Note: Descriptions are shown in the official language in which they were submitted.
81796801
USE OF INDEPENDENT MEASUREMENTS IN MAGNETIC RANGING
CROSS REFERENCE TO RELATED APPLICATIONS
loom This application claims priority to U.S. Provisional Patent
Application Serial No.
61/903,205, filed November 12, 2013.
FIELD OF THE INVENTION
[0002] Disclosed embodiments relate generally to drilling and surveying
subterranean
boreholes such as for use in oil and natural gas exploration and more
particularly to
methods for improving the accuracy of the relative location of two wells via
making
multiple independent magnetic ranging measurements between a twin well and a
target
well in a well twinning operation.
BACKGROUND INFORMATION
[0003] Magnetic ranging techniques are commonly utilized in twin well
drilling
applications. For example, in steam assisted gravity drainage (SAGD)
applications twin
horizontal wells having a predetermined, consistent vertical separation
distance,
commonly in the range from about 4 to about 10 meters, are drilled. During
production
steam is injected into the upper well to heat the tar sand. The heated heavy
oil contained
in the tar sand and condensed steam is then recovered from the lower well. The
success of
such heavy oil recovery techniques is often dependent upon successfully
drilling precisely
positioned twin wells having a predetermined relative spacing in the
horizontal
injection/production zone. Positioning the wells either too close or too far
apart may
severely limit production, or even result in no production, from the lower
well.
[0004] Several magnetic ranging techniques have been used in SAGD well
twinning
operations. In general these techniques involve deploying a magnetic target
(source) in
one well and sensing the magnetic field emanating from the target in the other
well. The
intensity and direction of the measured field is then used to compute a
distance and
direction to the target. SAGD ranging operations have made use of various
magnetic
sources including active
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sources such as an AC solenoid or a DC electromagnet, and passive sources such
as
permanently magnetized casing. Active source methodology is designed to
implicitly remove
the effects of the earth field while explicit techniques are used for passive
ranging methods.
While these techniques have been used with varying degrees of success, there
is room for
further improvement.
[0005] For example, when using an AC solenoid, the magnetic field is
attenuated by the
casing in the target well. The extent of the attenuation is not generally
known with precision
and can be influenced by many factors including the electrical and magnetic
properties of the
casing, the casing geometry including the thickness and the slot
configuration, and the relative
position of the AC solenoid in the casing. As such the calculated distance to
the target is
prone to error.
100061 When using a DC electromagnet, multiple measurements are made at
different
source excitation states. Errors may arise if the magnetic sensors or the
electromagnet move
between acquisitions corresponding to different excitation states or if the
data acquisition
times are not correctly synchronized with respect to the excitation states.
Errors can also
result when using either an AC solenoid or a DC electromagnet from undetected
hardware
failure such as current leakage due to worn or damaged insulation.
[0007] When using magnetized casing, the distance between the wells may be
in error if the
intensity of the casing magnetization differs from expectation. Moreover, all
known ranging
techniques are subject to the risk of errors caused by human failure.
[0008] Therefore a need remains for a method to reduce errors during well
twinning
operations and to verify well placement during drilling such that the well
trajectory may be
corrected in real time.
SUMMARY
[0009] A method for magnetic ranging includes deploying a drill string
having a magnetic
field sensor in a drilling well and a magnetic ranging tool having at least
first and second
axially spaced electromagnets in a target well. A first magnetic field
measurement is made
when the first and second electromagnets are de-energized. A second magnetic
field
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81796801
measurement is made when the first electromagnet is energized and the second
electromagnet is de-energized. A third magnetic field measurement is made when
the first
electromagnet is de-energized and the second electromagnet is energized. The
magnetic
field measurements are processed to compute (i) a distance and/or a direction
from the
drilling well to the target well and (ii) a quality parameter.
loom A method for magnetic ranging and calibrating a premagnetized
casing string
includes deploying a drill string having a magnetic field sensor in a drilling
well and a
magnetic ranging tool having at least one electromagnet in a target well. The
target well
further includes a premagnetized casing string deployed therein. A first
magnetic field
measurement is made when the electromagnet is de-energized and a second
magnetic field
measurement is made when the electromagnet is energized. The first and second
magnetic
field measurements are then processed to compute a calibration factor for the
magnetized
casing string.
loom A method for computing magnetic pole strength for magnetized casing
includes
magnetizing a casing string and deploying the magnetized casing string in a
target well.
