U.S. patent application number 14/439081 was filed with the patent office on 2016-01-28 for target well ranging method, apparatus, and system.
The applicant listed for this patent is HALLIBURTON ENERGY SERVICES, INC.. Invention is credited to Paul R. Rodney.
Application Number | 20160025887 14/439081 |
Document ID | / |
Family ID | 53479444 |
Filed Date | 2016-01-28 |
United States Patent
Application |
20160025887 |
Kind Code |
A1 |
Rodney; Paul R. |
January 28, 2016 |
TARGET WELL RANGING METHOD, APPARATUS, AND SYSTEM
Abstract
A ranging signal and a reference signal are generated. The
reference signal has a lower frequency than the ranging signal. The
reference signal is transmitted through a geological formation to
be received by a ranging tool in a ranging well while the ranging
signal is launched down a target well. The reference signal is
reconstructed in the ranging well and a signal that is a
combination of the ranging signal launched from the target well and
noise are received in the ranging well. The received signal may be
in the form of magnetic or electric field values or changes in
these fields. The reconstructed reference signal, in combination
with the received signal, is used to produce a filtered ranging
signal. A relative location of the target well can then be
determined in relation to the ranging well based on the filtered
ranging signal. The location information can be used to direct
drilling operations.
Inventors: |
Rodney; Paul R.; (Spring,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HALLIBURTON ENERGY SERVICES, INC. |
Houston |
TX |
US |
|
|
Family ID: |
53479444 |
Appl. No.: |
14/439081 |
Filed: |
December 27, 2013 |
PCT Filed: |
December 27, 2013 |
PCT NO: |
PCT/US2013/078107 |
371 Date: |
April 28, 2015 |
Current U.S.
Class: |
324/339 |
Current CPC
Class: |
E21B 47/02 20130101;
G01V 3/28 20130101; E21B 47/13 20200501; G01V 3/38 20130101 |
International
Class: |
G01V 3/38 20060101
G01V003/38; G01V 3/28 20060101 G01V003/28 |
Claims
1. A method for ranging between a target well and a ranging well,
the method comprising: generating a clock signal; generating a
ranging signal from the clock signal; generating a reference signal
from the clock signal; launching the ranging signal from the target
well; transmitting the reference signal; receiving the reference
signal with a first receiver in the ranging well; reconstructing
the clock signal as a reconstructed clock signal; using one of the
first receiver or a second receiver in the ranging well, receiving
a signal that is a combination of noise and of the ranging signal
launched from the target well; producing a filtered ranging signal
using the reconstructed clock signal in combination with the
received signal; and determining a relative location of the target
well in relation to the ranging well based on the filtered ranging
signal.
2. The method of claim 1 wherein reconstructing the clock signal
comprises: receiving a propagated reference signal; generating a
pair of quadrature signals from the received propagated reference
signal; and generating a rectangular signal from the received
propagated reference signal.
3. The method of claim 2 further comprising integrating the
magnetic field over a time period determined by the rectangular
signal.
4. The method of claim 3 wherein the time period comprises an
integer multiple of an inverse of a fundamental frequency of the
clock signal.
5. The method of claim 1 further comprising steering a drill bit
while drilling the ranging well in response to the relative
location.
6. The method of claim 1 wherein generating the reference signal
comprises: multiplying a frequency of the clock signal by an
integer "m" to produce a result; and dividing the result by an
integer "n" to produce the reference signal wherein m/n<1.
7. The method of claim 1 further comprising monitoring a magnetic
field generated by the target well in response to the ranging
signal using one of a magnetometer or a gradiometer in the ranging
well.
8. The method of claim 1 wherein transmitting the reference signal
comprises launching the reference signal into a surface of a
geological formation that includes the target well and the ranging
well.
9. The method of claim 8 wherein launching the reference signal
into the surface comprises launching the reference signal with at
least one of: a loop antenna coupled to the surface or a plurality
of ground contacts coupled to the surface.
10. (canceled)
11. The method of claim 1 further comprising: launching the
reference signal to a well head of a well casing of the ranging
well; creating a signal that contains the reference signal and the
ranging signal by summing the reference signal and the clock
signal; and launching the signal that contains the reference signal
and the ranging signal down a well head of a casing of the target
well.
12. A system, comprising: a signal generator circuit to generate
both a ranging signal and a reference signal based on a clock
signal, wherein the ranging signal has a first frequency, and the
reference signal has a second frequency, wherein the ranging signal
is coupled to a target well, and wherein the second frequency is
less than the first frequency; a downhole tool; at least one of a
magnetometer or a gradiometer attached to the downhole tool, the at
least one of the magnetometer or the gradiometer to provide
detected signals in the form of magnetic or electric field values
or changes in the magnetic or electric fields that, in combination
with a reconstructed reference signal, produce a filtered ranging
signal in response to the ranging signal being launched down the
target; and a signal processor to monitor the detected signals in a
ranging well to determine a relative location of the ranging well
in relation to the target well based on the filtered ranging
signal.
13. The system of claim 12 further comprising: an oscillator to
generate the clock signal; and a frequency scaling circuit coupled
to the oscillator to generate the reference signal by multiplying
the clock signal by an integer "m" and dividing by an integer "n"
to produce the reference signal wherein m/n<1.
14. (canceled)
15. The system of claim 14 further comprising a transmitter of the
reference signal coupled to the frequency scaling circuit.
16. The system of claim 15 wherein the transmitter of the reference
signal comprises at least one of: a loop antenna, a plurality of
dipole antennas, a power amplifier that can be used to couple the
frequency scaling circuit to a casing of the ranging well, or a
summing amplifier coupled to the clock circuit and the frequency
altering circuit to generate a signal that contains the reference
signal and the ranging signal by summing the reference signal and
the clock signal.
