Note: Descriptions are shown in the official language in which they were submitted.
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WELLBORE E-FIELD WIRELESS COMMUNICATION SYSTEM
Field of the invention
[0001] The present invention relates to the technical field of establishing
communication
links between surface or land-based equipment and instrumentation arranged in
a
wellbore. More specifically the invention relates to wireless communication in
an annulus
of the wellbore, where the annulus may extend into one or more lateral
wellbores.
Background art
[0002] Wireless downhole sensor technology is being deployed in numerous oil
and gas
wells. In prior art, system components are inductively coupled, which enables
remote
placement of autonomous apparatus in the wellbore without the need to for any
cable
connection, cord or battery to neither power nor communicate. These systems
make use
of a pair of inductive coils where one of the coils usually is casing
conveyed, i.e. arranged
in the wellbore as part of the casing or liner program, and the other coil is
tubing
conveyed, which means that it is inserted into the wellbore as part of the
completion
program. Thus, the pair of coils have to be aligned, usually as part of the
completion
program, so that they are within a certain distance required for the magnetic
field from
one coil to be detected by the other coil and vice-versa.
[0003] The inductive coils typically consist of a conductor wound around a
core. On the
sender side a magnetic field will be generated when an electric current is
applied to the
zo conductor, while on the receiver side a voltage across the conductor coil
will be
generated when the magnetic field from the sender attracts the receiver coil.
We may
say that the receiver coil is harvesting from the sender.
[0004] In prior art, power harvesting has been used to provide power to the
remote side
of the inductive wireless link to power a remote wellbore instrument, so that
the
instrument has sufficient power to transmit data from the remote wellbore
instrument,
e.g. sensor data back to the tubing conveyed coil.
[0005] The tubing conveyed coil may in turn be connected to a surface control
system
aboard a platform or ship by a downhole cable, and the control system will
eventually
receive the information from the remote wellbore instrument so that it can be
used to
analyze the properties of the wellbore or the surrounding formation.
[0006] One problem related to the system of prior art is that the range of the
inductive
wireless link is limited, and that alignment of the inductive coils is
critical for
establishment of the link. This may slow down the progress to run and set a
completion
program for the wellbore due to the inherent need of proximity between the
inductive
couplers involved.
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[0007] A further problem is related to the amount of information that can be
carried
over the inductive wireless link. Information or data is usually in digital
form and
modulated over the low frequency inductive field that works as a carrier.
[0008] US patent 5,008,664 discloses an apparatus employing a set of inductive
coils to
transmit AC data and power signals between a downhole apparatus and apparatus
of the
surface of the earth.
[0009] European patent application EP 0678880 Al discloses an inductive
coupling
device for coaxially arranged tubular members, where the members an be
telescopically
arranged and the liner member has a magnetic core assembly constructed from
magnetic
iron with cylinder sloped ends and the outer member has an annular magnetic
assembly
aligned with the core assembly.
[0010] US patent 4,806,928 discloses a inner and outer coil assemblies
arranged on
ferrite cores arranged on a downhole tool with an electrical device and a
suspension
cable for coupling the electrical device to a surface equipment via the coil
assemblies.
[0011] Of specific interest for this kind of communication systems, is the
possibility for
establishing communication with wellbore instruments in lateral wellbores.
Lateral
wellbores are important for improving production and exploit nearby
occurrences of
petroleum in the formation.
[0012] International patent publication W02001198632 Al and US patent
application
U52011011580 Al discloses the use of inductive wireless links for establishing
communication between a mother wellbore and lateral wellbores. However, in
addition to
the problems related to prior art above, a new problem related to arrangement
of the
inductive coils appears. Due to the nature of the lateral junctions, it is
difficult to avoid
that they become obstacles for the inductive wireless link, so that it it
becomes hard to
establish a reliable communication.
[0013] US2013/0168081 discloses two-way communication with a downhole tool
string.
