DRILLING COLLISION AVOIDANCE
APPARATUS, METHODS, AND SYSTEMS
BACKGROUND
[0001] Currently, it is relatively difficult and potentially expensive to steer a
drill bit through a field crowded with producing oil wells as a part of
constructing a new well, without interrupting production of any of the other
wells in the field.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] FIG. 1 illustrates an embodiment of one or more magnetic dipole
transmitter configurations.
[0003] FIGs. 2A and 2B illustrate an embodiment of a ranging system
showing various electrode placements.
[0004] FIGs. 3A-3C illustrate various dipole gradient measurement
embodiments.
[0005] FIGs. 4A and 4B illustrate various embodiments for drilling strategies
in accordance with the embodiments of FIGs. 1 and 2.
[0006] FIG. 5 illustrates a flowchart of an embodiment of a method for
ranging.
[0007] FIGs. 6A-6G illustrate various embodiments of wireline and loggingwhile-
drilling electrode configurations.
[0008] FIGs. 7A and 7B illustrate various embodiments of electrode-array
configurations.
[0009] FIG. 8 illustrates a wireline system.
[0010] FIG. 9 illustrates a drilling rig system.
DETAILED DESCRIPTION
[0011] As the easy-to-access and easy-to-produce hydrocarbon resources
depleted the remaining wells are more difficult to access. Moreover, the world
hydrocarbon demand is continuously growing, Meeting this demand can use
development of more advanced recovery procedures such as a steam assisted
gravity drainage (SAGD) application. SAGD addresses the mobility problem of
the heavy oil wells by injection of high pressure and high temperature steam
which reduces viscosity of the oil and allows easy extraction. This injection is
performed from a wellbore (e.g., injector well, ranging well) that is drilled
parallel to the producing well (e.g., target well) at a distance on the order of a
few meters from each other. The placement of the ranging well should be
achieved with very small margin in distance, since getting it too close would
expose the producing well to very high pressure and temperature and getting it
too far would reduce efficiency of the process. Traditional surveying techniques
may suffer from a widening cone of uncertainty as the well gets longer and they
cannot achieve the precision in placement that is used in this application.
[0012] Various embodiments disclosed herein utilize a combination of
magnetic dipole transmitters and electrode-based voltage measurements. The
magnetic dipole transmitters can be located downhole and/or at the surface of a
geological formation. When compared to a surface-type excitation, the various
embodiments do not produce unwanted induction signals at the receivers due to
magnetic sensor rotation with respect to earth. This allows ranging
measurements to be taken while rotating which can remove one of the
limitations of ranging applications.
[0013] As used herein, a target well may be an abandoned or a producing oil
or gas well which exists in a field and is to be avoided by a later well being
drilled. A target well may also be an existing well that has blown out, and is to
be intercepted at a selected depth below the surface of the earth by a relief
borehole. Alternatively, the target well may represent some other anomaly
located in the earth, such as an electrically conductive geological formation, a
well pipe, a drill string in an uncased well, or some other electrically conductive
material which may be a target for interception or avoidance. For purposes of
this disclosure, such material will be referred to as the target well or the target
well pipe.
[0014] Near the target well may be a second borehole that is being drilled, and
which is to be directed so as either to intersect the target well or to avoid it. For
convenience, the second borehole will be referred to as a ranging well. The
ranging well typically begins at a wellhead at the surface of the earth, and may
be relatively close to the wellhead of the target well, or may be spaced by a
distance. At the wellhead, subsections or drill collars are secured end to end to
form a drill string, and are lowered into the well as drilling progresses in a
conventional manner. Drilling mud can be supplied to the interior bore of the
string by way of fittings, again in conventional manner.
