A SINGLE WIRE GUIDANCE SYSTEM FOR RANGING USING
UNBALANCED MAGNETIC FIELDS
Background
[0001] Magnetic ranging refers to well positioning that provides relative
direction and distance of one well with respect to another. Several technologies
for ranging from a ranging well to a remote casing in a target well are based
upon launching a current at a known frequency from a power supply at the
earth's surface down the casing of the target well and receiving a signal radiated
from that casing in the ranging well.
[0002] The power supply at the surface typically employs a cable coupled to a
weight bar (to provide downhole contact to the well casing) to deliver the current
downhole so that magnetic fields can be generated surrounding the target well.
The downhole contact between the weight bar and the casing results in the
current flowing uphole through the casing. Sensors in the ranging well (e.g.,
drilling well) may measure the magnetic fields so that distance and direction
between the target well and ranging well can be determined.
[0003] One problem with this method is that the current flowing uphole is in an
opposite direction to the cable current direction. The magnetic field generated by
each current flow has the effect of reducing the total magnetic field received at
the sensors in the ranging well. Thus, it may be difficult to measure the resulting
magnetic field in the ranging well.
Brief Description of the Drawings
[0004] FIG. 1 is a diagram of an example single wire guidance system
incorporating a spiral configuration, according to aspects of the present
disclosure.
[0005] FIG. 2 is a diagram of an example single wire guidance system
incorporating the cable terminated downhole on the outside of a target well
casing, according to aspects of the present disclosure.
[0006] FIG. 3 is a diagram of an example single wire guidance system
incorporating the spiral configuration on the outside of the target well casing,
according to aspects of the present disclosure.
[0007] FIG. 4 is a diagram of an example two cable system, according to
aspects of the present disclosure.
[0008] FIG. 5 is a diagram of an example shielded cable with a metal exterior
over an insulator, according to aspects of the present disclosure.
[0009] FIG. 6 is a diagram of an example shielded cable with a triangular metal
core, according to aspects of the present disclosure.
[0010] FIG. 7 is a diagram of an example shielded cable with an insulator
exterior, according to aspects of the present disclosure.
[0011] FIG. 8 is a diagram of an example shielded cable with a cylindrical
conductive material wrapped with an insulated wire, according to aspects of the
present disclosure.
[0012] FIG. 9 is a diagram of an example shielded cable with a rectangular
conductive material wrapped by an insulated wire, according to aspects of the
present disclosure.
[0013] FIG. 10 is a diagram of an example shielded cable apparatus for
implementing a method for ranging, according to aspects of the present
disclosure.
[0014] FIG. 11 is a plot showing the unbalanced magnetic field densities in the
x-direction and the z-direction in accordance with the shielded cable apparatus of
FIG. 10.
[0015] FIG. 12 is a plot showing the total magnetic field density at sensor point
P in accordance with the shielded cable apparatus of FIG. 10.
[0016] FIG. 13 is a diagram of an example wireline system embodiment,
according to aspects of the present disclosure.
[0017] FIG. 14 is a diagram of an example drilling rig system embodiment,
according to aspects of the present disclosure.
[0018] FIG. 15 is a flowchart illustrating an example method for ranging
between a target well and a ranging well using unbalanced magnetic fields,
according to aspects of the present disclosure.
Detailed Description
[0019] The embodiments described herein operate to provide information that
assists in determining relative distance and direction of a well being drilled near
at least one other well. For example, determining a location of a target well in
relation to a ranging well. The ranging well may also be referred to as the
drilling well.
[0020] A "target well" may be defined herein as a well, the location of which is
to be used as a reference for the construction of another well. The other well may
be defined as a "ranging well." Other embodiments may reverse this terminology
since the embodiments are not limited to any one well being the target well and
any one well being the ranging well. The ranging may be used in steam assisted
gravity drainage (SAGD), well intersection, relief well intersection, well
avoidance, or any other usage where ranging, maintaining, avoiding, or
intersecting between two wells is desirable.
[0021] As used herein, unbalanced magnetic fields are defined as two or more
magnetic fields that have a different field pattern. For example, the magnetic
fields may have different directions and/or different amplitudes.