A wireless control signal may be issued through the pressure riser to the
downhole tool
to cause the downhole tool string to activate the component. The wireless
control signal
may involve an acoustic signal, an optical signal, and/or an electromagnetic
signal such
as electrical dipole coupling or magnetic dipole coupling.
[0014] US20110030946 discloses the use of antennas for transmitting
electromagnetic
waves from a casing in a wellbore to the surface.
[0015] In US4839644 A, a system and method are disclosed for wireless two-way
communication in a cased borehole having tubing extending therethrough. A
downhole
communications subsystem is mounted on the tubing. The downhole subsystem
includes
a downhole antenna for coupling electromagnetic energy in a TEM mode to and/or
from
the annulus between the casing and the tubing.
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[0016] US 6070662 A discloses a method and apparatus for establishing
communication
in a cased wellbore with a data sensor that has been remotely deployed, prior
to the
installation of casing in the wellbore, into a subsurface formation penetrated
by the
wellbore. Communication is established by installing an antenna in an opening
in the
casing wall.
[0017] However, all of the above systems suffer from low efficiency, which
becomes
even more apparent in applications where two way communication is established
using
power harvesting on the remote side. Further, the above prior art are
sensitive to
alignment of the antennas to obtain the necessary connectivity, and complex
completion
programs have to be run to align the completion with the casing.
Short summary of the invention
[0018] A main object of the present invention is to disclose a method and a
system for
improving the signal transfer and energy efficiency of the signal and power
transmission
between wireless transmitters and receivers of wireless links inside the
wellbore.
[0019] The invention is a wellbore E-field wireless communication system where
the
signal transfer and energy efficiency is improved compared to systems
described in prior
art.
[0020] The wellbore E-field wireless communication system comprises;
a first E-field antenna and a second E-field antenna, wherein the first
antenna, and the
second antenna are both arranged in a common compartment of a wellbore and
further
arranged for transferring a signal between a first connector of the first E-
field antenna
and a second connector of the second E-field antenna by radio waves, wherein
that the
second E-field antenna is arranged in a lateral wellbore, and the first and
second E-field
antennas comprises a first dipole antenna or a first toroidal inductor,
wherein the system
comprises a metallic resonator surrounding the first antenna and the second
antenna.
[0021] The first and second E-field antennas (11, 21) are electric dipoles.
Electric
dipoles set up an electric field (Ec) that will propagate through a medium as
waves, e.g.
radio waves. While the electric field as disclosed by the invention is created
around an
electrically charged particle, i.e. the electric dipole, the magnetic field
used for the
wireless link in prior art is created around the coil involved by the
modulated magnetic
field. Although the electric and magnetic fields are interrelated as known
from Maxwell's
equations, efficiency of the wireless link can be significantly improved by
using the E-field
for communication. However, to take advantage of the properties of the E-
field, at least
the sender antenna has to be an electric dipole, as discussed later in the
document.
[0022] The metallic resonator further improves the efficiency of the system
over the
prior art. In an embodiment the metallic resonator comprises existing casing
and tubing
inside the wellbore, where the resonator extends into a lateral wellbore with
an E-field
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antenna. An electric field set up by one of the antennas in the resonator will
then
propagate inside the resonator and reach the other antenna.
[0023] A further advantage of the invention is that the requirements for
alignment and
proximity between the sender and receiver pair of couplers are less strict
than for prior
art inductive couplers. As long as the antennas are located inside the
resonator, their
alignment is not critical. In this way superior connectivity can be
established between a
mother wellbore and a lateral wellbore without spacing out or similar
techniques known
from prior art.
[0024] According to prior art, alignment of the wellbore completion inside a
casing of a
wellbore requires specific procedures for spacing out the completion so that
the downhole
magnetic dipoles are properly aligned to establish wireless connectivity, as
the wellbore
completion is set and the tubing hanger is landed inside the wellhead housing
of the well.
