
the surface of the earth in the vicinity of said borehole.
.Iaddend. .Iadd.12. A method as defined in claim 6 wherein the
earth's magnetic field data is determined in terms of horizontal
and vertical components of said field. .Iaddend. .Iadd.13. A method
as defined in claim 5 wherein said instrument is provided located
in a drill string in said borehole, said instrument being located
between the lower drill string end connecting to a drill bit and an
upper drill string end connecting to the
surface. .Iaddend. .Iadd.14. A method as defined in claim 5 further
including providing said surveying instrument positioned in
non-magnetic material having a length no shorter than a length L
determined by the equation: ##EQU14## .Iaddend. where P.sub.U is
the magnetic pole of magnetic material above said non-magnetic
material, P.sub.L is the magnetic pole of magnetic material below
said non-magnetic material, d is the displacement of the poles
P.sub.U and P.sub.L from the axis of the instrument, B.sub.n is the
north component of the earth's magnetic field at the borehole,
and
.delta..psi. is an acceptable error in the azimuth angle. .Iadd.15.
A method as defined in claim 14 wherein the transverse components
of the local magnetic field are determined by utilizing sensing
means included in such a surveying instrument and located at least
one third of said length of said non-magnetic material from an end
of said non-magnetic material.
.Iaddend. .Iadd.16. A method of determining the orientation of a
surveying instrument in a borehole, comprising the steps of:
a. determining the inclination angle of the instrument in the
borehole;
b. determining the highside angle of the instrument in the
borehole;
c. providing data indicative of the earth's magnetic field at the
borehole;
d. determining two transverse components of the local magnetic
field perpendicular to the longitudinal axis of the instrument in
the borehole;
e. calculating, without directly measuring, a value for the
component of the local magnetic field along the longitudinal axis
of the instrument in the borehole utilizing the earth's magnetic
field data; and
f. determining a value for the azimuth angle of the instrument
utilizing said local magnetic field components, the inclination
angle and the highside angle, and without utilizing a directly
measured value for the local magnetic field component along the
longitudinal axis of the
instrument in the borehole. .Iaddend. .Iadd.17. A method according
to claim 16 wherein said value for the component of the local
magnetic field along the longitudinal axis of the instrument is
determined also utilizing the inclination angle and an
approximation of the azimuth angle of the instrument. .Iaddend.
.Iadd.18. A method according to claim 17 wherein the approximation
of the azimuth angle of the instrument is determined from the
inclination angle, the highside angle, the transverse components of
the local magnetic field perpendicular to the longitudinal axis of
the instrument, and earth's magnetic field data. .Iaddend.
.Iadd.19. A method according to claim 17 comprising the additional
steps of determining a further value for the component of the local
magnetic field along the longitudinal axis of the instrument
utilizing the previously determined value for the azimuth angle,
and determining a new, more accurate value for the azimuth angle
utilizing said further value for the component of the local
magnetic field along the longitudinal axis of the instrument.
.Iaddend. .Iadd.20. A method according to claim 19 comprising the
additional steps of repeating the steps of claim 19 until the
obtained values of azimuth angle converge to within an acceptable
error. .Iaddend. .Iadd.21. A method according to claim 16 which
comprises:
a. utilizing the inclination angle, the highside angle, earth's
magnetic field data and the transverse components of the local
magnetic field perpendicular to the longitudinal axis of the
instrument to obtain an approximate value of the azimuth angle for
the instrument;
b. utilizing the inclination angle and said approximate value for
the azimuth angle also in determining said value for the component
of the local magnetic field along the longitudinal axis of the
instrument; and
c. so determining a more accurate value for the azimuth angle.
.Iaddend. .Iadd.22. A method according to claim 21 comprising the
additional steps of utilizing such more accurate value for the
azimuth angle as an approximate value for the azimuth angle to
determine a further value for the component of the local magnetic
field along the longitudinal axis of the instrument, and
determining a new, more accurate value for the azimuth angle using
said further value for the component of the local magnetic field
along the longitudinal axis of the instrument as in claim 21.
.Iaddend. .Iadd.23. A method according to claim 22 comprising the
additional steps of repeating the steps of claim 22 until the
obtained values of azimuth angle coverage to within an acceptable
error. .Iaddend. .Iadd.24. A method according to claim 16 wherein
the local magnetic field components are determined by utilizing
sensing means included in such a surveying instrument. .Iaddend.