17. An apparatus comprising: a reconstruction circuit to receive a
signal and generate a reconstructed reference signal from the
received signal, the reconstructed reference signal based on a
clock signal; at least one of a magnetometer or a gradiometer to
couple to the reconstruction circuit, the at least one of the
magnetometer or the gradiometer to provide detected signals in the
form of magnetic or electric field values or changes in the
magnetic or electric fields that, in combination with a
reconstructed reference signal to produce a filtered ranging signal
in response to the ranging signal being launched down a target
well; and a signal processor, coupled to the at least one
magnetometer or the gradiometer, to monitor the detected signals in
a ranging well to determine a location of the ranging well in
relation to the target well based on the filtered ranging
signal.
18. The apparatus of claim 17 wherein the circuit to receive the
signal and generate the reconstructed reference signal comprises
one of a resonant circuit or a bandpass filter to filter out noise
from the reference signal wherein the resonant circuit comprises a
capacitor coupled in parallel.
19. (canceled)
20. The apparatus of claim 18 wherein the circuit to receive the
signal and generate the reconstructed reference signal comprises: a
frequency multiplier circuit, coupled to one of the resonant
circuit or the bandpass filter, to multiply the frequency of the
received signal by an integer "n" to produce a result; and a
frequency divider circuit to divide the resulting frequency by an
integer "m" wherein m/n<1.
21. The apparatus of claim 20 further comprising an integrator
period generator, coupled to the frequency divider circuit, to
generate an integrated signal wherein the integrated signal is
derived from the reconstructed reference signal over a time period
of 1/f.sub.0 wherein f.sub.0 is a fundamental frequency of the
reference signal.
22. The apparatus of claim 21 further comprising a lock-in
amplifier, coupled to the at least one magnetometer or the
gradiometer and the reconstruction circuit, to generate the
filtered ranging signal based on the detected signals, representing
the magnetic or electric field values or changes in the magnetic or
electric fields or the gradiometer, generated in response to the
ranging signal being launched down the target well.
23. The apparatus of claim 21 further comprising: at least one
analog-to-digital converter coupled to the at least one
magnetometer or the gradiometer and the reconstruction circuit; and
a digital signal processor coupled to the at least one A/D
converter to generate the filtered ranging signal based on the
detected signals representing the magnetic or electric field values
or changes in the magnetic or electric fields.
24. The apparatus of claim 17 further comprising a bandpass filter
coupled to an output of the at least one magnetometer or the
gradiometer to remove effects of the reference signal from detected
signals.
25. (canceled)
26. A transmitter apparatus comprising: an oscillator circuit that
generates a clock signal having a clock frequency; a frequency
scaler circuit, coupled to the oscillator circuit, that generates a
reference signal having a lower frequency than the clock frequency;
a ranging signal generation circuit, coupled to the oscillator
circuit, to generate a ranging signal having the clock frequency,
the ranging signal coupled to a target well; and a transmitter
circuit, coupled to the frequency scaler circuit, configured to
transmit the reference signal into a surface of a geological
formation.
27. The transmitter apparatus of claim 26 wherein the transmitter
circuit is at least one of: a loop antenna coupled to the surface
or a plurality of dipole antenna coupled to the surface.
28. (canceled)
Description
BACKGROUND
[0001] Currently, it is desirable to know the location of a target
well in relation to a ranging well that may be used for operations
such as steam assisted gravity drainage (SAGD).
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] FIG. 1 illustrates an embodiment of a system for target well
ranging.
[0003] FIG. 2 illustrates another embodiment of a system of target
well ranging.
[0004] FIG. 3 illustrates another embodiment of a system of target
well ranging.
[0005] FIG. 4 illustrates another embodiment of a system of target
well ranging.
[0006] FIG. 5 illustrates an embodiment of a circuit for generating
a reference signal in accordance with the systems of FIGS. 1-4.
[0007] FIG. 6 illustrates an embodiment of an apparatus for
reconstructing a received reference signal and detecting a magnetic
or electric field in accordance with the embodiments of FIGS.
1-4.
[0008] FIG. 7 illustrates a representative plot of lock-in
amplifier output signals in accordance with the embodiment of FIG.
6.
[0009] FIG. 8 illustrates an enlarged view of the plot of signals
presented in FIG. 7.
[0010] FIG. 9 illustrates a representative plot of a ratio of the
signal amplitudes at the ranging tool in accordance with the
embodiment of FIG. 6.
[0011] FIG. 10 illustrates a plot of the ratio of the signal
amplitudes at the ranging tool having an increased ranging
frequency in accordance with the embodiment of FIG. 6.
[0012] FIG. 11 illustrates wireline and drilling embodiments of a
system for target well ranging.
DETAILED DESCRIPTION
[0013] The various embodiments described herein operate to provide
information that assists in verifying 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.
[0014] For purposes of clarity, a "target well" will be defined as
a well, the location of which is to be used as a reference for the
construction of another well. The other well will 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. In most
embodiments, the location of the ranging well is fairly well known,
the location of the target well is not as well known, and it is the
distance from the ranging well to the target well that is to be
determined.
[0015] 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 the earth's surface down the casing of
the target well and receiving a signal radiated from that casing in
the ranging well. Due to propagation through different geological
formations, the received signals can be very weak, especially
considering that several technologies rely on measuring a magnetic
gradient across the relatively small diameter of a logging or
drilling tool. Subsequently described embodiments can operate to
increase the signal-to-noise ratio in a signal received by a
magnetic ranging tool, resulting in increased accuracy of
determining a distance between the ranging well and the target well
as well as boosting the range at which target wells can be reliably
detected in order to plan well intersection or well collision
avoidance.
[0016] At close range, the present embodiments can enable operation
at higher frequencies than were previously possible. Some
embodiments can be easily generalized to other ranging tools that
make use of signals from a well to which ranging occurs (e.g., a
target well). Examples of such tools include those that range using
electric fields, or a combination of electric and magnetic fields.
The embodiments herein can be useful when ranging to wells through
highly conductive geological formations.
[0017] The embodiments for target well ranging, illustrated in
FIGS. 1-4, include some common characteristics. For example, the
embodiments generate a ranging signal directly from a clock signal,
generated by a master oscillator, and a reference signal based on
the clock signal. The resulting reference signal is at a
significantly lower frequency than the ranging signal. An
apparatus, coupled to a downhole tool, includes reconstruction
circuitry to reconstruct the reference signal from a received
signal that was propagated to the downhole tool.