Magnetic dipoles have to be aligned so that the B-field from a sender can
penetrate the
coil of the receiver. It is well known that the strength of the B-field around
a magnetic
dipole has a certain propagation, and that the field is strongest in specific
directions
relative the coil.
[0025] Space out can be understood as the process required to add exactly the
necessary tubings to the top of the wellbore completion as this is lowered
into the
wellbore casing. At the end of the wellbore completion program the wellbore
completion
is landed and terminated in a tubing hanger in a wellhead housing. If the
wellbore
completion is to long the tubing has to be lifted up to remove some of the
tubing. If it is
to short, more tubing has to be added.
[0026] If however, the present invention is used, the completion program may
be
simplified since the alignment is less critical, which in turn can reduce the
time both for
planning and conducting the wellbore completion program.
[0027] Another advantage of the invention is that the pair of electric dipoles
according
to the invention can be placed a longer distance away from each other than for
magnetic
dipoles according to prior art.
[0028] A further advantage is that the electric dipoles can communicate even
when
there are intermediate obstacles, as long as they are in the same annulus.
[0029] In a number of wellbore applications, such as for e.g. establishing
communication between a mother wellbore and lateral wellbores, this adds a lot
of
flexibility. A sender can be arranged attached or integrated to the tubing
wall of the
completion, and a receiver may be attached to the tubing wall of the lateral
bore. Even
when they are not directly opposite each other, or there are obstacles between
them,
such as edges of the casing where the lateral bore branches off, the sender
and receiver
pair will be able to establish a reliable wireless power and communication
link.
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[0030] Another application where the use of the invention is advantageous, is
to set up
communication between sender and receiver pairs at different depths along the
motherbore or a lateral bore. This can be important if measurements have to be
performed at different locations, such as formation measurements at two
levels.
5 Figure captions
[0031] The attached figures illustrate some embodiments of the claimed
invention.
[0032] Fig. 1 illustrates in a sectional view a wireless electric transfer
system according
to an embodiment of the invention with toroidal inductor antennas arranged in
an
annulus of a wellbore.
[0033] Fig. 2 illustrates in the same way as in Fig. 1 a wireless electric
transfer system
according to an embodiment of the invention where the toroidal inductor
antennas are
arranged at the same height.
[0034] Fig. 3 illustrates in a simplified sectional view toroidal inductor
antennas with
stand-alone cores arranged in the mother wellbore and a lateral wellbore.
[0035] Fig. 4 illustrates the same as in Fig. 3, where the antennas are
toroidal inductor
antennas arranged about a motherbore tubing (101) and a lateral tubing (201).
[0036] Fig. 5 illustrates in a simplified sectional view a wireless electric
transfer system
according to an embodiment of the invention with dipole antennas arranged in
an
annulus of a wellbore.
zo [0037] Fig. 6 illustrates the same as in Fig. 5, where the tubing is used
as an active
element of the dipole antenna.
[0038] Fig. 7 illustrates in a sectional view a wireless electric transfer
system according
to an embodiment of the invention comprising a resonator wherein the antennas
are
arranged.
[0039] Fig. 8 illustrates in a sectional view the system according to the
invention in a
multi-lateral wellbore (2) with an open hole formation.
Embodiments of the invention
[0040] The invention will in the following be described and embodiments of the
invention will be explained with reference to the accompanying drawings.
[0041] Figure 1 illustrates in a simplified cross sectional drawing an
embodiment of the
wellbore E-filed wireless communication system (1). The wellbore (2) comprises
an inner
tool, tubing, liner or casing (101) and an outer tubing, liner or casing
(102). In between
the inner tool, tubing, liner or casing (101) and an outer tubing, liner or
casing (102)
there is defined a compartment (210).
[0042] It will be understood from the following description of the
communication system
(1) that it is not important in any of the embodiments whether the
compartment, or
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annulus (210) is delimited by an inner tool, tubing, liner or casing (101) on
one side or
an outer tubing, liner or casing (102) on the other side, as long as an
annulus (210) is
defined between the tool, tubing, liner or casing elements. For simplicity,
tubing (101) is
used to denote inner tool, tubing, liner or casing (101) and casing is used to
denote outer
tubing, liner or casing (102).