[0015] A ranging device can include two parts: (1) a magnetic dipole
transmitter apparatus that generates alternating currents (AC) on a target well
pipe and (2) a receiver with electrodes that measure the absolute and differential
voltages, due to these pipe AC currents, from electrodes deployed at a wellhead,
at a shallow depth near the surface of the geological formation, and/or in contact
with the target well. The magnetic dipole transmitter apparatus can be deployed
as part of a logging tool downhole in the ranging well. Relative distance and
direction from the ranging well to the target well can be determined by analyzing
the measured voltages based on absolute and magnetic dipole differential
excitations.
[0016] Magnetic dipole transmitters, operating at relatively low frequencies
(e.g., 0.02-250 Hz) can be used to induce the AC on the target well. One or more
magnetic dipole transmitters can be used as part of the magnetic dipole
transmitter apparatus to generate the AC.
[0017] FIG. 1 illustrates an embodiment of a magnetic dipole transmitter
apparatus 206 configuration using one magnetic dipole transmitter 103, as
represented by Hy 2, located in a ranging well 100 to induce an AC current on a
target well 120. In an embodiment where measurements are being taken while
the transmitter is rotating, a single physical magnetic dipole transmitter 103 can
be used to synthesize four magnetic dipole results 101-104 at different rotation
angles if the measurements are binned. The four magnetic dipole results 101-104
are represented by Hxi, Hx2, Hyi, and Hy2. FIG. 1 shows the synthesized magnetic
dipole transmitters 101, 102, 104 are represented by dotted lines, while the
physical magnetic dipole transmitter 103 is represented by a solid line.
Alternatively all magnetic dipole transmitters can be physical. For example, each
magnetic dipole transmitter 101-104 can be a physical magnetic dipole
transmitter. While a more electrically complicated system, having physical
magnetic dipole transmitters may reduce errors due to synthesis operations.
[0018] The magnetic dipole transmitters 101-104 can be located towards an
outer surface of the logging tool. If more than one physical magnetic dipole
transmitter 101-104 is used, they can be separated radially from other magnetic
dipole transmitters and located a substantially equal distance from an axial
centerline 130 of the ranging well 100 but on an opposite side of the ranging
well 100 from an opposing magnetic dipole transmitters.
[0019] The magnetic dipole transmitters 101-104 can induce closed electric
field lines in the geological formation and currents across the target well pipe
120. Induced currents, at such low relative frequencies, can reach distances
greater than 10000 feet. Thus, such an implementation works well for a SAGD
application.
[0020] Magnetic dipole transmitters 101-104 are, in some instances, operated
in a differential mode, where voltage readings at different dipole rotation angles
are subtracted. In order to improve the voltage signal levels corresponding to the
differential readings, magnetic dipole transmitters can be placed as far as
possible from the axial centerline of the tool. In addition, magnetic dipoles in
opposite sides of the tool axis, (i.e., those that are separated by 180 degrees) can
be used in subtraction. Another embodiment to obtain differential excitation is to
place an antenna 101, 103 with opposite windings on two sides of the tool. This
can physically balance the currents and help calibration of the tool.
[0021] In order to improve the drill bit steering performance, magnetic dipole
transmitters 101-104 can be placed as close as possible to the bit (e.g., next to it).
In the SAGD application, a drill string disposed in the ranging well 100 may be
substantially parallel to the target pipe 120, so placement of the magnetic dipole
transmitters 101-104 may be less important in terms of steering performance.
Other embodiments place the magnetic dipole transmitters elsewhere on the drill
string, such as in the bit.
[0022] FIGs. 2A and 2B illustrate an embodiment of a ranging system
showing various electrode placements. The ranging system can include a
receiver and controller 201 coupled to a plurality of electrodes 202, 203. The
receiver and controller 201 can include a voltmeter to measure absolute and
differential voltages between the electrodes 202, 203. The receiver and controller
201 can also include control circuitry for controlling operation of the system as
well as executing any ranging methods, such as that illustrated in FIG. 5. The
embodiments of FIGs. 2A and 2B are for purposes of illustration only as other
systems and other locations for the electrodes can be used.