[0022] The present embodiments generate unbalanced magnetic fields so that
Eq. (1) below will not be zero or too small to be measureable. One method for
generating the unbalanced magnetic fields includes introducing different
orientations of cable winding in or around the casing instead of a straight cable
along the wellbore, as illustrated in FIGs. 1-4 and discussed subsequently.
Another method includes introducing magnetic shielding (e.g., high permeability
mu-metal) in the cables, as illustrated in FIGs. 5-9 and discussed subsequently.
Other methods may combine the different orientations of cable windings with
the magnetic shielding. For example, the high permeability mu-metal may be
used in a spiral cable configuration. In another example, any of the cables of
FIGs. 5-9 may be used in any of the configurations of FIGs. 1-4.
[0023] To represent a magnetic field generated by a cable in a casing, the total
current flowing in the cable may be represented by Ic. The current flowing in the
casing back to ground may be represented by 1{(b) having an azimuthal angle b
with respect to the target well. Consequently, with a separation R between
sensors in the ranging well and the casing in the target well, the magnetic field H
received at the sensors may be expressed by
H = (1)
2 R
[0024] The cable current and the casing current are very similar due to the well
head ground, as described by
360°
(2)
o°
[0025] Part of the casing current may disappear or be reduced due to lossy pipe
properties and/or the current leaking to geological formations. In such situations,
I c n Eq. (2) is larger than the total casing current in all azimuthal directions. Eq.
(2) may then be representative of a very weak or no magnetic field at Eq. (1)
such that wireline sensors in the ranging well may not be able to measure the
field and determine a distance and/or direction to the target well during a
wireline operation.
[0026] FIG. 1 is a diagram of an example single wire guidance system
incorporating a spiral configuration, according to aspects of the present
disclosure This embodiment may use a typical cable 100 configured in a spiral
and/or one of the subsequently discussed magnetically shielded cables, for
example using high permeability mu-metals.
[0027] FIG. 1 shows a cable 100 spirally wound within a casing 102 of a target
well. The spiral cable 100 (e.g., solenoid cable) is terminated downhole at the
bottom of the casing 102 by a termination 107. The termination 107 may be a
weight bar that is in electrical contact with the casing 102 or some other
electrically conductive termination between the cable 100 and the casing 102.
[0028] The cable 100 is coupled to a power supply 110 on the surface of a
formation 130 through which the target well and a ranging well 103 are drilled.
The power supply 110 provides the current I c through the cable 100. The power
supply ground 111 may be grounded to a well head, which is electrically
connected to the casing, or to the geological formation 130. The termination 107
between the spiral cable 100 and the casing 102 results in casing current I t( )
that returns to the power supply ground 111.
[0029] The ranging well 103 may include sensors 105 (e.g., sensors included in
a wireline logging tool or included in a drill string, e.g. as part of a bottom hole
assembly (BHA)) to measure the magnetic field produced at the target well. The
sensors 105 may include triaxial magnetometers or gradient sensors. The sensors
105 are located a distance R (see Eq. (1)) from the target well spiral cable 100.
[0030] The spiral cable configuration 100 produces magnetic fields 1 1 in
different directions as compared to the magnetic fields 120 from the casing
current. FIG. 1 defines that the z-direction is along the wellbore and the xdirection
is in the direction from the drilling well to the target well. The
magnetic fields 120 at the sensors with respect to the casing current I t(p) will be
in the y-direction, whereas the magnetic fields 121 from spiral cabling 100 will
be in both the y and z-directions.
[0031] The y-directional field from the spiral cabling 100 is typically similar
but opposite in sign to the y-directional field 120 from casing current l .
Therefore, the total magnetic y-directional fields at the sensors 105 will
disappear in Eq. (1). On the other hand, the more turns the spiral cabling 100
has, the more unbalanced the y-directional and z-directional fields will be. In one
or more embodiments, such as when spiral cabling 100 that has many turns
and/or a relatively large radius for each turn, Eq. (1) will not be valid. Thus it is
possible to acquire a significant total field (both y-directional and z-directional
fields) downhole from the cable current lc in FIG. 1 and such total field can be
detected by different sensor configurations 105 in the ranging well 130.