.Iadd.25. A method according to claim 16 wherein the earth's
magnetic field data is determined by utilizing sensing means
included in such a survey instrument. .Iaddend. .Iadd.26. A method
according to claim 16 wherein the earth's magnetic field data is
determined at the surface of the earth in the vicinity of said
borehole. .Iaddend. .Iadd.27. A method according to claim 16
wherein said instrument is provided located in a drill string in
said borehole, said instrument being located between a lower drill
string end connecting to a drill bit and an upper drill string end
connecting to the surface. .Iaddend. .Iadd.28. Apparatus for
determining the orientation of a surveying instrument in a
borehole, comprising:
a. means for determining the inclination angle for the instrument
in the borehole;
b. means for determining the highside angle of the instrument in
the borehole;
c. means for storing data indicative of the earth's magnetic field
at the borehole;
d. means for determining two transverse components of the local
magnetic field perpendicular to the longitudinal axis of the
instrument in the borehole;
e. first calculating means for calculating, without directly
measuring, a value for the component of the local magnetic field
along the longitudinal axis of the instrument in the borehole
utilizing the earth's magnetic field data; and
f. second calculating means for calculating a value for the azimuth
angle of the instrument utilizing said local magnetic field
components, the inclination angle and the highside angle, and
without utilizing a directly measured value for the local magnetic
field component along the longitudinal axis for the instrument in
the borehole. .Iaddend.
Description
This invention relates to the surveying of boreholes and to the use
of a shorter nonmagnetic drill collar for housing the surveying
instrumentation. It is particularly concerned with the
determination of the azimuth angle of a borehole using a shorter
nonmagnetic drill collar.
At present "pivoted compass" single shot and multishot instruments
are used for determination of azimuth angle. However, with such
instruments, the necessary correction to compensate for the
modification of the earth's magnetic field in the vicinity of the
instruments can only be performed by assuming the size and
direction of the error field caused by the instrument, requiring a
knowledge of the magnetic moment of the compass magnet and using
instrumentation located in a nonmagnetic drill collar having a
minimum length of 30 feet and in some areas of the world, as much
as 120 feet. The procedure for determination of the azimuth angle
is necessarily empirical and use of the lengthy nonmagnetic collar
is troublesome.
In Russell et al., U.S. Pat. No. 4,163,324, there is disclosed a
method for determination of the azimuth angle of a borehole in
which it is assumed that the error vector which modifies the
earth's magnetic vector at the instrument is in the direction of
the borehole at the survey location. The instrument can be mounted
in a nonmagnetic housing in the form of a drill collar with the
other components of the drill string above and below the instrument
being typically constructed of magnetic materials. The effect of
this assumption is that the magnitude of the error vector can be
determined from the difference between the true and apparent values
of the components of the earth's magnetic field in a single
direction which is not perpendicular to the axis of the
borehole.
In the method of Russell et al. for determining the orientation of
the surveying instrument in the borehole, the steps include
determining the inclination angle of the instrument at the location
thereof in the borehole, sensing, at said location, at least one
vector component of the local magnetic field to determine the local
magnetic field in the direction of a primary axis of the instrument
aligned with the borehole, determining the azimuth angle of the
instrument relative to the apparent magnetic north direction at
said location, ascertaining the true horizontal and vertical
components of the earth's magnetic field at the location of the
borehole and determining the correction to be applied to the
apparent azimuth angle from the true and apparent values for the
horizontal and vertical components of the earth's magnetic
field.