[0023] Both FIGs. 2A and 2B show the target well 204 and the ranging well
205. The magnetic dipole transmitter apparatus 206, that comprises the one or
more magnetic dipole transmitters 101-104, is shown located in the ranging well
205. The magnetic dipole transmitter apparatus 206 can generate an AC on the
ranging well pipe 205 in order to generate an electric field 207 that can create the
voltage differential as measured by the electrodes 202, 203.
[0024] In the embodiment illustrated in FIG. 2A, the first electrode 202 is
located at the surface of the geological formation. The second electrode 203 is
connected to the target well 204. For example, the electrode 203 can be
connected by insulated wire to the well-head or area surrounding the well head
for the target well 204. If connected to the area surrounding the target well head,
the electrode can be located at relatively shallow depths (e.g., < 6 m). In this
embodiment, the voltage measurement is made across the pipe through the
wellhead and shallow geological formations.
[0025] In the embodiment illustrated in FIG. 2B, the first electrode 202 is
located at the surface of the geological formation. The second electrode 203 is
located in the target well pipe 204. In this embodiment, the voltage measurement
is made using an insulated cable that is deployed in the target pipe, preferably in
the vicinity of the area that is targeted for SAGD drilling.
[0026] In both embodiments illustrated in FIGs. 2A and 2B, the measured
voltage difference is indicative of the magnitude of currents induced on the
target well pipe 204. In order to minimize the resistance of the load that is
connected to the source, the electrode contact can be designed to reduce
electrode resistance as much as possible. Reducing the load can improve the
signal to noise ratio of the voltage measurement. In the embodiment of downhole
electrodes, a mechanical clamp can be used. In a horizontal or substantially
horizontal section of a well, a long piece of conductive material, as part of the
electrode, can be coupled to a low side of the pipe and can be pushed against the
pipe with the help of gravity.
[0027] An unexpected variation in the pipe current distribution may result in
relatively small measured voltages between the electrodes or reversing of the
sign of the measured voltage. The problem with the small measured voltages can
be addressed by utilizing a different excitation frequency of the AC or a position
of the magnetic dipole transmitter apparatus that is expected to produce a
different current distribution on the ranging well pipe. The problem with the
voltage sign reversal can be detected based on a comparison between past
voltage measurements or different frequency or moving the transmitter positions.
[0028] Excitation of the target well by a single magnetic dipole transmitter can
be illustrated by the following equations where r is the distance the single
magnetic dipole transmitter is from the z-oriented target well pipe. The voltage
difference across two far away end points of the target well pipe can be
approximated as follows:
where is the approximate voltage, E is the electric field vector, C is the contour
along pipe, Ez is the z-component of the electric field at the pipe position that is
closest to the magnetic dipole and K is a proportionality factor that depends on
formation and pipe properties. In the case of a single x-axis directed magnetic
dipole transmitter in the geometry as shown in FIG. 1, the voltage generated
across two points in a homogeneous formation can be written as
KIJ s (
An
A
where x =r cos(#) , y =r sin(#) , b is the wave number, r is the radial distance,
Qis the relative orientation angle. Similarly, voltage due to y-directed magnetic
dipole transmitter can be approximated as
Hy KI cos ) V
Aw
(3) KIJ
-x(x2 +y
An
[0029] A "differential" excitation of a y-directed magnetic dipole
combined with a y-directed magnetic dipole of opposite direction (as shown in
FIG. 1) would produce:
KIJAx
cos(2#)
Aw
[0030] Similarly, differential excitation of the two x-directed magnetic
dipoles would produce:
cos(2 ) (5) 4p
[0031] It can be observed that by analyzing the absolute measurements
with x-directed and y-directed magnetic dipoles, it is possible to determine the
orientation of the pipe as follows
Q=angle(VHy ,-V Hx ) (6)
[0032] By taking a ratio of the absolute measurements to differential
excitation measurements for both the x-directed and y-directed excitations, it is
possible to obtain the distance to pipe as follows
cos(2 )
(8)
sin(0)
[0033] Although equations (7) and (8) both provide a distance to the
target well pipe, they have complementing numerical behavior. Equation (7) is
numerically most stable when q=180&, while equation (8) is most stable when
0=90+ 180& ( s an integer) due to size of the denominator. The choice of
equation for distance calculation is best made based on whichever equation is
more stable for a given range of Q.