[0032] FIG. 2 is a diagram of an example single wire guidance system
incorporating the cable terminated downhole on the outside of a target well
casing, according to aspects of the present disclosure. The cable 200 may include
a straight cable (i.e. , non-spiral) or one of the cable embodiments of FIGs. 5-9.
[0033] FIG. 2 illustrates the casing 202 surrounded by an insulating concrete
layer 2 13 . The cable 200 is embedded in the insulating concrete layer 213 and
coupled to the casing at a cable termination point 207. The termination point 207
may be any location along the target well casing 202 according to various
embodiments.
[0034] The cable 200 is further coupled to a power supply 2 10 on the surface of
the formation 230 through which the wells are drilled. The power supply 2 10
provides the current lc through the cable 200. The power supply ground 2 11 may
be grounded to a well head, which is electrically connected to the casing, or to
the geological formation 230.
[0035] The power supply 210 supplies the current lc through the cable 200 to
the termination point 207. The current then returns to ground on the casing as
represented by return current 1{(b ) -
[0036] The ranging well 203 includes sensors 205 (e.g., magnetometers,
gradient sensors) that are located a distance R from the center of the target well
casing 202. The sensors 205 may be included in a wireline logging tool or
included in a drill string, e.g. part of a BHA.
[0037] Using one of the cables of FIGs. 5-9, current may be delivered to the
end of the target well to generate larger unbalanced total fields as compared to
the fields due to the same current of the typical straight cable. The resulting
magnetic fields are received at the sensors 205 in ranging well owing to the
advantages of unbalance magnetic fields between current I c at the cable 200 and
the current I (b ) at the casing 202.
[0038] FIG. 3 is a diagram of an example single wire guidance system
incorporating the spiral configuration on the outside of the target well casing,
according to aspects of the present disclosure. This embodiment adopts similar
wiring methods as illustrated in FIGs. 5-9 as cable winding methods integrated
with the insulating cement layer 313.
[0039] One end of the spirally wound cable 300 is coupled to a power supply
310 on the surface. The target well and the ranging well 303 are drilled into a
geological formation 330. The power supply 310 is further grounded to either the
well head of the target well or the geological formation 330. The power supply
310 supplies the cable current lc-
[0040] The spiral cable 300 is terminated on the casing 302 at a termination
point 307. The termination point 307 is shown at the bottom of the casing 302
but may be located anywhere on the casing 302. The connection of the cable 300
to the casing 302 enables the casing current I t ( ) to return to the power supply
ground.
[0041] The cable 300 of FIG. 3 may be a typical cable (with conductor and
insulator inside) or a magnetically shielded cable. The cement layer 313 provides
both insulation of the cable 300 from the casing 302 as well as stabilization of
the cable 300 with respect to the casing 302. The system of FIG. 3 provides the
unbalanced magnetic fields as described previously. Since the cable 300 is
permanently installed with cement 313 around the casing 302, the cable may be
accessible anytime and can be used for other purposes. For example, the cable
300 may be built with fibers for well monitoring purposes.
[0042] FIG. 4 is a diagram of an example two cable system, according to
aspects of the present disclosure. Both the current source cable 400 and the
current return cable 4 11 are located within the target well casing 402.
[0043] The system of FIG. 4 includes the current source cable 400 coupled to
an output of a power supply 410. The current return cable 411 is coupled to the
power supply' s return.
[0044] As in previous embodiments, the target well casing 402 and the ranging
well 403 are located in a geological formation 430. The ranging well 403
includes sensors 405 located a distance R from the center of the target well
casing 402.
[0045] A first cable 400 of the two cables is a magnetically shielded cable such
as those shown in FIGs. 5-9. The second cable 4 11 of the two cables is normal,
unshielded cable. The cables 400, 411 have no connection to the target well
casing 402 so that no casing current will be generated. The sensors 405 in the
ranging well 403 measure the unbalanced magnetic fields 420 where a majority
of the magnetic fields are generated from the normal, unshielded cable. Either
one of the current source cable 400 or the current return cable 411 may be the
magnetically shielded cable as long as the other cable is the unshielded cable.