An axial component of a magnetic field internal to the magnetized casing
string is
measured and processed to compute the magnetic pole strength of the magnetized
casing
string.
loot fa] Some embodiments disclosed herein provided a method for magnetic
ranging
comprising: (a) deploying a drill string in a drilling well, the drill string
including at least
one magnetic field sensor; (b) deploying a magnetic ranging tool in a target
well, the
magnetic ranging tool including first and second axially spaced
electromagnets; (c)
making a first magnetic field measurement with the magnetic field sensor when
the first
and second electromagnets are de-energized; (d) making a second magnetic field
measurement with the magnetic field sensor when the first electromagnet is
energized and
the second electromagnet is de-energized; (e) making a third magnetic field
measurement
with the magnetic field sensor when the first electromagnet is de-energized
and the second
electromagnet is energized; and (f) processing the first, second, and third
magnetic field
measurements to compute (i) a distance and/or a direction from the drilling
well to the
target well and (ii) a quality parameter.
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[0011b] Some embodiments disclosed herein provided a method for magnetic
ranging and
calibrating a premagnetized casing string comprises: (a) deploying a drill
string in a
drilling well, the drill string including at least one magnetic field sensor;
(b) deploying a
magnetic ranging tool in a target well, the magnetic ranging tool having at
least one
electromagnet, the target well being cased with a premagnetized casing string;
(c) making
a first magnetic field measurement with the magnetic field sensor when the
electromagnet
is de-energized; (d) making a second magnetic field measurement with the
magnetic field
sensor when the electromagnet is energized; and (e) processing the first and
second
magnetic field measurements to compute a calibration factor for a magnetized
casing
string.
lootici Some embodiments disclosed herein provided a method for calibrating a
magnetized casing string comprising: (a) magnetizing a casing string; (b)
deploying the
magnetized casing string in a target well; (c) measuring an axial component of
a magnetic
field internal to the magnetized casing string as deployed in (b); and (d)
processing the
axial component of the magnetic field measured in (c) to compute a magnetic
pole strength
of the magnetized casing string.
[0011d] Some embodiments disclosed herein provided a method for magnetic
ranging
comprising: (a) deploying a drill string in a drilling well, the drill string
including at least
one magnetic field sensor; (b) deploying a magnetic ranging tool in a target
well, the
magnetic ranging tool including first and second axially spaced
electromagnets; (c)
making a first magnetic field measurement with the magnetic field sensor when
the first
electromagnet is energized with a first polarity and the second electromagnet
is de-
energized; (d) making a second magnetic field measurement with the magnetic
field sensor
when the first electromagnet is energized with a second polarity and the
second
electromagnet is de-energized; (e) making a third magnetic field measurement
with the
magnetic field sensor when the first electromagnet is de-energized and the
second
electromagnet is energized with a first polarity; (f) making a fourth magnetic
field
measurement with the magnetic field sensor when the first electromagnet is de-
energized
and the second electromagnet is energized with a second polarity; and (g)
processing the
first, second, third, and fourth magnetic field measurements to compute (i) a
distance
and/or a direction from the drilling well to the target well and (ii) a
quality parameter.
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[0012] This summary is provided to introduce a selection of concepts that
are further
described below in the detailed description. This summary is not intended to
identify key
or essential features of the claimed subject matter, nor is it intended to be
used as an aid in
limiting the scope of the claimed subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For a more complete understanding of the disclosed subject matter,
and
advantages thereof, reference is now made to the following descriptions taken
in
conjunction with the accompanying drawings, in which:
[0014] FIGS. 1A and 1B depict prior art arrangements for a well twinning
operation.
[0015] FIG. 2 depicts a magnetic ranging tool having first and second
electromagnets
(solenoids).
[0016] FIGS. 3A and 3B depict flow charts of related methods of magnetic
ranging with
DC magnetic source.
[0017] FIG. 4 depicts a flow chart of a method for calibrating pre-
magnetized casing.
[0018] FIG. 5 depicts a flow chart of a method for computing a pole
strength of pre-
magnetized casing.