According to the invention of this Application, there is provided
an improved method for determining the orientation of a surveying
instrument in a borehole including the steps of determining the
inclination angle .[.of the instrument at the location in the
borehole, determining the high side angle of the instrument at the
location, determining the true horizontal and vertical components
of the earth's magnetic field at the location, determining the
components of the local magnetic field perpendicular to the
longitudinal axis of the instrument at the location, determining
the azimuth angle for the instrument relative to the apparent
magnetic north direction at the location..]. .Iadd.and the highside
angle of the instrument in the borehole, determining two transverse
components of the local magnetic field perpendicular to the
longitudinal axis of the instrument in the borehole, determining a
value for the component of the local magnetic field along the
longitudinal axis of the instrument, and determining a value of
azimuth angle utilizing the local magnetic field components, the
inclination angle and the highside angle. The step of determining
the component of the local magnetic field along the longitudinal
axis of the instrument may be accomplished by utilizing the
inclination angle and data indicative of the earth's magnetic field
at the borehole. An approximate value of the azimuth angle may also
be utilized in determining a value for the component of the local
magnetic field along the longitudinal axis of the instrument, which
may then be used to determine a more accurate value for the azimuth
angle. Such an iteration process may be carried out until the
obtained values of azimuth angle converge to within an acceptable
error. .Iaddend.
The inclination and highside angle are preferably determined by
measuring the gravity vector at the instrument. This may be done
using three accelerometers which are preferably orthogonal to one
another and are conveniently arranged such that two of them sense
the components of gravity in the two directions that the fluxgates
sense the components of the local magnetic field.
.[.In another embodiment of this application, a system positioned
in a drill collar is disclosed for determining the orientation of a
downhole instrument in a borehole comprising: means for determining
inclination angle of the instrument at a location in the borehole;
means for determining the highside angle of the instrument at the
location; means for determining the true horizontal and vertical
components of the earth's magnetic field at the borehole; means for
determining two components of the local magnetic field
perpendicular to the direction of the longitudinal axis of the
instrument at the location, means for determining the azimuth angle
of the instrument relative to magnetic north direction at the
location, the drill collar being constructed of nonmagnetic
material, and having a minimum length, L, which is determined by:
##EQU1##
.Iadd.A survey instrument according to the present invention,
including means for determining the inclination angle and highside
angle of the instrument and means for determining components of the
local magnetic field perpendicular to the direction of the
longitudinal axis of the instrument, may be positioned within a
pipe string in a borehole and utilized to determine the orientation
of the instrument within the borehole by being placed within
non-magnetic material in the pipe string to allow for magnetic
field measurements. The non-magnetic material may be extended to a
determinable length to separate the survey instrument from magnetic
material in the pipe string for the purpose of avoiding magnetic
field components transverse to the longitudinal axis of the
instrument due to magnetic interference from pipe string members.
.Iaddend.
The determination of the azimuth angle of an instrument in a
borehole, in accordance with the invention, will now be described
in more detail with reference to the accompanying drawings in
which:
FIG. 1 is a schematic elevational view of a drill string
incorporating a survey instrument in accordance with the
invention.
FIG. 2 is a schematic perspective view illustrating a
transformation between earth-fixed axes and instrument-fixed
axes.
FIGS. 3 to 5 are diagrams illustrating, in two dimensions, the
various stages of the transformation shown in FIG. 2.
FIG. 6 is a block schematic diagram illustrating the instrument
shown in FIG. 1.
FIG. 7 illustrates typical error in calculated azimuth as a
function of collar length for the Gulf Coast region.
FIG. 8 is a schematic view of the survey instrument located in a
drilling collar.
Referring to FIG. 1, a drill string comprises a drilling bit 10
which is coupled by a nonmagnetic drill collar 12 and a set of
drill collars 14, which may be made of magnetic material, to a
drill string or pipe 16. The nonmagnetic drill collar 12 .[.of a
predetermined length.]. contains a survey instrument 18 in
accordance with the invention. As shown in FIG. 6, the survey
instrument 18 comprises a fluxgate section 22 and an accelerometer
section 24. The accelerometer section 24 comprises three
accelerometers arranged to sense components of gravity in three
mutually orthogonal directions, .[.once.]. .Iadd.one .Iaddend.of
which is preferably coincident with the longitudinal axis of the
drill string. The fluxgate section 22 comprises two fluxgates
arranged to measure magnetic field strength in two of the three
mutually orthogonal directions namely along axes OX and OY as will
be described with reference to FIG. 2. Additionally, the survey
instrument comprises associated signal processing apparatus as will
be described hereinafter with reference to FIG. 6.
The instrument sensors measure local field components within a
"nonmagnetic" drill collar 12 which is itself part of the drill
string, the collar being located close to the drilling bit 10. The
outputs from the two mutually orthogonal fluxgates comprise the
components B.sub.x and B.sub.y of the local magnetic field along
the axes OX and OY respectively. The outputs from the three
accelerometers in the accelerometer section 24 comprise the
components g.sub.x, g.sub.y, and g.sub.z of the local gravitation
field along the axes OX, OY and OZ.