[0046] FIGs. 5-9 illustrate various embodiments for magnetically shielded
cables. These embodiments may utilize a cable with: (a) intrinsic magnetic
shielding materials, (b) intrinsic solenoid wiring or extrinsic spiral winding
around a cylindrical material (such as mu-metal), or (c) other intrinsic or
extrinsic wiring orientations (e.g., cylindrical, triangular, rectangular, or other
shapes for wiring). A mu-metal may be defined as a nickel-iron alloy.
[0047] FIG. 5 is a diagram of an example shielded cable with a metal exterior
over an insulator, according to aspects of the present disclosure. The illustrated
embodiment includes a metal enclosure 501 (e.g., steel) to protect the conductor
504 and shielding 503. An insulator 502 insulates the conductor 504 and
shielding 503 from the metal enclosure 501. The shield 503 (e.g., metal, mumetal)
is wrapped around the metal conductor 504 (e.g., copper). In an
embodiment, the shield 503 is spirally wrapped around the metal conductor 504.
[0048] FIG. 6 is a diagram of an example shielded cable with a triangular metal
core, according to aspects of the present disclosure. The illustrated embodiment
includes a triangular metal core 602 (e.g., metal, mu-metal) around which is
wrapped a metal conductor 603 (e.g., copper) that provides magnetic shielding
of the metal core 602. An insulator 601 encloses the cable for protection of the
cable as well as insulation of the conductor 602 and shielding 603 from other
metal contact. The metal conductor 603 may be spirally wrapped around the
metal core 602.
[0049] FIG. 7 is a diagram of an example shielded cable with an insulator
exterior, according to aspects of the present disclosure. The illustrated
embodiment includes a cylindrical metal core 702 (e.g., metal, mu-metal) around
which is wrapped a metal conductor 703 (e.g., copper) that provides magnetic
shielding of the metal core 702. An insulator 701 encloses the cable for
protection of the cable as well as insulation of the metal conductor 702 and
shielding 703 from other metal contact. The metal conductor 703 may be spirally
wrapped around the metal core 702.
[0050] FIG. 8 is a diagram of an example shielded cable with a cylindrical
conductive material wrapped with an insulated wire, according to aspects of the
present disclosure. The illustrated embodiment includes a metal core 802 (e.g.,
copper) enclosed by an insulator 801 to form an insulated wire. The insulated
wire is spirally-wrapped around a cylindrical conductive material 800 (e.g.,
metal, mu-metal) to form the cable. The insulated wire may be spirally wrapped
around the cylindrical conductive material 800.
[0051] FIG. 9 is a diagram of an example shielded cable with a rectangular
conductive material wrapped by an insulated wire, according to aspects of the
present disclosure. The illustrated embodiment includes a metal core 902 (e.g.,
copper) enclosed by an insulator 901 to form an insulated wire. The insulated
wire is wrapped around a rectangular conductive material 900 (e.g., metal, mumetal)
to form the cable. The insulated wire may be spirally wrapped around the
rectangular conductive material 900.
[0052] The various shapes and compositions of the embodiments illustrated by
FIGs. 5-9 are for purposes of illustration only. Other shapes and compositions
may be used for magnetically shielded cables.
[0053] FIG. 10 is a diagram of an example shielded cable apparatus for
implementing a method for ranging, according to aspects of the present
disclosure. Parameters from this apparatus, as modeled using the following
, porated into Eq. (1) in order to generate the
plots ofFIGs. 11 and 12.
[0054] Fig. 10 shows that the cable 1000 has two sections 1001, 1002 with
spiral wiring. Each section 1001, 1002 is assumed to have a length of L. In
addition, there is a separation distance of S between the two spiral wiring
sections 1001, 1002.
[0055] One spiral wiring 1001 is in a counterclockwise direction and the other
spiral wiring 1002 is in clockwise direction. The inner conductor 1005 may be
used as a current inject path and the outer conductor 1001, 1002 may be used as
a current return path. Which conductor is the current return path and which
conductor is the current injection path is interchangeable. An injection path can
be a return path by changing the current direction (i.e., applying positive voltage
to one path and negative voltage to the other path).