[0018] Embodiments can include one or more magnetometers, magnetic
gradiometers, or electric field sensors to measure field values or
changes in field values induced by the target well based on the
ranging signal being transmitted down the target well. The relative
location of the target well can then be determined in relation to
the ranging well based on a relationship between the reconstructed
reference signal and magnetic or electric field values or changes
in these fields, as discussed subsequently. The relative location
of the target well can include relative distance and angle from the
ranging well.
[0019] The ranging signal that is transmitted on a casing of the
target well can be generated directly from a master oscillator. The
reference signal is derived from the same master oscillator.
However, the frequency of the reference signal is different from
that of the ranging signal driving the casing. The reference signal
is obtained by using a frequency multiply and divide circuit (e.g.,
frequency altering circuit).
[0020] If the frequency of the reference signal was only an
integral fraction of the master oscillator frequency, harmonic
distortion in this signal might easily be confused with the ranging
signal to be detected. That is, such a harmonic can corrupt the
received ranging signal. This can also corrupt the reference to a
lock-in amplifier used in the system (to be described in more
detail later). Hence, in various embodiments, the reference signal
is generated by dividing the frequency of the master oscillator by
an integer "n" and multiplying by a different integer "m", where
m/n<1. For example, if the ranging signal has a frequency of 5
Hz, a signal at 5/7 Hz can first be generated, and then a signal at
2*5/7 Hz can be generated from that. The fundamental and first
three harmonics of such a signal are 1.4285 Hz, 2.8571 Hz, 4.28571
Hz, and 5.7143 Hz.
[0021] Signals at any of these frequencies can be easily separated
from the 5 Hz ranging signal emanating from the casing of the
target well. The fundamental (1.4285 Hz in this example) can
propagate through the earth with considerably less signal loss than
the 5 Hz ranging signal.
[0022] Lock-in amplifiers, as described subsequently, can achieve a
separation of signals that are far closer in frequency than those
of the example used above, even if signal loss through the earth is
severe. Thus, it may be possible to provide a reference signal at
an even lower frequency than this example without causing signal
corruption of the ranging signal. For example, using a ratio of
2/17 of the base frequency, the fundamental and first 8 harmonics
of a 5 Hz signal are 0.5882 Hz, 1.1765 Hz, 2.3529 Hz, 2.9412 Hz,
3.5294 Hz, 4.1177 Hz, 4.7059 Hz, and 5.2941 Hz.
[0023] FIG. 1 illustrates an embodiment of a system for target well
ranging. This system uses a transmitter circuit comprising a large
area current loop 150 (e.g., loop antenna) to launch the reference
signal into the geological formation above the ranging well 101.
The loop antenna 150 can be simply a large conductor near or in
contact with the surface of the geological formation.
[0024] The system comprises the master oscillator 120 which is a
precision clock. It is desirable that the frequency of this clock
be relatively stable, as defined subsequently. The ability of the
subsequently described lock-in amplifiers to discriminate against
noise is a function of the stability of the reference signal,
generated from the master oscillator 120, and of the integration
time used in the lock-in amplifier. Thus, the more stable the
clock, the better the discrimination performance of the lock-in
amplifiers.
[0025] The master oscillator 120 is coupled to a power amplifier
122 (e.g., balanced input/output amplifier) that has an output
coupled to the casing 130 of the target well 100 and an output
coupled to a grounded electrode 124. The power amplifier 122
provides the power necessary to launch the ranging signal down the
target well 100.
[0026] A frequency scaler circuit 121 (e.g., frequency divider and
multiplier) is coupled to the master oscillator 120. The frequency
scaler circuit 121 provides the down-converted reference signal for
transmission. The frequency scaler circuit 121 divides the
frequency of the clock signal from the master oscillator 120 by the
first integer "n" and multiplies the result by the second integer
"m" such that m/n<1.
[0027] A power amplifier 123 (e.g., balanced input/output
amplifier) is coupled to the output of the frequency scaler circuit
121. The power amplifier 123 provides the power used to launch the
reference signal into the geological formation above the ranging
and target wells 100, 101. An embodiment of the reference signal
generator and transmitter circuitry 120, 121, 123 is described
subsequently with reference to FIG. 5.
[0028] The system further comprises a ranging tool 103 in the
ranging well 101. The ranging tool 103 can be a downhole tool that
is part of the drill string in the ranging well 101. The ranging
tool 103 includes the apparatus for sensing the magnetic or
electric fields for determining a relative location of the target
well 100 from the ranging well 101. The apparatus for sensing the
magnetic or electric fields is described subsequently in greater
detail with reference to FIG. 6.
[0029] FIG. 2 illustrates another embodiment of a system for target
well ranging. This system uses a transmitter circuit comprising one
or more ground contacts (e.g., dipole transmitters) 250, 251 to
launch the reference signal into the geological formation above the
target well 200 and the ranging well 201.
[0030] The system comprises the master oscillator 220 that is
coupled to the power amplifier 222 having an output coupled to the
casing 230 of the target well 200 and an output coupled to a
grounded electrode 224. The power amplifier 222 (e.g., balanced
input/output amplifier) provides the power used to launch the
ranging signal down the target well 200.
[0031] The frequency scaler circuit 221 (e.g., frequency divider
and multiplier) is coupled to the master oscillator 220. The
frequency scaler circuit 221 provides the down-converted reference
signal for transmission. The frequency scaler circuit 221 provides
a signal at a frequency that is obtained by dividing the frequency
of the clock signal from the master oscillator 220 by a first
integer "n" and multiplying the frequency of the result by a second
integer "m" such that m/n<1.
[0032] The power amplifier 223 (e.g., balanced input/output
amplifier) is coupled to the output of the frequency scaler circuit
221. The power amplifier 223 provides the power used to launch the
reference signal, through the ground contacts 250, 251, into the
geological formation above the ranging and target wells 200, 201.
An embodiment of the reference signal generation circuitry 220,
221, 223 is described subsequently with reference to FIG. 5.