The five output components g.sub.x,g.sub.y,.Iadd.g.sub.z,
.Iaddend.B.sub.x .[., .]. and B.sub.y .[.and By.]. are in the form
of proportional voltages which are applied to a circuit processing
unit 26 comprising analog to digital converters. The outputs
g.sub.x,g.sub.y, and g.sub.z from the analog to digital converters
in the circuit processing unit 26 are ultimately processed through
a digital computing unit 28 to yield values of highside angle .phi.
and inclination .theta.. This computing operation may be performed
within the survey instrument and the computed values stored in a
memory section 30 which preferably comprises one or more
solid-state memory packages. However, instead of storing four
values .phi., .theta., B.sub.x and B.sub.y it will usually be more
convenient to provide the memory section 30 with sufficient
capacity to store the five outputs from the analog to digital
converters in the circuit processing unit 26 and to provide the
computing unit 28 in the form of a separate piece of apparatus to
which the instrument is connected after extraction from the
borehole. Alternatively, the values may be directly transferred to
the surface units via conventional telemetry means (not shown).
The instrument 18 may also comprise a pressure transducer 32
arranged to detect the cessation of pumping of drilling fluids
through the drill string, this being indicative that the survey
instrument is stationary. The measurements are preferably made when
the instrument is stationary. Other means of detecting the
nonmovement of the instrument may be used such as motion
sensors.
Power for the instrument may be supplied by a battery power pack
34, downhole power generator or power line connected with a surface
power supply unit.
The preferred form of the invention, using two fluxgates and three
accelerometers as described above, has the advantage of not
requiring any accurately pivoted components, the only moving parts
being the proof masses of the accelerometers.
FIG. 2 shows a borehole 20 and illustrates various reference axes
relative to which the orientation of the borehole 20 may be
defined. A set of earth-fixed axes (ON, OE and OV) are illustrated
with OV being vertically down and ON being a horizontal reference
position. A corresponding instrument-case-fixed set of axes OX, OY
and OZ are illustrated where OZ is the longitudinal axis of the
borehole (and therefore of the instrument case) and OX and OY,
which are in a plane perpendicular to the borehole axis represented
by a chain-dotted line, are the two above-mentioned directions in
which the accelerometers and fluxgates are oriented.
A spatial survey of the path of a borehole is usually derived from
a series of measurements of an azimuth angle .psi. and an
inclination angle .theta.. Measurements of (.theta., .psi.) are
made at successive stations along the path, and the distance
between these stations is accurately known. The set of case-fixed
orthogonal axes OX, OY and OZ are related to an earth-fixed set of
axes ON, OE and OV through a set of angular rotations (.psi.,
.theta., .phi.). Specifically, the earth-fixed set of axes (ON, OE,
OV) rotates into the case-filled set of axes (OX, OY, OZ) via three
successive clockwise rotations; through the azimuth angle .psi.
about OV shown in FIG. 3; through the inclination angle .theta.,
about OE shown in FIG. 4; and through the highside angle .phi.,
about OZ shown in FIG. 5. .[.In.]. .Iadd.If .Iaddend.U.sub.N,
U.sub.E and U.sub.V are unit vectors in the ON, OE and OV
directions respectively, then the vector operation equation is:
which represents the transformation between unit vectors in the two
frames of reference (ONEV) and OXYZ) where: ##EQU2## The vector
operation equation for a transformation in the reverse direction
can be written as,
The computing operation performed by the computing unit 28 will now
be described. The first stage is to calculate the inclination angle
.theta. and the highside angle .phi.. Use of the vector operation
equation 5 to operate on the gravity vector; ##EQU3## yields
gravity components in the OXYZ frame
Thus, the highside angle .phi. can be determined from
.Iadd.and the inclination angle from ##EQU4## .Iaddend. The next
step is to obtain the value of B.sub.n and B.sub.v, the true
horizontal and vertical components of the earth's magnetic field,
respectively, from published geomagnetic survey data .Iadd.or
otherwise.Iaddend.. .[.If geomagnetic survey data is not available,
the probe itself.]. .Iadd.The probe itself, or a similar sensor
with at least one fluxgate or the like, .Iaddend.may be used to
measure B.sub.n and B.sub.v the measurement being made at a
location close to the top of the borehole but sufficiently remote
from any ferromagnetic structure which may cause the true earth's
magnetic field to be modified. .Iadd.By "true" is meant magnetic
measurements not influenced by magnetic material of the drill
string.