[0056] The following parameter assumptions are used for modeling the
apparatus of FIG. 10 only to generate the plots ofFIGs. 11 and 12 and are not
limiting on any other examples herein: radius of the spiral cable = 2.54
centimeters, L = 4 meters, S = L, the injected current is 1 Amp, and the density
of the spiral cable is D=N/L=200 =number of turns/meter. The sensor (e.g.,
magnetometer, wireline tool) is assumed to be 5 meters away in the x direction
from the cable. Z=0 meter for point P that represents the position of the sensor
between the two spiral wiring sections 1001, 1002.
[0057] FIG. 11 is a plot showing the unbalanced magnetic field densities in the
x-direction and the z-direction in accordance with the shielded cable apparatus of
FIG. 10. FIG. 12 is a plot showing the total magnetic field density at sensor point
P in accordance with the shielded cable apparatus of FIG. 10. Both show
Amps/meter (A/m) for the magnetic field in the z-direction. It can be seen that
the maximum field is approximately 0.57 A/m (or ~716nT) for the illustrated
embodiment. This is a relatively significant magnetic field for a typical ranging
application.
[0058] The embodiments ofFIGs. 10, 11, and 12 are only for purposes of
illustration of a typical ranging embodiment. Other embodiments may have
different parameters that generate different magnetic fields at the sensor location
P.
[0059] FIG. 13 is a diagram showing a wireline system 1364 and FIG. 14 is a
diagram showing a drilling rig system 1464. The systems 1364, 1464 may thus
comprise portions of a wireline logging tool body 1320, including the abovedescribed
sensors, as part of a wireline logging operation or of a down hole tool
1424, including the above-described sensors, as part of a down hole drilling
operation.
[0060] FIG. 13 illustrates a well that may be used as a ranging well or a target
well. In this case, a drilling platform 1386 is equipped with a derrick 1388 that
supports a hoist 390. If this well is used as the target well, the sensors in the
wireline logging tool 1320 and the illustrated cable maybe replaced with one or
more of the previously discussed embodiments (e.g., spiral cable, intrinsic
magnetic shielding materials, intrinsic solenoid wiring or extrinsic spiral
winding around a cylindrical material (such as mu-metal), or other intrinsic or
extrinsic wiring orientations (e.g., cylindrical, triangular, rectangular, or other
shapes for wiring)).
[0061] Drilling oil and gas wells is commonly carried out using a string of drill
pipes connected together so as to form a drillstring that is lowered through a
rotary table 1310 into a wellbore or borehole 13 12. Here it is assumed that the
drillstring has been temporarily removed from the borehole 1312 to allow a
wireline logging tool 1320, such as a probe or sonde, to be lowered by wireline
or logging cable 1374 (e.g., slickline cable) into the borehole 1312. Typically,
the wireline logging tool 1320 is lowered to the bottom of the region of interest
and subsequently pulled upward at a substantially constant speed. In one or more
embodiments, the borehole 1312 of FIG. 3 may represent a ranging well to the
target well of FIG. 14. When this well is used as a ranging well, the wireline
logging tool 1320 may include the sensors to measure the magnetic field
produced from the target well.
[0062] During the upward trip, at a series of depths, various instruments may
be used to perform measurements on the subsurface geological formations 1314
adjacent to the borehole 1312 (and the tool body 1320), including measurements
of the magnetic field produced at the target well. The wireline data may be
communicated to a surface logging facility 392 for processing, analysis, and/or
storage. The logging facility 1392 maybe provided with electronic equipment,
such as a controller, for various types of signal processing. The controller 1396
may be coupled to the ranging tool and configured to determine and decouple the
total magnetic field to a relative range and direction from the ranging well to the
target well. Similar formation evaluation data may be gathered and analyzed
during drilling operations (e.g., during LWD/MWD operations, and by
extension, sampling while drilling).
[0063] In some embodiments, the tool body 1320 is suspended in the wellbore
by a wireline cable 1374 that connects the tool to a surface control unit (e.g.,
comprising a workstation 1354). The tool maybe deployed in the borehole 1312
on coiled tubing, jointed drill pipe, hard wired drill pipe, or any other suitable
deployment technique.