[0034] The above-described embodiments can be used in SAGD
applications in practice. However it can be seen from both of equations (7) and
(8) that when 9=45+90k, the nominator goes to zero, that indicates that the
denominator (the measurement) will also have to go to zero. This is also a
numerically unstable condition that comprises a "blind spot" for the differential
excitation measurement. This is an area in which the measurements made not be
as useful or accurate as other areas.
[0035] FIGs. 3A-3C illustrate various embodiments for 3-, 4- and 8-
dipole excitation measurement configurations, respectively. The different dipole
excitation measurement configurations are for purposes of illustration only.
There are no limitations on either the number of magnetic dipole transmitters
used to generate the different configurations or the number of measurement
configurations.
[0036] The 3-magnetic dipole transmitter embodiment illustrated in
FIGs. 3A and the 4-magnetic dipole transmitter embodiment illustrated in FIG.
3B can suffer from the blind spot problem. Each of the magnetic dipole
transmitters are represented by Hy, Hx i , Hx2 for the 3-magentic dipole transmitter
embodiment of FIG. 3A and Hxi , Hx2, Hy i , and Hy2 for the four-magnetic dipole
transmitter embodiment of FIG. 3B. In the case of a target that is in x-axis or yaxis
direction, the locations having the highest numerical stability points are
indicated as locations 301-308. The areas that are in the middle of these
locations 301-308, i.e., locations that make 45 degree angle with the x-axis and
y-axis, may be considered to be blind spots.
[0037] In the third configuration, illustrated in FIG. 3C, the highest
sensitivity directions for two sets of dipoles are indicated as locations 310-313
and locations 320-323, respectively, if the target is in x-axis or y-axis direction.
If the target is making a 45 degree angle with x-axis or y-axis however, locations
320-323 have the highest sensitivity. As a result, the configuration of FIG. 3C
can produce stable results in all of the measurements, while the configurations of
FIGs. 3A and 3B may produce unstable results based on the relative orientation
of the target well pipe. In a case where the measurements are taken while the
drill string is rotated, a large number of measurements can be made at different
rotation angles producing the variation that is sufficient to remove selected blind
spots.
[0038] Each of the magnetic dipole transmitters, when activated (e.g.,
energized), have magnetic moments. As known in the art, a magnetic moment is
a quantity that determines the force that the magnetic dipole transmitters can
exert on electric currents and the torque that a magnetic field will exert on the
magnetic dipole transmitter. A magnetic dipole transmitter can have a magnetic
moment that is in an opposite direction from a magnetic moment of a radially
separated magnetic dipole transmitter.
[0039] For example, of the four magnetic dipole transmitters Hxi, H 2,
Hyi, and Hy 2 of FIG. 3B, Hxi and HX2 are radially separated from each other and
generate magnetic moments in an opposite direction from each other. Similarly,
Hyi and Hy 2 are radially separated from each other and generate magnetic
moments in an opposite direction from each other. In an embodiment, the
radially separated magnetic dipole transmitters may be a substantially equal
distance from an axial centerline of the ranging well and in a substantially
opposite direction when compared to the other magnetic dipole transmitter of the
radially separated magnetic dipole transmitters.
[0040] FIGs. 4A and 4B illustrate various embodiments for drilling strategies
using the ranging system and ranging methods disclosed herein. FIG. 4A shows
a triangulation approach where multiple orientation (Q) measurements 400-403
can be made. These measurements 400-403 can be overlaid on survey data to
triangulate the position of the ranging well 205 in relation to the target well 204,
as shown.