It will be appreciated that any data indicative of the earth's
magnetic field may be determined. For example, the combination of
the total magnetic field strength and the field dip angle is
equivalent to the combination of the north and vertical components
of the field (the east component is always zero). The present
calculations to obtain a correct azimuth may be effected in terms
of any equivalent field data. Also, where the survey instrument or
other sensor is used to determine "true" field data, the
measurements need not be absolute, that is, the fluxgates need only
be calibrated relative to each other. .Iaddend.
The azimuth angle, .psi., is calculated using an .[.iteration
loop.]. .Iadd.iterative procedure in which .Iaddend.the input
values .[.being.]. .Iadd.are .Iaddend.the highside angle .phi.,
inclination angle .theta., and the magnetic field components
B.sub.x, B.sub.y, .Iadd.B.sub.v .Iaddend.and B.sub.n. The initial
value of azimuth angle, .[..theta.o.]. .Iadd..psi..sub.o .Iaddend.,
is calculated from ##EQU5## Successive values of azimuth angle,
.[..psi.n.]. .Iadd..psi..sub.n .Iaddend., may be used to determine
B.sub.z by equation:
Using B.sub.z, the azimuth angle, .psi., may be determined using
the equation ##EQU6## Equations (12) and (13) are convenient to
mechanize in a computing step until (.psi..sub.n+1 -.psi..sub.n)
approaches a small preselected value. Measurement of the local
magnetic and gravitational field components in the instrument
case-fixed frame thus provides sufficient information to determine
the azimuth value.
.Iadd.Measurements by the fluxgates must be made through
non-magnetic material. Consequently, the drill collar 12 in the
immediate vicinity of the fluxgate section 22 of the survey
instrument 18 must be made of non-magnetic material. The remainder
of the drill collar 12 and the drill string in general may be
constructed of magnetic material, and a correct value for azimuth
achieved with the foregoing method provided the effect of the
magnetic material on the fluxgate measurements lies only along the
longitudinal axis OZ of the survey instrument. This will be the
case provided the drill string members, such as drill collars and
drill pipe, which contribute to the measurable error in the
fluxgate measurements, are cylindrically symmetric, for example, so
that the magnetic poles of such members so interfering lie along
the longitudinal axis of the sensor instrument 18.
The source of the field of the magnetic material of a pipe string
member is distributed in an annular region which is the pipe or
drill collar itself; there is no source of magnetic field along the
axis of the pipe or drill collar, which is hollow. Any anisotropies
in the drill string member, for example due to lack of
concentricity between the inside diameter and the outside diameter
of the member, variation in the material density of the member,
etc., may, but won't necessarily, cause the effective magnetic pole
at the end of the member to be off-axis, resulting in transverse
field components along the longitudinal axis of the drill string at
the survey instrument. At some distance from the end of a magnetic
section of drill collar or drill pipe, for example many diameters
of the drill string member away, the fluxgates may nevertheless
sense only a point pole along the tool axis due to the magnetic
material if the transverse field due to the pipe string member is
sufficiently weak to be undetectable at such distance. However, if
a transverse field is generated by the drill string member, and if
the drill string member is sufficiently close to the sensor
instrument 18 that the fluxgates detect the transverse field, the
assumption that the magnetic flux influence due to the magnetic
material in the drill string lies only along the longitudinal axis
of the survey instrument fails.