[0064] Referring to FIG. 14, it can be seen how a system 1464 may also form
a portion of a drilling rig 1402 located at the surface 1404 of a well 1406. The
drilling rig 1402 may provide support for a drillstring 1408. The drillstring 1408
may operate to penetrate the rotary table 1310 for drilling the borehole 1312
through the subsurface formations 1314. The drillstring 1408 may include a drill
pipe 1418 and a bottom hole assembly 1420, perhaps located at the lower portion
of the drill pipe 1418.
[0065] The bottom hole assembly 1420 may include drill collars 1422, a down
hole tool 1424, and a drill bit 1426. The drill bit 1426 may operate to create the
borehole 1312 by penetrating the surface 1404 and the subsurface formations
1314. The down hole tool 1424 may comprise any of a number of different types
of tools including sensors used to measure magnetic fields, as described
previously, MWD tools, LWD tools, and others. The sensors may be used to
measure the magnetic fields and relay the information to a controller 1396 that
may then control the direction and depth of the drilling operation in order to
range to the target well.
[0066] During drilling operations, the drillstring 1408 (perhaps including the
drill pipe 1418 and the bottom hole assembly 1420) may be rotated by the rotary
table 1310. Although not shown, in addition to, or alternatively, the bottom hole
assembly 1420 may also be rotated by a motor (e.g., a mud motor) that is located
down hole. The drill collars 1422 may be used to add weight to the drill bit 1426.
The drill collars 1422 may also operate to stiffen the bottom hole assembly 1420,
allowing the bottom hole assembly 1420 to transfer the added weight to the drill
bit 1426, and in turn, to assist the drill bit 1426 in penetrating the surface 1404
and subsurface formations 1314.
[0067] During drilling operations, a mud pump 1432 may pump drilling fluid
(sometimes known by those of ordinary skill in the art as "drilling mud") from a
mud pit 1434 through a hose 1436 into the drill pipe 1418 and down to the drill
bit 1426. The drilling fluid can flow out from the drill bit 1426 and be returned
to the surface 1404 through an annular area 440 between the drill pipe 1418 and
the sides of the borehole 1312. The drilling fluid may then be returned to the
mud pit 1434, where such fluid is filtered. In some embodiments, the drilling
fluid can be used to cool the drill bit 1426, as well as to provide lubrication for
the drill bit 1426 during drilling operations. Additionally, the drilling fluid may
be used to remove subsurface formation cuttings created by operating the drill
bit 1426.
[0068] The workstation 1354 and the controller 1396 may include modules
comprising hardware circuitry, a processor, and/or memory circuits that may
store software program modules and objects, and/or firmware, and combinations
thereof. The workstation 1354 and controller 1396 maybe configured to control
the direction and depth of the drilling, by executing instructions, in order to
perform ranging from a target well using the method for ranging using
unbalanced magnetic fields as described subsequently. For example, the
controller 1396 maybe configured to determine and decouple the total magnetic
field to a relative range and direction from the ranging well to the target well.
For example, in some embodiments, such modules may be included in an
apparatus and/or system operation simulation package, such as a software
electrical signal simulation package, a power usage and distribution simulation
package, a power/heat dissipation simulation package, and/or a combination of
software and hardware used to simulate the operation of various potential
embodiments.
[0069] FIG. 15 is a flowchart showing an embodiment of a method for ranging
between a target well and a ranging well using unbalanced magnetic fields. In
block 1501, a current is injected, by a power supply, downhole through an
injection path (e.g., spiral cable). The current may be a direct current or some
form of alternating current (e.g., clock signal, sine wave).
[0070] In block 1503, a return path is provided for the current. The return path
may be the target well casing or another cable. The return path is coupled to the
ground of the power supply.
[0071] In block 1505, the unbalanced magnetic fields from the injection path
and the return path are measured. The measurement maybe accomplished from
the ranging well during a wireline operation as shown in FIG. 13 or a
MWD/LWD operation as shown in FIG. 14 and discussed previously.
[0072] In block 1507, the total magnetic field is measured. The total magnetic
field is received at the sensors with the presence of current in the injection path
and the return path. Since the first and the second magnetic fields are
unbalanced, the sensors pick up a total magnetic field strong enough to
determine the relative distance and direction between the target well and the
drilling well.