[0033] The system further comprises the ranging tool 203 in the
ranging well 201. The ranging tool 203 can be a downhole tool that
is part of the drill string in the ranging well 201. The ranging
tool 203 includes the apparatus for sensing the magnetic or
electric fields for determining a relative location of the target
well 200 from the ranging well 201. The apparatus for sensing the
magnetic or electric fields is described subsequently in greater
detail with reference to FIG. 6.
[0034] FIG. 3 illustrates another embodiment of a system for target
well ranging. This system uses an electrode 351 in the surface and
an electrode 350 coupled to the casing 331 of the well containing
the ranging tool 303 (e.g., ranging well) in order to launch the
reference signal into the geological formation above the target
well 300 and the ranging well 301, as well as down the length of
the ranging well 301.
[0035] The system comprises the master oscillator 320 that is
coupled to the power amplifier 322 (e.g., balanced input/output
amplifier) that has an output coupled to the casing 330 of the
target well 300 and an output coupled to a grounded electrode 324.
The power amplifier 322 provides the power used to launch the
ranging signal down the target well 300.
[0036] A frequency scaler circuit 321 (e.g., frequency divider and
multiplier) is coupled to the master oscillator 320. The frequency
scaler circuit 321 provides the down-converted reference signal for
transmission. The frequency scaler circuit 321 divides the clock
signal from the master oscillator 320 by a first integer "n" and
multiplies the result by a second integer "m" such that
m/n<1.
[0037] The power amplifier 323 (e.g., balanced input/output
amplifier) is coupled to the output of the frequency scaler circuit
321. The power amplifier 323 provides the power used to launch the
reference signal into the geological formation above the ranging
and target wells 300, 301 as well as down the ranging well 301. The
power amplifier 323 has an output coupled to an electrode 351 in
the ground and an output coupled to an electrode 350 coupled to the
well casing of the ranging well 301. An embodiment of the reference
signal generation circuitry 320, 321, 323 is described subsequently
with reference to FIG. 5.
[0038] The system further comprises a ranging tool 303 in the
ranging well 301. The ranging tool 303 can be a downhole tool that
is part of the drill string in the ranging well 301. The ranging
tool 303 includes the apparatus for sensing the magnetic or
electric fields for determining a relative location of the target
well 300 from the ranging well 301. The apparatus for sensing the
magnetic or electric fields is described subsequently in greater
detail with reference to FIG. 6.
[0039] FIG. 4 illustrates another embodiment of a system for target
well ranging. This system uses a summing amplifier 424 to add the
reference signal to the ranging signal prior to coupling the
resulting summed signal to the casing 430 of the target well
400.
[0040] The system comprises a master oscillator 420 that is coupled
to the summing amplifier 424, which in turn has an output coupled
to a power amplifier 422 (e.g., balanced input/output amplifier).
An output of the power amplifier 422 is coupled to the casing 430
of the target well 400 and another output of the power amplifier
422 is coupled to a grounded electrode 425. The power amplifier 422
provides the power used to launch the ranging signal down the
target well 400.
[0041] A frequency scaler circuit 421 (e.g., a frequency divider
and multiplier) is coupled to the master oscillator 420. The
frequency scaler circuit 421 provides the down-converted reference
signal for transmission. The frequency scaler circuit 421 divides
the clock signal from the master oscillator 420 by a first integer
"n" and multiplies the result by a second integer "m" such that
m/n<1.
[0042] A first amplifier 423 (e.g., balanced input/output
amplifier) is coupled to the output of the frequency scaler circuit
421. An output of the amplifier 423 is coupled to an input of the
summing amplifier 424 so that the summing amplifier 424 can add the
reference signal to the ranging signal prior to launching the
summed result down the target well 400. A power amplifier 426 has
an input coupled to an output of the amplifier 423, an output
coupled to a grounded electrode 427, and an output coupled to the
well casing 450 of the ranging well 401. The power amplifier 426
provides the power used to launch the reference signal down the
ranging well 401. An embodiment of the reference signal generation
circuitry 420, 421, 423 is described subsequently with reference to
FIG. 5.
[0043] The system further comprises a ranging tool 403 in the
ranging well 401. The ranging tool 403 can be a downhole tool that
is part of the drill string in the ranging well 401. The ranging
tool 403 includes the apparatus for sensing the magnetic or
electric fields for determining a relative location of the target
well 400 from the ranging well 401. The apparatus for sensing the
magnetic or electric fields is described subsequently in greater
detail with reference to FIG. 6.
[0044] The embodiments of FIGS. 1-4 provide different ways for
transmitting the reference signal. For example, some embodiments
might be appropriate when safety requirements would preclude
embodiments in which the transmitted signal, often a high energy
signal, might present a hazard to rig personnel or an explosion
hazard.
[0045] FIG. 5 illustrates an embodiment of the reference signal
generating circuitry. The embodiment of FIG. 5 is for purposes of
illustration only as the reference signal can be generated in other
ways. This circuit generates the reference signal having a
frequency of f.sub.0m/n cycles per second, where f.sub.0 is the
frequency of the master oscillator (e.g., precision clock).
[0046] The circuit is comprised of the master oscillator 520 that
generates the clock signal. A multiplying circuit 501 multiplies
the frequency f.sub.0 by the integer "m". The result, mf.sub.0, is
then input to a dividing circuit 502 that divides the result by the
integer "n" to generate a reference signal having a frequency of
f.sub.0m/n.
[0047] A conversion circuit 503 takes the clock signal from the
dividing circuit 502 and converts it to a sine wave having a
frequency off where f=f.sub.0m/n. In another embodiment, the
conversion circuit 503 may be left out by simply filtering the
output of the dividing circuit 502. The sine wave from the
conversion circuit 503 is input to a filter (e.g., lowpass,
bandpass) 504 that eliminates unwanted frequencies proximate to the
reference frequency. An amplifier 505 provides the power necessary
for transmission of the signal to a load 506 according to one or
more of the above-described embodiments. The load 506 can be a
transmitter 150, ground contacts 250, 251, a casing connection 350
and ground connection 351, or a casing connection 450 and ground
connection 427.