[0019] FIG. 6 depicts a running tool for running casing into a wellbore.
DETAILED DESCRIPTION
[0020] FIG. 1A schematically depicts one example of a well twinning
operation such as
a SAGD twinning operation. Common SAGD twinning operations include drilling a
horizontal twin well 20 a substantially fixed distance substantially directly
above a
horizontal portion of the target well 30 (e.g., not deviating more than about
1-2 meters up
or down or to the left or right of the lower well). In the depicted
embodiment, the lower
(target) well 30 is drilled first, e.g., near the bottom of the oil-bearing
formation, using
conventional directional drilling and measurement-while-drilling (MWD)
techniques.
However, the disclosed embodiments are not limited in regard to which of the
wells is
drilled first. In the method shown on FIG. 1A, a high strength electromagnet
34 is
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81796801
positioned in the cased target well 30 while drilling the upper (twin) well
20. The
electromagnet 34 may be positioned in the cased target well 30 via, for
example, a tractor
32, coiled tubing (not shown), or on the end of a drill string (not shown). An
MWD tool
26 (including a tri-axial magnetic field sensor) deployed in the drill string
24 near drill bit
22 measures the magnitude and direction of the magnetic field. The solenoid
may be
energized with direct current (DC) or alternating current (AC). If the
excitation is DC, a
minimum of two measurements are required, the measurements corresponding to
different
excitation states. The difference between these two measurements is
independent of any
constant background magnetic field. The magnetic field measurements are
processed to
estimate the separation distance between the two wells and a direction from
the target well
30 to the twin well 20 as described in U.S. Patent 4,710,708 and U.S. Reissue
Patent
RE36,569.
[0021] FIG. 1B schematically depicts another example of a well twinning
operation. As
in the example depicted on FIG. 1A, the lower (target) well 30 is drilled
first using
conventional directional drilling and MWD techniques. The lower wellbore 30 is
then
cased using a plurality of premagnetized tubulars to form a magnetized casing
string 35.
In the embodiment shown, drill string 24 includes at least one ti-axial
magnetic field
measurement sensor 28 deployed in close proximity to the drill bit 22. Sensor
28 is used
to measure the magnetic field about target well 30 as the twin well is
drilled. Such
measurements of the passive magnetic field are then utilized to compute the
distance and
direction to the target well 30 and to guide continued drilling of the twin
well 20 along a
predetermined path relative to the target well 30 (e.g., as described in U.S.
Patents
7,617,049, 7,656,161, and 8,026,722).
[0022] FIG. 2 depicts a magnetic ranging tool 60 for use in a magnetic
ranging operation
such as depicted on FIGS. 1A and 1B. Magnetic ranging tool 60 includes first
and second
axially spaced solenoids (electromagnets) 64 and 66 deployed in a tool body
62. Each
solenoid 64 and 66 includes a solenoid winding wound about a substantially
cylindrical
magnetic core. The solenoids 64 and 66 may be configured to be energized in
such a
manner as to distinguish the magnetic field from each solenoid (e.g., the
first solenoid 64
may be energized while the second solenoid 66 is de-energized and visa a versa
or the
solenoids may be energized with AC at different frequencies). The solenoids 64
and 66
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are aligned with the tool axis 65 (and are therefore intended to be aligned
with the
borehole axis when deployed in a target well) and have an axial separation
distance d.
[0023] The
distance and direction between the twin well and target well may be
computed independent of the measured magnetic field intensity. U.S. Patent
8,063,641
discloses a method by which the direction of the measured magnetic field
induced by a
first solenoid, the direction of the measured magnetic field induced by a
second solenoid,
and the axial distance between the first and
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second solenoids may be used to compute the distance and direction between a
twin well and
a target well without relying on magnetic field strengths.
[0024] FIG. 3A depicts a flow chart of a possible sequence of measurements
demonstrating
one method 100 of magnetic ranging using magnetic ranging tool 60. Method 100
includes
making a first magnetic field measurement when both solenoids 64 and 66 are de-
energized
(off) at 102. A second magnetic field measurement is made at 104 when the
first solenoid 64
is energized (on) and the second solenoid 66 is de-energized. A third magnetic
field
measurement is made at 106 when the first solenoid 64 is de-energized and the
second
solenoid 66 is energized. The magnetic field measurements are processed at 108
to compute a
distance and/or a direction to the target well and a quality parameter.