[0041] FIG. 4B shows how ranging can be performed when a reliable distance
measurement is available. In this embodiment, there is no need for triangulation
and the ranging well 205can be drilled without spiraling around the target well
204. This embodiment, can be used to help regulate the distance between wells
using multiple relative distance measurements 420-423 between the ranging well
205 and the target well 204.
[0042] In an embodiment where well intercept is desired, both the embodiment
of FIGs. 4A and 4B can be used. In a SAGD application, the embodiment of
FIG. 4B can be used since the optimum position for the injector is above the
producer due to gravity considerations.
[0043] FIG. 5 illustrates a flowchart of an embodiment of an operation of the
ranging system as discussed previously. Initially, one of the ranging well (e.g.,
injector well) or the target well (e.g., producer well) is drilled. Traditionally, the
producer well is drilled first since it has to be placed in the reservoir at the
optimum position to yield a greater amount of hydrocarbons. A well placement
tool such as azimuthal propagation resistivity tools or an ultra-deep reading
resistivity tool can be used to place the producer well at selected distances from
adjacent layers in a reservoir. Survey data can be collected in the drilling of this
first well to aid the guiding of the second well.
[0044] The second well drilling can then be started with the build section
guided either with survey information or absolute or gradient information from
the ranging tool. After the build section, a procedure to keep the second well
parallel and at desired distance to the first well is taken. The disclosed ranging
system can allow substantially the same distance or it can follow a prescribed or
controlled varying distance based on local characteristics of the formations.
[0045] The low noise level that is desired for ranging can be achieved by
stopping drilling while ranging measurements are taken. The time between the
drilling stop and ranging start can be optimized to reduce noise due to wobbling
and also minimize idle time. Similarly duration of ranging activity can be
selected to reject electrical system and magnetic environment noise while
reducing idle time.
[0046] During ranging, the magnetic dipole transmitters and electrode
receivers are activated substantially simultaneously 500 from the surface either
automatically or with manual operation. During the excitation with the magnetic
dipole transmitters, the electrodes may be activated from the surface or by a
downhole algorithm that detect the currents of the pipe.
[0047] Differential voltage signals, generated due to the AC in the ranging
well pipe, are measured 503 between the first and second electrodes. The
absolute and differential voltages can then be determined 504 and the relative
direction and relative distance to the target well determined 505 using the
equations as described above. The drilling well path can be adjusted using the
calculated relative direction and relative distance to the target well 506. This
method can repeat for as long as the well is being drilled and ranging is desired.
[0048] Since the direction and distance results are based on the receiver's
coordinate system, a transformation can be done to convert the results to earth or
another coordinate system that can be used in geosteering of the drill bit. The
Earth's magnetic field or gravity information can be used to measure receiver
orientation and achieve the transformation mentioned above.
[0049] The ranging procedure described above can be performed at certain
depth intervals that improve geosteering accuracy performance and reduce rig
time. Apriori information can be used to adjust the interval. For example, if the
survey data of the first well indicates that the well is expected to be flat, intervals
between ranging measurements can be extended. If the well is expected to have
dog-legs, ranging measurements can be performed more rapidly. Near the end of
a well, currents behave differently compared to other sections since the flow path
of current is modified. In order to avoid adverse effects, the first well can be
drilled longer than the second well. Based on these scenarios, it is possible to
switch to different processing techniques. For example, if it is desired that the
second well follow a path that is far from the first well, ranging based on the
absolute value can be used locally.
[0050] FIGs. 6A-6G illustrate various embodiments of LWD and wireline
electrode configurations. FIGs. 6A-6E illustrate LWD embodiments while FIGs.
6F and 6G illustrate wireline embodiments.