[0048] In another embodiment, the reference signal can be generated
in the digital domain. In the case of a digital implementation,
where the reference signal exists in digital form, the digital form
of the reference signal is converted to an analog format using a
digital-to-analog converter. The resulting reference signal is
bandpass-filtered in order to remove the effects of the
discretization of the signal. Generally, a bandpass or a lowpass
filter may also be used with analog implementations of multipliers
and dividers since nonlinearities may exist in these circuits.
[0049] FIG. 6 illustrates a block diagram of an embodiment of an
apparatus for reconstructing a received reference signal and
detecting a magnetic or electric field in accordance with the
embodiments of FIGS. 1-4. The apparatus of FIG. 6 can be located in
a ranging tool as part of the drill string in the ranging well.
FIG. 6 illustrates how a lock-in amplifier can be used with the
reference and ranging signals to improve the signal-to-noise ratio
over that obtained with prior art filtering techniques for ranging
signals that do not make use of a reference signal.
[0050] The bock diagram of FIG. 6 is for purposes of illustration
only as the reference signal can be reconstructed using other
embodiments (e.g., digitally) and the illustrated magnetometers can
be replaced with gradiometers to detect an electric field. Other
embodiments can use different quantities of magnetometers and/or
gradiometers depending on the granularity desired for detecting the
reference signal.
[0051] The magnetic or electric fields detected by the apparatus
arise from fields induced in the target well as a result of the
ranging signal being launched down the target well. Thus, the
signal received as detected magnetic or electric fields can be
considered to comprise the ranging signal in combination with
noise. This noise can be filtered using the reconstructed reference
signal to produce a filtered ranging signal, as described
subsequently.
[0052] The apparatus of FIG. 6 comprises a solenoidal or toroidal
antenna 600 in parallel with a capacitor 601 to form a filtering
element 602 (e.g., resonant circuit). The resonant frequency of
this circuit is set to a frequency f.sub.1 (i.e., the fundamental
frequency) of the reference signal. In another embodiment, a
bandpass filter can be used as the filtering element 602. The
intent is to avoid corruption of the reference signal by the
ranging signal. The filtering element 602 also serves to improve
the detection of the reference signal by filtering out noise near
the reference frequency.
[0053] An amplifier 603 is coupled to the output of the filtering
element 602, which may comprise a resonant circuit or filter. The
amplifier 603 outputs the amplified received signal at a frequency
of f.sub.1 to a frequency multiplier circuit 605 that outputs a
signal having a frequency n*f.sub.1, where "n" is an integer as
defined earlier. The output of the multiplier circuit 605 is input
to a frequency divider circuit 607 that develops a signal at a
frequency of n*f.sub.1/m, where "m" is an integer as defined
earlier. Since n*f.sub.1/m=f.sub.0, this creates a signal at
precisely the frequency of the signal that is launched onto the
target well. Neglecting propagation delay, which is small in
comparison to signal acquisition time, this signal will track slow
drifts in the reference frequency f.sub.0.
[0054] Three signals are generated from this signal at frequency
f.sub.0. Two of these signals 610, 611 are in quadrature. That is,
one can be represented as an amplitude times a sine wave at
frequency f.sub.0 with a certain phase shift while the other can be
represented as an amplitude times a cosine wave at frequency
f.sub.0 with the same phase shift (or a sine wave with an
additional phase shift of 90.degree. over that of the first sine
wave). The third signal 612, shown as a rectangular wave, is
generated from an integrator period generator circuit 609 that
generates a rectangular wave having a period that is an integer
multiple of 1/f.sub.0. The output signal 612 is used to set the
time over which two integrator circuits 642, 643 in each of a
plurality of lock-in amplifiers 620-623 operate.
[0055] The two quadrature signals 610, 611 are coupled to the
plurality of lock-in amplifiers 620-623 as reference signals. The
rectangular wave output signal from the integrator period generator
circuit 609 is also coupled to each of the lock-in amplifiers
620-623. Outputs of a 3-axis magnetometer 630-633 are coupled to
respective lock-in amplifiers 620-623.
[0056] The magnetometers 630-633 are used to detect the magnetic
field. At least one magnetometer is used to detect the reference
signal while at least one magnetometer (or at least one magnetic
gradiometer) is used to detect the ranging signal. The
magnetometers 630-633 can be flux gate magnetometers,
magnetoresistive magnetometers, spin exchange relaxation-free
(SERF) or similar atomic magnetometers (or any other devices with a
similar use) as well as inductors or toroids when used to receive a
time varying magnetic field. It is possible to receive the
reference and ranging signals with the same magnetometer(s). The
magnetometers can be vector magnetometers (i.e., magnetometers that
have a preferred sense axis) or scalar magnetometers (i.e.,
magnetometers that measure the magnitude of the magnetic field). In
some ranging applications, multiple axis magnetometers can be used.
The magnetometers associated primarily with detection of the
ranging signal are generally vector type magnetometers.
[0057] The outputs of the lock-in amplifiers 620-623 are
transmitted to one or more signal processors 650-653 to monitor the
detected signals in the ranging well to determine a relative
location of the ranging well in relation to the target well based
on the relationship between the detected magnetic fields (e.g.,
filtered ranging signal) based on measurements made at a plurality
of positions in the ranging well. Another embodiment can use only a
single signal processor with multiple inputs for processing the
signals. In another embodiment, an inference can be made from a
single measurement in the ranging well based on a magnetic
gradient. The reference signal can be used as a reference frequency
for the lock-in amplifiers.
[0058] In operation, using magnetometer1 630 as an example, the
output of a first magnetometer 630 is amplified and possibly
bandpass filtered and then fed to a pair of mixers 640, 641. Each
mixer 640, 641 has two inputs and multiplies both of these inputs
together to produce an output signal. The second input to one of
the mixers is the sine wave 610 at frequency f.sub.0, while the
second input to the other mixer is the cosine wave 611 at frequency
f.sub.0. To better illustrate the operation of the mixers 640, 641,
and later of the integrators 642, 643, signal terminology and
equations will now be introduced.