[0073] In block 1509, a relative distance and direction of the ranging well to
the target well is determined based on the total magnetic field. The range from
the well can then be used to steer the ranging well during the drilling operation.
When the magnetic field increases, the ranging well is getting closer to the target
well. When the magnetic field decreases, the ranging well is getting farther from
the target well. In addition, design of gradient sensors with tri-axial component
measurements can be utilized to directly determine the relative distance.
[0074] Example 1 is a method for ranging between a target well and a ranging
well, the method comprising: generating a downhole current through a current
injection path, wherein the current injection path generates a first magnetic field;
receiving a return current through a return path, wherein the return path
generates a second magnetic field, wherein the first and second magnetic fields
are unbalanced with respect to each other; and measuring the first and second
magnetic fields.
[0075] In Example 2, the subject matter of Example 1 can further include
measuring the total magnetic fields from the first and second magnetic fields.
[0076] In Example 3, the subject matter of Examples 1-2 can further include
decoupling the total magnetic field to a relative distance and direction from the
ranging well to the target well.
[0077] In Example 4, the subject matter of Examples 1-3 can further include
the injection path and the return path are exchangeable.
[0078] In Example 5, the subject matter of Examples 1-4 can further include
wherein generating the downhole current through the current injection path
comprises generating the downhole current through a spiral cable.
[0079] In Example 6, the subject matter of Examples 1-5 can further include
wherein the spiral cable is coupled to a casing of the target well such that the
casing is the return path.
[0080] In Example 7, the subject matter of Examples 1-6 can further include
wherein the spiral cable is located inside or outside of the casing of the target
well.
[0081] In Example 8, the subject matter of Examples 1-7 can further include
wherein generating the downhole current through the current injection path
comprises generating the downhole current through an intrinsically magnetically
shielded cable.
[0082] Example 9 is a system for ranging between a target well and a ranging
well, the system comprising: a current injection path associated with a target
well casing, wherein the current injection path is configured to generate a first
magnetic field; a return path coupled to the current injection path, wherein the
return path is configured to generate a second magnetic field such that the first
and second magnetic fields are unbalanced with respect to each other; and the
current injection path and current return path are exchangeable.
[0083] In Example 10, the subject matter of Example 9 can further include
wherein the current injection path or the current return path comprises a spiral
cable.
[0084] In Example 11, the subject matter of Examples 9-10 can further
include, wherein the spiral cable is embedded in concrete around the exterior of
the target well casing.
[0085] In Example 12, the subject matter of Examples 9-11 can further
include, wherein the current injection path or the current return path comprises
the target well casing.
[0086] In Example 13, the subject matter of Examples 9-12 can further include
wherein the spiral cable comprises a high permeability mu-metal cable.
[0087] In Example 14, the subject matter of Examples 9-13 can further include
wherein the spiral cable comprises a mu-metal wire wrapped around a
conductive core.
[0088] In Example 15, the subject matter of Examples 9-14 can further include
wherein the spiral cable comprises a conductive wire wrapped around a mumetal
core.
[0089] In Example 16, the subject matter of Examples 9-15 can further include
wherein the spiral cable comprises a core having a shape of one of a triangle, a
cylinder, or a rectangle.
[0090] In Example 17, the subject matter of Examples 9-16 can further include
wherein the current injection path is a first cable and the return path is a second
cable.
[0091] In Example 18, the subject matter of Examples 9-17 can further include
wherein the first cable is a spiral cable and the second cable is a straight cable.
[0092] In Example 19, the subject matter of Examples 9-18 can further include
wherein the current injection path is a spiral cable located inside or outside of the
target well casing and the return path is the target well casing.
[0093] Example 20 is a system comprising: a target well comprising a casing;
a power supply coupled to the casing and configured to launch a current
downhole through an injection path and receive a return current from a return
path, wherein the injection path generates a first magnetic field and the return
path generates a second magnetic field that is unbalanced with respect to the first
magnetic field; a ranging tool in a ranging well, the ranging tool configured to
measure a total field from the first and second unbalanced magnetic fields; and a
controller coupled to the ranging tool, the controller configured to determine and
decouple the total magnetic field to a relative range and direction from the
ranging well to the target well.