To the extent that the drill string introduces field components in
a transverse direction, for example along one or both of the OX and
OY axes, measurable at the fluxgates, the value of the azimuth
determined by the foregoing method will be incorrect. However, the
correct azimuth may be determined by eliminating the transverse
field components due to the drill string from sensing by the
fluxgates. This can be done by separating the magnetic material in
the drill string from the fluxgates a sufficient distance so that
the fluxgates cannot detect the transverse field effects generated
by the magnetic material of the drill string. Such separation
between the fluxgates and the magnetic material of the drill string
may be achieved by lengthening the section of non-magnetic material
in which the sensor instrument 18 is located. The minimum length of
non-magnetic material, such as may be provided by the drill collar
12, that is necessary to prevent transverse magnetic fields from
destroying the validity of the assumption that the only field
effects due to the drill string lie along the longitudinal axis of
the sensor instrument 18 may be calculated. The length of non-
magnetic drill collar need to avoid error due to drill string
interference transverse to the longitudinal axis of the survey
instrument is small compared to that needed to avoid error in the
longitudinal direction without the method of the present invention.
.Iaddend.
The length of the nonmagnetic drill collar may be determined as a
function of the tolerable transverse error field B.sub.err, as
shown in FIG. 8 in which survey instrument 18 is located within the
drill collar 12 having a minimum length, L, and an outer diameter,
OD. The transverse field error will be created by the proximity of
the magnetic material in the drill string 16 above and the drill
collar or bit 10 below. The magnetic material of these two sources
will create poles, P.sub.U and P.sub.L, respectively. In the worst
case, the poles may be assumed to be displaced from center by
The transverse error field may be determined by ##EQU7## where
.eta. is the angle between the axis and the poles having a vertex
at the survey instrument 18. Therefore:
The error caused in the azimuth angle in radians is determined by
expanding the azimuth angle in a Taylor series as a function of the
transverse field (B.sub.t). ##EQU8## Therefore, the error in
azimuth, .delta..psi., is given by
By definition,
.Iadd.where B.sub.T is the earth's magnetic field strength.
.Iaddend.
Therefore:
B.sub.t is approximately constant between about 20,000 and 60,000
.mu.T as determined from (for example) pages 75-76 of the U.S.
Geological Survey publication by E. B. Fabiano, N. W. Peddie. D. R.
Barraclough and A. Zunde entitled "International Geomagnetic
Reference Field 1980: Charts and Grid Values".
From Equation (12),
Using average values, <B.sub.z /B.sub.t >.apprxeq.1, ##EQU9##
then
By definition, B.sub.err =(.delta.B.sub.t
/.delta..psi.).delta..psi.(21)
From equation (21)
From Equation (16), ##EQU10## Solving equation (23) for L,
##EQU11## For .[..vertline.P.sub.U .vertline.+.vertline..sub.L
.vertline.=2000 micro Webers.]. .Iadd..vertline.P.sub.U
.vertline.+.vertline.P.sub.L .vertline.=2000 micro Webers
.Iaddend.and a collar having an outer diameter of 71/2", d, from
equation (14), equals 0.013 in. Equation (14) may vary slightly
with configuration of collar.
For an acceptable error in azimuth angle, .psi., of 0.25 degrees in
the Gulf Coast, the minimum nonmagnetic collar length is
L=6.4 ft.
FIG. 7 illustrates the error incurred in the calculation of azimuth
angle as a function of collar length, L, for B.sub.n equals 25
micro Tesla, a value for the Gulf Coast region. As the length of
non-magnetic collar is increased, the extraneous transverse
magnetic field strength is reduced and the calculated azimuth
approaches the true azimuth.
Therefore a minimum L of between about 5 to 7 feet will result in a
calculated azimuth angle falling within the acceptable error region
of FIG. 7 for the Gulf Coast. Other collar lengths will be
calculated accordingly for different regions, collar configuration
and outside diameter.
Using this determination, a system of this invention for
determining the orientation of a downhole instrument in a borehole
would comprise a means for determining inclination angle of the
instrument at a location thereof in said borehole; a means for
determining the highside angle of said instrument at said location;
.[.a means for determining the true horizontal and vertical
components of the earth's magnetic field at the location of the
borehole;.]. a means for determining components of the local
magnetic field perpendicular to the direction of a primary axis of
the instrument aligned with the borehole at said location, said
drill collar being constructed of non-magnetic material, and having
a minimum length, L, determined as follows: ##EQU12##
Numerous variations and modifications may obviously be made in the
apparatus herein described without departing from the present
invention. Accordingly, it should be clearly understood that the
forms of the invention described herein and shown in the figures of
the accompanying drawings are illustrative only and are not
intended to limit the scope of the invention.
* * * * *