[0059] Setting S.sub.m as the output of a magnetometer that is used
as one input to a mixer:
S.sub.m=A.sub.rsin(2.pi.f.sub.0t+.theta.)+N.sub.r(t)+N.sub.i(t),
[0060] where A.sub.r is the amplitude of the ranging signal
received at the ranging tool and may include a gain factor; [0061]
f.sub.0 is the frequency in Hz of the ranging signal; [0062] t is
the time in seconds; [0063] .theta. is a phase factor related to
the time reference and the propagation delay of the signal from the
casing of the target well to the ranging tool; [0064] N.sub.r(t) is
the received noise at the magnetometer; and [0065] N.sub.i (t) is
the instrumentation noise added by the electronics at the
magnetometer input to the mixer.
[0066] The sinusoidal input to the mixer, R.sub.s is given by:
R.sub.S=Bsin(2.pi.f.sub.0t+.phi.)+N(t); [0067] where B is the
amplitude of the sinusoidal output of the circuit that regenerated
the signal at frequency f.sub.0 from the received signal at
f.sub.1; [0068] .phi. is a phase factor, similar to .theta.. Note
that .phi. and .theta. will vary slowly with time as the ranging
tool is moved in the borehole; and [0069] N(t) is the electronic
noise from the circuit that regenerated the signal at frequency
f.sub.0.
[0070] Using the above equations for S.sub.m and R.sub.S, the
output of the first mixer is given by:
S m ( t ) * R s ( t ) = N ( t ) ( N i ( t ) + N r ( t ) ) + 1 2 A r
B cos ( .theta. - .PHI. ) + sin ( 2 .pi. tf 0 ) ( cos ( .theta. ) A
r N ( t ) + cos ( .PHI. ) BN i ( t ) + cos ( .PHI. ) BN r ( t ) ) +
cos ( 2 .pi. tf 0 ) ( sin ( .theta. ) A r N ( t ) + sin ( .PHI. )
BN i ( t ) + sin ( .PHI. ) BN r ( t ) ) + B 2 A r cos ( 4 .pi. tf 0
- .theta. - .PHI. ) . ##EQU00001##
[0071] By a simple trigonometric identity, the product of the two
components at frequency f.sub.0 is a constant+a component at
frequency f.sub.0+a component at a frequency of 2*f.sub.0. The
individual noise terms are multiplied by sine and cosine terms at
frequency f.sub.0 or by each other. The output of the second mixer
641 is calculated in an analogous fashion.
[0072] The outputs of the two mixers 640, 641 are then sent to
integrators 642, 643 and integrated over N cycles with period
1/f.sub.0, that is for a time of N/f.sub.0 seconds. In integrating
over an integer number of cycles at frequency f.sub.0, the terms at
a frequencies f.sub.0 and 2*f.sub.0 become 0. If the noises are
truly random noises, only the component that is coherent with
frequency f.sub.0 makes any contribution to the integral and, so,
the noise component from these terms is greatly reduced. Likewise,
assuming that the noise terms are not correlated, the first term, a
product of noise terms, will be greatly reduced upon integration.
This leaves only the term 1/2A.sub.rB cos(.theta.-.phi.) from the
result in the above equation and a similar term 1/2A.sub.rB
sin(.theta.-.phi.) from the integrator 643.
[0073] The outputs of the integrators 642, 643 are sensed by
electronics after the period of N/f.sub.0, the integrators 642, 643
are reset and another integration period of N/f.sub.0 seconds
commences (there are ways of dealing with these outputs as analog
signals; in some embodiments, an analog to digital converter would
be used along with signal processing hardware and software to
accomplish this task).
[0074] The resulting sampled data streams have random noise reduced
by the square root of N with respect to the noise at the inputs to
the integrators 642, 643. The root mean square of the sum of the
outputs from the two integrators 642, 643 will be proportional to
the output of the magnetometer 1 630, but with significantly
reduced noise. The ratio of the amplitudes of the two signals from
the integrators 642, 643 provides some information about the
propagation of the signals through the earth, but are not usually
of interest for ranging. In some embodiments, a sudden change in
this ratio (which is the tangent of the phase of the ranging signal
with respect to that of the recovered signal at frequency f.sub.0
used for the reference) provides an indication of a significant
change at the target well or a significant change in the relation
between the target well and the ranging tool, so the phase or ratio
of phases may be monitored.
[0075] In some embodiments, all or part of the lock-in amplifiers
620-623 can be implemented using A/D converters and digital signal
processing. In that case, the digital samples should be made at
some high multiple of the reference frequency f.sub.0 that can be
generated with another frequency multiplier.
[0076] Criteria for specifying the frequency separation between the
ranging and reference signals, and for specifying the stability of
the master oscillator, can be determined as described
subsequently.
[0077] The selectivity of the lock-in amplifier can be examined by
calculating its output when the reconstructed reference frequency
differs from the frequency of the signal at the other input to the
lock-in amplifiers 620-623 (in the illustrated example, this is the
output of one of the other magnetometers 631-633).
[0078] The output of the lock-in amplifier for two such signals is
shown in FIG. 7 for integration times of 1/f.sub.1, 10/f.sub.0,
100/f.sub.0 and 1,000/f.sub.0. To make the comparisons easier to
understand, each output has been divided by the output of the
lock-in amplifier when there is no difference between the signals.
The abscissa of FIG. 7 is the fractional difference in frequency
between the two signals. For example, at an abscissa value of 0.1,
the signal input from the ranging magnetometer is 1.1*f.sub.0, and
an abscissa value of -0.1 corresponds to a signal from the ranging
magnetometer of 0.9*f.sub.0. As can be seen in the figure, the
selectivity of the lock-in amplifier increases dramatically as the
integration time is increased.
[0079] The same curves are presented in FIG. 8, but the range of
frequencies is reduced, covering only 0.01 times (normalized to
f.sub.0) that which is shown in FIG. 7. Even over the greatly
reduced range of FIG. 8, the curve corresponding to integration
over 1,000 cycles can be difficult to interpret. A local maximum of
0.01 occurs near a frequency offset of 0.001 (i.e. 1.001*f.sub.0).