Alternatively an AC
source may be used, in which case the need for a de-energized solenoid
measurement is
eliminated. Applicants note that numbering of the steps has been added for the
purpose of
annotating that a magnetic field measurement corresponds to a set of
conditions to which the
first and second solenoid are subjected and such numbering is not intended to
suggest the
order in which such magnetic field measurements must be taken. Indeed, such
magnetic field
measurements may be taken in any order.
[0025] FIG. 3B depicts a flow chart of another possible sequence of
measurements
demonstrating a second method 120 of magnetic ranging using magnetic ranging
tool 60. A
first magnetic field measurement is made at 124 when the first solenoid 64 is
energized with a
first polarity (e.g., positive) and the second solenoid 66 is de-energized. A
second magnetic
field measurement is made at 126 when the first solenoid 64 is energized with
a second
polarity (e.g., negative) and the second solenoid 66 is de-energized. A third
magnetic field
measurement is made at 128 when the first solenoid 64 is de-energized and the
second
solenoid 66 is energized with a first polarity (e.g., positive). A fourth
magnetic field
measurement is made at 130 when the first solenoid 64 is de-energized and the
second
solenoid 66 is energized with a second polarity (e.g., negative). Method 120
may include
another, fifth, magnetic field measurement when both solenoids 64 and 66 are
de-energized
(off) at 122. The magnetic field measurements are processed at 132 to compute
a distance
and/or a direction to the target well and a quality parameter. Applicants note
that numbering
of the steps has been added for the purpose of annotating that a magnetic
field measurement
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corresponds to a set of conditions to which the first and second solenoid are
subjected and
such numbering is not intended to suggest the order in which such magnetic
field
measurements must be taken. Indeed, such magnetic field measurements may be
taken in any
order.
[0026] Methods
100 and 120 permit the distance and direction from the twin well to the
target well to be computed by at least three independent ranging measurements.
For example,
the intensity and direction of the measured magnetic field when the first
solenoid is energized
and the second solenoid is de-energized may be processed to provide a first
estimate of the
distance and direction to the target well. The intensity and direction of the
measured
magnetic field when the second solenoid is energized and the first solenoid is
de-energized
may be processed to provide a second independent estimate of the distance and
direction to
the target well. In addition, the method of U.S. Patent 8,063,641 provides a
third independent
estimate of the distance and direction to the target well. Since the magnetic
sources are
independent (two distinct solenoids), they therefore provide a quality check
on errors which
may be present in only one of the sources, such as source placement under a
casing coupling
or insulation failure causing a partial short-circuit the solenoid. 1
[0027] All the
independent measurements described above have an associated level of
uncertainty. These uncertainties may be estimated, for example, from the noise
levels of the
various magnetic field measurements as well as from other systematic factors
associated with
each of the measurements. When multiple methods are utilized (as described
above), the
discrepancies between computed distances and directions may be compared with
the estimates
of the corresponding uncertainties. Such comparisons may be used to further
compute a joint
quality parameter, a single maximum-likelihood distance and direction, and/or
an estimate of
an improved uncertainty associated with the maximum-likelihood distance and
direction.
[0028] In one
embodiment, two independent measurements of distance D1 and D2 have
corresponding uncertainties described by variances 01 and 01. A maximum-
likelihood
distance Dmi, may be estimated, for example as follows:
D o-2+D a2
Diva = 22 22
Equation 1
0-1+0-2
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[0029] A corresponding joint variance o-41, may be estimated, for example
as follows:
2
2 1 2
CIML 2 2
Equation 2
al +02
[0030]
Moreover, differences between two or more calculated ranging distances may be
used as a quality check. For example, if one or more of the differences
exceeds a
predetermined threshold such as two or three standard deviations, it may be
concluded that the
accuracy of at least one of the measurements is compromised. A numerical
estimate of
confidence in the result may be derived using a statistical test of
significance, such as a two-
sample t-test. If the results fail to meet the required confidence, corrective
action may be
taken immediately, before the well is placed incorrectly.