[0051] The electrodes 601, 602 can be located between 6-90 m away from the
transmitters. The electrodes 601, 602 are electrically connected to the drill string
with gap subs that may or may not be separating the electrodes 601, 602. This
arrangement can increase the voltage measurement by removing the current
short between the electrodes 601, 602 and increase the effective outside resistive
load of the electrode system. Similarly gap subs 603-605 can be placed above or
below the drill string to avoid any direct coupling between the receivers and the
transmitters.
[0052] For example, FIG. 6A shows a gap sub 603 between the electrodes 601,
602. FIGs. 6C and 6D show alternating electrodes 601, 602 with gap subs 603-
605. FIG. 6E shows gap subs 603, 604 on either side of the electrodes 601, 602
that are separated from each other. FIG. 6B shows the electrodes 601, 602
simply separated from each other, without the use of any gap subs.
[0053] In all of the embodiments of FIGs. 6A-6E, the electrodes are in
electrical contact with the borehole fluid and the geological formation. These
embodiments also have the electrodes in electrical contact with a tool mandrel
and with the geological formation through the mud.
[0054] FIGs. 6F and 6G show a wireline 620 inside a LWD configuration
where a wireline tool with the electrode receiver 601 is lowered into the LWD
drill string 620. In the embodiments of FIG. 6F and 6G, the voltage between the
measurement electrode 601 and the surface of the geological formation is
measured. It is also possible to measure the voltage difference between two
wireline electrodes (not shown). Gap subs 603 may be used on the drill string
620 may also help reduce transmitter receiver direct coupling effects.
[0055] An unexpected relatively large variation in the pipe current distribution
can produce either small measured voltages between the electrodes or flipping of
the sign of the measured voltage. The problem with the small measured voltages
can be addressed by utilizing a different excitation frequency for the AC or a
different position of the magnetic dipole transmitter apparatus that can produce a
different current distribution on the ranging well pipe.
[0056] Voltage sign reversal can be detected based on a comparison between
past measurements or different frequency or source positions. FIGs. 7A and 7B
show embodiments with an electrode array configuration where more than two
electrodes 701-703 are used in the voltage measurement. A gap sub 705 can be
used in various locations of the electrode array.
[0057] FIG. 8 illustrates a wireline system 864 embodiment as part of a
target well 812, as illustrated in FIGs. 2A and 2B. FIG. 9 illustrates a drilling rig
system 964 embodiment as part of a ranging well 912, as illustrated in FIGs. 2A
and 2B. During a drilling operation of the ranging well 912, as illustrated in FIG.
9, it may be desirable to know the distance between the ranging well 912 and the
already drilled target well 812 of FIG. 8.
[0058] The system 864 of FIG. 8 may comprise portions of a tool body
870 as part of a wireline logging operation that includes one or more of the
electrodes 800 as described previously. The system of FIG. 9 may comprise a
downhole tool 924, as part of a downhole drilling operation, that includes the
magnetic dipole transmitter apparatus as described previously.
[0059] FIG. 8 shows a drilling platform 886 that is equipped with a
derrick 888 that supports a hoist 890. Drilling of oil and gas wells is commonly
carried out using a string of drill pipes connected together so as to form a drilling
string that is lowered through a rotary table 810 into a wellbore or borehole 812.
Here it is assumed that the drilling string has been temporarily removed from the
borehole 812 to allow a wireline logging tool body 870, such as a probe or
sonde, to be lowered by wireline or logging cable 874 into the borehole 812.
Typically, the tool body 870 is lowered to the bottom of the region of interest
and subsequently pulled upward at a substantially constant speed.
[0060] During the drilling of the nearby ranging well, measurement data
can be communicated to a surface logging facility 892 for storage, processing,
and/or analysis. At least one of the above-described electrodes 800 for ranging
between the ranging well and a target well may be part of the wireline logging
tool body 870. The logging facility 892 may be provided with electronic
equipment 854, 896 for various types of signal processing, which may be used
by any one or more of the electrodes 800. Similar formation evaluation data may
be gathered and analyzed during drilling operations (e.g., during LWD
operations, and by extension, sampling while drilling).