That is, a signal at frequency 1.001*f.sub.0 is reduced to 0.01 of
the amplitude obtained with the same signal but with a frequency of
f.sub.0. This is a rejection of 40 dB. At a frequency offset of
0.01 (i.e. 1.01*f.sub.0), it is reduced by 60 dB. For most ranging
applications, it is reasonable to use an integration time of 1,000
cycles of the ranging signal. For example, with a 5 Hz ranging
signal, the integration time over 1,000 cycles is 3 minutes, 20
seconds. Shorter integration times can be used.
[0080] In a typical embodiment, the frequency of the reference
oscillator is sufficiently stable that the amplitude output of the
lock-in amplifier of the ranging signal does not drift to less than
0.99 of its optimal value. This corresponds to a frequency offset
of .+-.0.0000391*f.sub.0. A typical ranging signal has a frequency
of around 5 Hz. It takes 200 seconds to produce 1,000 cycles at 5
Hz. Therefore, a clock would have to drift by as much as 1 part in
a million/second in order to produce such a frequency shift. Clocks
of this quality or better are readily available.
[0081] Several embodiments also take into account the effect of
receiving the frequency downshifted reference signal with the
ranging signal in a magnetometer set up to receive the ranging
signal. That is, the reference signal may be received at one or
more of the ranging magnetometers and may be stronger than the
ranging signal at these magnetometers. Since the frequency
downshifted reference signal is not an integral sub-harmonic of the
ranging signal, this should not create a problem as long as care is
taken to properly separate its frequency (and that of any of its
harmonics) from f.sub.0. In an embodiment, a bandpass filter can be
inserted between each ranging magnetometer and the input to the
mixers in the lock-in amplifier.
[0082] Several embodiments also deal with preventing components of
the ranging signal, received at the magnetometer used to receive
the frequency downshifted signal, from corrupting the reconstructed
signal at frequency f.sub.0. This is important since an error may
otherwise be introduced in re-creating a signal at frequency
f.sub.0. It is thus recommended that the antenna for receiving the
frequency downshifted reference signal be put in resonance with a
capacitor or that the output of the antenna be bandpass filtered.
Even though the frequency downshifted signal may be stronger than
the ranging signal, the frequency downshifted and ranging signals
should be separated, in some embodiments, at the input to that part
of the system which reconstructs a reference signal at frequency
f.sub.0.
[0083] In addition to, or as an alternative to using a resonant
circuit or a filter, if the frequency downshifted signal is
launched down the casing of the borehole that contains the ranging
tool, it will hop to the drill pipe in the borehole, and can be
detected with a toroidal antenna or as a voltage across a resistive
gap. The toroid or resistive gap may not be sensitive to many forms
of ranging signals. In particular, it may not be sensitive to a
signal generated by launching current down the casing of the target
well when the ranging and target wells are approximately
parallel.
[0084] Considerations of propagation through the geological
formation or the well casings can have an effect on the appropriate
frequency of the reference signal and of the ranging signal to be
used in a given application. Since f.sub.1<<f.sub.0, the
attenuation of the reference signal at frequency f.sub.1 will be
less than that at frequency f.sub.0. The degree of difference will
depend on the specific implementation, well profile, formation
resistivities, and frequencies. For the following examples,
consider a system wherein the reference signal is generated at
0.5882 Hz, the frequency of the ranging signal on the target well
is 5 Hz, and both signals are launched onto the target well, while
the return for the signal at frequency f.sub.1 is attached to a
remote grounding point.
[0085] The ratio of the signal amplitudes at the ranging tool is
shown in FIG. 9 for geological formation resistivities of 1 and 0.2
ohm meters as a function of distance from the ranging tool to the
target well. In this figure, it is assumed that both signals have
the same current in the target well. In this case, at a range of
about 90 meters, and with a resistivity of 0.2 ohm meter, the
signal at 0.5882 Hz is a factor of 1.8 times stronger than the
signal at 5 Hz. In a 1 ohm meter formation, it is about 1.7 times
stronger than the ranging signal at a range of 200 meters. As the
range is decreased, the signal strength ratio decreases. However,
the signal strength ratio is an increasing function of the ratio of
f.sub.0/f.sub.1. Hence, it is possible to increase the frequency of
the oscillator as the range is decreased and thus provide better
resolution than would otherwise be possible.
[0086] The effect of increasing the ranging frequency to 25 Hz is
shown in FIG. 10. In the past, it has not been desirable to
increase the frequency of the ranging frequency because of
attenuation, even though resolution is improved with frequency. In
the illustrated embodiment, at a range of 90 meters, the ratio of
the amplitude of the reference signal to the ranging signal is 5.5,
or 14.8 dB which, with other filters in the circuit as described
earlier, may be sufficient to provide improved performance when a
lock-in amplifier is used.
[0087] If the signal is launched as illustrated in the embodiments
shown in FIG. 3 or 4, the analysis is analogous to that used for an
electromagnetic (EM) telemetry downlink, and the ratio of the
signal received as the low frequency reference signal to the
ranging signal can be considerably improved over embodiments where
both signals are only launched on to the target well. EM downlink
signals can typically be received at depths of 10,000 feet.
Downlinks are typically strongest when the return leg of the
downlink is the casing of a second well, as illustrated in the
embodiment of FIG. 4.
[0088] In other embodiments, the downhole ranging tool can be
designed to receive commands from the earth's surface and select
different filters so as to make operation at different frequencies
possible. The reference signal can be modulated, included in a
separate mud pulse, or EM downlinks may be used to accomplish such
an embodiment.
[0089] There may be situations where the reference signal is no
stronger than the ranging signal. In this case, and where it may be
determined that the lock-in amplifier may actually be adding noise
to the system, intelligence can be added to detect such a
condition, so that the lock-in amplifiers are bypassed.
[0090] Launching a signal into a casing is an operation that is
known by one of ordinary skill in the art. For example, a target
well may have a casing. The 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, or may 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 well may represent some other anomaly located in the earth,
such as an electrically conductive geological formation, 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.
[0091] While above embodiments discuss launching the ranging signal
on the target well casing, other embodiments are not limited to
this method of launching the ranging signal. For example, the
ranging signal may be launched using a wireline or using an
electromagnet.