[0031] The
following example of a positional uncertainty computation is based on an
approximation by a dipole with moment m under the assumption that the two
wells are
parallel. Radial and axial components Br and /3, of the measured magnetic
field may be
expressed, for example, as follows:
3mrz
Br = (r2 z2)2
Equation 3
s
r2-2z2
B =
Equation 4
z 0.2 +z2 )2.5
[0032] When Br
and Bz are measured during a ranging operation, the position vector (r, z)
may be found by inverting Equations 3 and 4. It may also be noted that in
practice the
problem is three-dimensional, and that a third magnetometer measurement is
obtained by
which the circumferential direction to the source (tool face to target) may be
ascertained.
[0033]
Sensitivity to position or source strength may be found by differentiating the
dipole
equations given in Equations 3 and 4, for example, as follows:
813r 3mz(z2-4r2)
¨ = _________________________________________________________________
Equation 5
Sr
(r2 z2)3 s
SBr 3mr(r2 ¨4z2)
Equation 6
(r2 +z2)3.5
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SBr 3rz
Equation 7
sm (r2+z2)25
SBz 3mr(4z2¨r2)
Equation 8
Sr (r2+z2)3
SBz 3mz(2z2-3r2)
Equation 9
sz (r2+z2)35
SBz (r2-2z2)
Equation 10
sin (r2+z2)25
100341 By assuming small errors and using linearized equations:
SBr SBr SBr
eBr = er ez ¨sz em ¨am Equation
11
SBz SBz SBz
eBz = er ¨sr + ez ¨sz emm¨sm Equation
12
where eBr and eBz represent the errors in the Br and Bz measurements. The
position errors er
and ez may also be expressed, for example, as follows:
SBr eBr sz-.- aBz, (8Br aBz aBz SBr)
eBz 5z ¨ern orn' oz ¨orn' sz
er = 8B 8B 8B 8B Equation
13
az Sr - Sr az
SBr ez aBz __ (SBr aBz aBz SBr)
eBz sr eBr--+
orem sm. or ¨am' sr = Equation
14
SE SB 8B 513
Sr az - az Sr
[0035] The
sensitivities of the calculated radial distance er to various measurement
errors,
may be expressed, for example, as follows:
8Bz
er(Br) az
= Equation 15
673 673 6B 6B
eBr
az Sr Sr az
SBr
er(Bz) = az
eBz Equation
16
673 613 6B 6B
T, T, z
az Sr Sr az
SBr.SBz aBz8Br
er (n) Sm Sz am az
em = Equation 17
SBr SBz aBr aBz
az Sr ¨ Sr az
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[0036]
Equations 15-17 provide a means by which the measurement errors eB, and eBz
and
a source strength error (or a source strength modelling error) ern may be
transformed into
corresponding position errors e, and ez. The position errors may be used to
compute a
covariance matrix for each independent error source with the overall
uncertainty being
expressed in terms of the sum of these covariance matrices.
[0037] When
ranging by using the directions of fields (U.S. Patent 8,063,641) received
from two transmitters, the response may be estimate in terms of tangents of
the flux angles,
for example using dipole approximations as follows:
t1 = gzi = r2-2zT
Equation 18
Br, 3rzi
Bz2 r 2 -24
t2 = ¨ =¨ Equation 19
Br2 3rz2
where z1 and z2 represent the axial positions of the magnetic field sensor
with respect to the
first and second solenoids.
100381 In this
configuration, the dipole equations may be inverted explicitly, for example,
as follows:
4dz
r =
Equation 20
3 [ti-sign(mBr4tT 4-t2 +sign(mBr2 )jql
d+2 +sign(mBr2)\iq
Z1 = ________________________________________________________________
Equation 21
ti+sign(insr,),1q t2 -sign(mBr2 )jq 4
where dz is the known spacing between the solenoids (i.e., z2 ¨ z1). In this
example, the
radial position uncertainty may be found via differentiation, for example, as
follows:
er(t,) -3r 2 1-sign(mBrõ)'t1
=
Equation 22
t1 4dz jtT.-q
er(t2) 3r2 1-sign(mBr2)'t2
__________________________ = _____________________________________ Equation
23
t2 4dz
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Err(dz) _ 4
Equation 24
az
n. (7.115:-1).6'+--t+3 ig ira (m.srz.i,v
[0039] FIG. 4 depicts a flow chart of a method 150 for magnetic ranging
and calibrating
a magnetized casing string such as depicted on FIG. 1B. While the magnetic
intensity of
the casing string may be measured at the surface, there is generally no
verification of the
intensity after the casing is deployed in the target well. Method 150 is
intended to verify
the magnetic intensity and/or provide a calibration factor for the magnetic
intensity of the
casing string. At 152 a magnetic ranging tool including at least one
electromagnet (e.g.,
ranging tool 60 depicted on FIG. 2) is deployed in a target wellbore having a
pre-
magnetized casing string. A first magnetic field measurement is made at 154
when the
electromagnet(s) in the ranging tool are de-energized (off). This is used in
to estimate the
target location using a passive ranging with magnetized casing methodology.