[0061] FIG. 9 shows a system 964 that may also include a drilling rig
902 located at the surface 904 of a well 906. The drilling rig 902 may provide
support for a drill string 908. The drill string 908 may operate to penetrate a
rotary table for drilling a borehole 912 through subsurface formations 914. The
drill string 908 may include a Kelly 916, drill pipe 918, and a bottom hole
assembly 920, perhaps located at the lower portion of the drill pipe 918.
[0062] The bottom hole assembly 920 may include drill collars 922, a
downhole tool 924, and a drill bit 926. The drill bit 926 may operate to create a
borehole 912 by penetrating the surface 904 and subsurface formations 914. The
downhole tool 924 may comprise any of a number of different types of tools
including MWD (measurement while drilling) tools, LWD tools, and others.
[0063] During drilling operations, the drill string 908 (perhaps including
the Kelly 916, the drill pipe 918, and the bottom hole assembly 920) may be
rotated by the rotary table. In addition to, or alternatively, the bottom hole
assembly 920 may also be rotated by a motor (e.g., a mud motor) that is located
downhole. The drill collars 922 may be used to add weight to the drill bit 926.
The drill collars 922 may also operate to stiffen the bottom hole assembly 920,
allowing the bottom hole assembly 920 to transfer the added weight to the drill
bit 926, and in turn, to assist the drill bit 926 in penetrating the surface 904 and
subsurface formations 914.
[0064] During drilling operations, a mud pump 932 may pump drilling
fluid (sometimes known by those of skill in the art as "drilling mud") from a
mud pit 934 through a hose 936 into the drill pipe 918 and down to the drill bit
926. The drilling fluid can flow out from the drill bit 926 and be returned to the
surface 904 through an annular area 940 between the drill pipe 918 and the sides
of the borehole 912. The drilling fluid may then be returned to the mud pit 934,
where such fluid is filtered. In some embodiments, the drilling fluid can be used
to cool the drill bit 926, as well as to provide lubrication for the drill bit 926
during drilling operations. Additionally, the drilling fluid may be used to remove
subsurface formation 914 cuttings created by operating the drill bit 926.
[0065] In some embodiments, the system 964 may include a display 996
to present voltage information as measured by the electrodes 800 and generated
in response to the magnetic dipole transmitter apparatus 900. This information
can be used in steering the drill bit 926 during the drilling operation as described
previously. The system 964 may also include computation logic, perhaps as part
of a surface logging facility 992, or a computer workstation 954, to receive
signals from transmitters and receivers, and other instrumentation to determine
the distance to the target well 812.
[0066] It should be understood that the apparatus and systems of various
embodiments can be used in applications other than those described above. The
illustrations of systems 864, 964 are intended to provide a general understanding
of the structure of various embodiments, and they are not intended to serve as a
complete description of all the elements and features of apparatus and systems
that might make use of the structures described herein.
[0067] In the foregoing Detailed Description, it can be seen that various
features are grouped together in a single embodiment for the purpose of
streamlining the disclosure. This method of disclosure is not to be interpreted as
reflecting an intention that the claimed embodiments require more features than
are expressly recited in each claim. Rather, as the following claims reflect,
inventive subject matter lies in less than all features of a single disclosed
embodiment. Thus the following claims are hereby incorporated into the
Detailed Description and the figures, with each claim standing on its own as a
separate embodiment.

CLAIMS
What is claimed is:
1. A system comprising
a magnetic dipole transmitter to be located in a ranging well;
a voltage measurement device comprising a plurality of probes wherein
each probe is to be located on one of: a target well, the ranging well, or a surface
of a geological formation comprising the ranging well and the target well; and
a controller coupled to the voltage measurement device to calculate a
distance or relative direction between the target well and the ranging well based
on a voltage difference between the plurality of probes.