[0092] The bottom-most drill string subsection carries the drill
bit in many cases. The drill bit subsection may comprise a bent sub
which angles the drill bit with respect to the longitudinal axis of
the drill string to permit changes in the direction of drilling, or
may be a straight section for straight-ahead drilling. With the use
of a bent sub, the direction of drilling may be controlled by
rotating the entire drill string from the wellhead, thereby turning
the bent sub.
[0093] As is known, the drill bit may be driven by a motor which in
turn is driven by the flow of drilling mud down the drill string
bore. The mud flows out of the drill string at the bent sub and
around the location of the drill bit and flows up and out of the
well through the annular space around the outside of the string,
carrying the material loosened by the drill up and out of the well
through an outlet fitting.
[0094] Located within the drill string bore may be an alternating
magnetic field detector, or magnetometer. The detector may be
adapted to detect alternating magnetic fields produced in the earth
surrounding the target by alternating current flow induced in the
conductive material (such as a well casing) of the target well.
This current may be induced by way of an electrode located at the
surface of the earth very close to, or, electrically connected to,
the casing. The electrode induces a current flow I in the
electrically conductive target casing, and this vertically flowing
current produces a magnetic field surrounding the casing. The
magnetic field lines are perpendicular to the direction of current
flow I, and thus are generally horizontal when the well is
vertical. The field extends outwardly from the target well casing
to a distance dependent upon the magnitude of the current flow and
upon the nature of the strata surrounding the target well and the
relief borehole.
[0095] The electrode, which may be a conductive rod located in the
earth near the wellhead or may be a connector for securing a cable
to the metal casing, is connected by way of a cable to a source of
alternating current (AC). The other side of the AC source is
connected to a ground point at a location spaced away from the
target well by a distance sufficient to ensure that the current
will flow primarily in the casing. As the current flows downwardly
in the casing, it will gradually be dissipated outwardly into the
surrounding earth, and will return to the ground electrode, but
since the current flow in the casing is highly concentrated, as
compared to the current flow through the earth, the magnetic field
produced by the casing current will predominate and will be
detectable by the magnetometer.
[0096] A magnet gradiometer, which may be a highly sensitive
magnetic field detector, includes a pair of field sensors each
having two spaced, parallel legs. The legs may be ferrite rods,
each several inches long and surrounded by corresponding solenoidal
windings. The magnetic gradiometer may be located in a suitable
housing within the central opening of the drill string but
alternatively may be mounted on the exterior thereof, as in reduced
areas or notches formed on the outer surface of the drill string.
In this latter configuration, the two legs of each sensor are
mounted on diametrically opposite sides of the drill string so that
they are spaced apart by a distance approximately equal to the
diameter of the drill string. In either arrangement, the two legs
of each sensor have their axes of maximum sensitivity parallel to
each other, and perpendicular to the axis of the drill string,
while the axes of maximum sensitivity of the two sensors are
perpendicular to each other.
[0097] When a relief borehole is being drilled, the borehole is
initially directed toward the target well through the use of
conventional methods. The magnetometer can be located within the
drill string or on its surface, and is operable during the actual
drilling, although for greatest accuracy the drilling operation
would normally be momentarily halted while measurements are made.
The information obtained from the magnetometer may be used to
control the directional drilling of the relief borehole (as a
ranging well), among other uses, as described herein.
[0098] FIG. 11 illustrates a drilling rig system 1164 embodiment
that can incorporate the above-described embodiments. Thus, system
1164 may comprise portions of a downhole tool 1124 as part of a
downhole drilling operation.
[0099] 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 1110 into a wellbore
or borehole 1112. A system 1164 may form a portion of a drilling
rig 1102 located at the surface 1104 of a well 1106. The drilling
rig 1102 may provide support for a drill string 1108. The drill
string 1108 may operate to penetrate a rotary table 1110 for
drilling a borehole 1112 through subsurface geological formations
1114. The drill string 1108 may include a Kelly 1116, drill pipe
1118, and a bottom hole assembly 1120, perhaps located at the lower
portion of the drill pipe 1118. In some embodiments, ranging tool
apparatus 103, 203, 303, 403 of FIGS. 1, 2, 3, 4, respectively, may
be carried as part of the drill string 1108 or the downhole tool
1124.
[0100] The bottom hole assembly 1120 may include drill collars
1122, a downhole tool 1124, and a drill bit 1126. The drill bit
1126 may operate to create a borehole 1112 by penetrating the
surface 1104 and subsurface geological formations 1114. The
downhole tool 1124 may comprise any of a number of different types
of tools including MWD (measurement while drilling) tools, LWD
tools, and others.
[0101] During drilling operations, the drill string 1108 (perhaps
including the Kelly 1116, the drill pipe 1118, and the bottom hole
assembly 1120) may be rotated by the rotary table 1110. In addition
to, or alternatively, the bottom hole assembly 1120 may also be
rotated by a motor (e.g., a mud motor) that is located downhole.
The drill collars 1122 may be used to add weight to the drill bit
1126. The drill collars 1122 may also operate to stiffen the bottom
hole assembly 1120, allowing the bottom hole assembly 1120 to
transfer the added weight to the drill bit 1126, and in turn, to
assist the drill bit 1126 in penetrating the surface 1104 and
subsurface formations 1114.
[0102] During drilling operations, a mud pump 1132 may pump
drilling fluid (sometimes known by those of skill in the art as
"drilling mud") from a mud pit 1134 through a hose 1136 into the
drill pipe 1118 and down to the drill bit 1126. The drilling fluid
can flow out from the drill bit 1126 and be returned to the surface
1104 through an annular area 1140 between the drill pipe 1118 and
the sides of the borehole 1112. The drilling fluid may then be
returned to the mud pit 1134, where such fluid is filtered. In some
embodiments, the drilling fluid can be used to cool the drill bit
1126, as well as to provide lubrication for the drill bit 1126
during drilling operations. Additionally, the drilling fluid may be
used to remove subsurface formation 1114 cuttings created by
operating the drill bit 1126.
[0103] 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.
* * * * *