One or more
active ranging field measurements are made at 156 using the methodologies
described
above. The active and passive ranging distances are compared and are then
processed at
158 to compute a calibration factor for the passive ranging method. The
calibration factor
may include, for example, a correction factor for the magnetic pole strength
of the
magnetized casing such that the calibrated pole strength yields the same
distance and
direction as that obtained using the electromagnetic.
[0040] It will be understood that method 150 may be performed at multiple
locations in
the target well so as to obtain multiple correction factors. After obtaining a
calibration
factor (or factors) the remainder of the twin well may be drilled ranging only
to the
premagnetized casing. Moreover, if similar calibration factors are obtained
for several
wells in a given project (e.g., on a given pad) it may be possible to omit the
verification/calibration procedure on subsequent wells.
[0041] FIG. 5 depicts a flow chart of a method 180 for calibrating a
magnetized casing
string for use in subsequent magnetic ranging operations. The casing string
may be
magnetized at 182, for example, as described in U.S. Patent 7,538,650. The
external
magnetic field is characterized along with at least one axial magnetic field
measurement
internal to the magnetized casing. The casing string may be deployed in the
target well (or
a portion of the target well) at 184. After deployment in the target well, at
least one
magnetic field measurement is made internal to the
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magnetized casing string at 186. The magnetic field measurement may include
multiple
magnetic field measurements made at multiple axial positions in the magnetized
casing string.
An axial component of the internal magnetic field at 182 and 186 is processed
at 188 to
compute a calibrated magnetic pole strength of the magnetized casing. The
computed
magnetic pole strength may then be used in subsequent magnetic ranging
measurements to
compute at least a distance between a twin well and the magnetized target
well.
[0042] FIG. 6 depicts a running tool 200 (such as the Schlumberger Long
Reach Running
Tool) configured for running wellbore casing into a previously drilled
wellbore. The running
tool may be fitted with an extension tube 210 including an axial magnetometer
215. Running
tool 200 may be used to run magnetized casing into a target wellbore using
conventional
means and to measure the axial component of the magnetic field internal to the
magnetized
casing, for example, when drawing the running tool back out of the target
well. Such
measurements may be made with minimum disturbance to rig activities. The
running tool 200
may be configured to record the axial component of the magnetic field with
time while
drawing the tool back out of the target well. The recorded times may then be
converted to
depths and the magnetic field versus depth profile may be used to compute one
or more
magnetic pole strengths of the magnetized casing string. The use of running
tool 200 and
method 180 obviates the need to access the twin and target wells
simultaneously. Moreover,
no wireline tools are required.
[0043] With reference again to FIGS 2, 3A, and 3B, it will be understood
that the described
ranging measurements are not limited to the use of multiple solenoid
transmitters and a single
magnetometer receiver to provide independent estimates of distance and
direction.
Independent distance and direction measurements may also be obtained using
multiple axially
spaced magnetometer receivers in combination with a magnetic ranging tool
having a single
solenoid transmitter. Likewise, independent distance and direction
measurements may also be
obtained using a combination including multiple solenoid transmitters and
multiple
magnetometer receivers. The analysis provided above is substantially
independent of whether
multiple solenoids and/or multiple axially spaced magnetometers are utilized.
Hence, the
disclosed embodiments are not limited in this regard.
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[0044] Although the use of independent measurements in magnetic ranging
certain
advantages thereof have been described in detail, it should be understood that
various
changes, substitutions and alternations can be made herein without departing
from the spirit
and scope of the disclosure as defined by the appended claims.
13