[0094] In Example 1, the subject matter of Example 20 can further include
wherein the injection path or the return path comprises a spiral cable that
terminates at the casing.
[0095] In Example 22, the subject matter of Examples 20-21 can further
include wherein the power supply is grounded via a well head of the target well
or a geological formation disposed proximate thereto.
[0096] The accompanying drawings that form a part hereof, show by way of
illustration, and not of limitation, specific embodiments in which the subject
matter may be practiced. The embodiments illustrated are described in sufficient
detail to enable those skilled in the art to practice the teachings disclosed herein.
Other embodiments may be utilized and derived therefrom, such that structural
and logical substitutions and changes may be made without departing from the
scope of this disclosure. This Detailed Description, therefore, is not to be taken
in a limiting sense, and the scope of various embodiments is defined only by the
appended claims, along with the full range of equivalents to which such claims
are entitled.

CLAIMS
What is claimed is:
1. A method for ranging between a target well and a ranging well, the
method comprising:
generating a downhole current through a current injection path, wherein
the current injection path generates a first magnetic field;
receiving a return current through a return path, wherein the return path
generates a second magnetic field, wherein the first and second
magnetic fields are unbalanced with respect to each other; and
measuring the first and second magnetic fields.
2. The method of claim 1, further comprising measuring the total magnetic
fields from the first and second magnetic fields.
3. The method of claim 2, further comprising decoupling the total magnetic
field to a relative distance and direction from the ranging well to the
target well.
4. The method of claim 1, further comprising the injection path and the
return path are exchangeable.
5. The method of claim 1, wherein generating the downhole current through
the current injection path comprises generating the downhole current
through a spiral cable.
6. The method of claim 5, wherein the spiral cable is coupled to a casing of
the target well such that the casing is the return path.
7. The method of claim 6, wherein the spiral cable is located inside or
outside of the casing of the target well.
8. The method of claim 1, wherein generating the downhole current through
the current injection path comprises generating the downhole current
through an intrinsically magnetically shielded cable.
9. A system for ranging between a target well and a ranging well, the
system comprising:
a current injection path associated with a target well casing, wherein the
current injection path is configured to generate a first magnetic
field;
a return path coupled to the current injection path, wherein the return
path is configured to generate a second magnetic field such that
the first and second magnetic fields are unbalanced with respect
to each other; and the current injection path and current return
path are exchangeable.
10. The system of claim 9, wherein the current injection path or the current
return path comprises a spiral cable.
11. The system of claim 10, wherein the spiral cable is embedded in concrete
around the exterior of the target well casing.
1 . The system of claim 10, wherein the current injection path or the current
return path comprises the target well casing.
13. The system of claim 10, wherein the spiral cable comprises a high
permeability mu-metal cable.
14. The system of claim 13, wherein the spiral cable comprises a mu-metal
wire wrapped around a conductive core.
15. The system of claim 13, wherein the spiral cable comprises a conductive
wire wrapped around a mu-metal core.
16. The system of claim 13, wherein the spiral cable comprises a core having
a shape of one of a triangle, a cylinder, or a rectangle.
17. The system of claim 9, wherein the current injection path is a first cable
and the return path is a second cable.
18. The system of claim 17, wherein the first cable is a spiral cable and the
second cable is a straight cable.
19. The system of claim 9, wherein the current injection path is a spiral cable
located inside or outside of the target well casing and the return path is
the target well casing.
20. A system comprising:
a target well comprising a casing;
a power supply coupled to the casing and configured to launch a current
downhole through an injection path and receive a return current
from a return path, wherein the injection path generates a first
magnetic field and the return path generates a second magnetic
field that is unbalanced with respect to the first magnetic field;
a ranging tool in a ranging well, the ranging tool configured to measure a
total field from the first and second unbalanced magnetic fields;
and
a controller coupled to the ranging tool, the controller configured to
determine and decouple the total magnetic field to a relative range
and direction from the ranging well to the target well.
21. The system of claim 20, wherein the injection path or the return path
comprises a spiral cable that terminates at the casing.
22. The system of claim 20, wherein the power supply is grounded via a well
head of the target well or a geological formation disposed proximate
thereto.