2. The system of claim 1wherein a first probe of the plurality of probes is
to be connected to a wellhead of the target well and a second probe of the
plurality of probes is to be connected to ground at the surface.
3. The system of claim 1wherein the plurality of probes are configured to
be connected to axially separated points in the target well.
4. The system of claim 1wherein the plurality of probes are to be connected
to axially separated points in the ranging well.
5. The system of claim 4 wherein the plurality of probes are to be connected
to a bottom hole assembly (BHA) and separated from the BHA by a gap sub.
6. The system of claim 1wherein the magnetic dipole transmitter is a first
magnetic dipole transmitter and wherein the system further comprises a second
magnetic dipole transmitter, the second magnetic dipole transmitter having a
magnetic moment in an opposite direction from a magnetic moment of the first
magnetic dipole transmitter.
7. The system of claim 6, wherein the second magnetic dipole transmitter is
a substantially equal distance from an axial centerline of the ranging well and in
a substantially opposite direction when compared to the first magnetic dipole
transmitter.
8. A method for ranging between a ranging well and a target well, the
method comprising:
activating a magnetic dipole transmitter to generate an alternating current
on the target well;
sensing a voltage difference between a pair of probes, wherein any one
probe of the pair of probes is conductively coupled to a string of pipe in the
target well, a string of pipe in the ranging well, or a surface of a geological
formation; and
determining at least one of a relative distance or a relative direction
between the ranging well and the target well based on the voltage difference.
9. The method of claim 8 further comprising adjusting a drilling operation
direction based on the relative distance or the relative direction.
10. The method of claim 8 wherein the magnetic dipole transmitter is a first
magnetic dipole transmitter, the method further comprising:
locating a second magnetic dipole transmitter in the ranging well, with a
magnetic moment in an opposite direction compared to a magnetic moment of
the first magnetic dipole transmitter, the second magnetic dipole transmitter
being radially separated from the first magnetic dipole transmitter.
11. The method of claim 10 wherein the second magnetic dipole transmitter
is excited substantially simultaneously with the first magnetic dipole transmitter.
12. The method of claim 9 further comprising generating an alternating
current on the target well with a second magnetic dipole transmitter.
13. The method of claim 11 further comprising generating an alternating
current on the target well with three or more magnetic dipole transmitters.
14. The method of claim 10 wherein activating the magnetic dipole
transmitter to generate an alternating current on the target well comprises
transmitting the signal from one of: a 3-dipole transmitter, a 4-dipole transmitter,
or an 8-dipole transmitter.
15. The method of claim 9 wherein activating the magnetic dipole transmitter
to generate an alternating current on the target well comprises transmitting the
alternating current from the magnetic dipole transmitter coupled to a bottom hole
assembly, a drilling assembly, or a wireline tool disposed within the ranging
well.
16. A method for ranging between a ranging well and a target well, the
method comprising:
generating an alternating current on the target well;
sensing a voltage difference between a pair of probes, wherein a first
probe of the pair of probes is conductively coupled to a geological formation and
a second probe of the pair of probes is coupled to one of the target well or the
ranging well; and
determining at least one of a relative distance or a relative direction
between the ranging well and the target well based on the voltage difference.
17. The method of claim 16 further comprising coupling the second probe of
the pair of probes to a wellhead of the target well.
18. The method of claim 16 further comprising locating the pair of probes at
axially separated points in the target well.
19. The method of claim 16 further comprising locating the pair of probes at
axially separated points in the ranging well.
20. The system of claim 16 further comprising locating the pair of probes in
a bottom hole assembly (BHA) wherein the probes are separated by a gap sub.
1. The method of claim 16 wherein the pair of probes are part of a wireline
tool.
22. The method of claim 16 further comprising sensing an absolute voltage between the pair of probes based on the alternating current on the target well.