[0043] An annulus (210) as described above is typical for modern wellbores and
this is
where communication according to the invention is typically set up. However,
the first
and second E-field antennas may be arranged in any compartment of a wellbore,
such as
in the bore of an open hole formation, or inside the tubing.
[0044] In an embodiment the wellbore E-field wireless communication system (1)
comprises a wellbore instrument (22) and a second E-field transceiver (20)
connected to
the wellbore instrument (22) and the second connector of the second antenna
(21).
[0045] The second E-field transceiver (20) and the wellbore instrument (22) is
in this
embodiment are separate or integrated remote devices.
[0046] In an embodiment the wellbore E-field wireless communication system (1)
comprises a control system (70) and a first E-field transceiver (10) connected
to the
control system (70) and the first connector of the first E-field antenna (11).
The control
system is typically a surface based system as illustrated in Fig. 1.
[0047] The wireless communication system (1) is arranged for transferring a
zo communication signal between the control system (70) and the wellbore
instrument (22)
via the first and second electric antennas (11, 21) by radio waves (Ec). Radio
waves
have by definition a frequency between 3 kHz and 300 GHz. In an embodiment the
communication signal transferred across the wireless communication system is
modulated onto a carrier wave with a radio frequency.
[0048] The First and second E-field transmitters (10, 20) are shown in the
compartment
(210). The first E-field transmitter (10) is connected to one end of a
downhole cable (9)
arranged to be connected in the other end to -, and communicate with the
downhole
control system (70). The second E-field transmitter (20) is connected to a
wellbore
instrument (22) arranged to receive commands from the downhole control system
(70)
and/or send signals to the downhole control system (70).
[0049] The first and second E-field transmitters (10, 20) are connected to
first and
second antennas (11, 21), respectively, arranged in the same compartment
(210). The
electric field (Ec) set up between the first and second E-field antennas (11,
21) is
illustrated as dotted lines in the figure.
[0050] The first E-field transmitter (10) may be connected to either end of
the cable
(9). In the embodiment where the first E-field transmitter (10) is connected
between the
cable (9) and the first antenna (11), the cable (9) will typically carry power
and
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information signals down to the downhole E-field transmitter (10) that is
responsible for
modulating power and information signal onto a carrier.
[0051] If the E-field transmitter (10) is arranged on, or close to the
surface, the
modulation has already been taken care of before propagating downhole, and the
cable
(9) will be an antenna feeding cable connected directly to the antenna.
Typically, a
coaxial cable can be used for this purpose. Impedance matching means may also
be
applied.
[0052] The first E-field transmitter may also be arranged anywhere between the
two
extremities, requiring a portion of the cable to transfer the "raw",
unmodulated signals,
and a second section to transfer the modulated signal. Different types of
cables may
therefore be required for the two sections.
[0053] Bidirectional communication may be set up by implementing transmitter
and
receiver pairs into transceivers on both sides of the wireless link, where the
same
antenna is used for both transmitting and receiving.
[0054] The wellbore instrument (22) may be any downhole instrument that
requires
communication with a downhole control system. An example is a sensor device
measuring typical annulus parameters, such as e.g. pressure. It may also be a
sensor
device for measuring formation parameters outside the casing as illustrated in
Fig. 1,
where the sensor is communicating with the second E-field transmitter (20) via
a
communication line through the casing (102).
[0055] In an embodiment the wellbore instrument (22) is an actuator for
actuating a
wellbore component, such as a valve in the wellbore (2).
[0056] In an embodiment the downhole cable (9) is arranged to transfer a
communication signal from the downhole control system (70) to the first E-
field
transmitter (10). Further, the first E-field transmitter (10) is arranged to
transfer the
communication signal to the second E-field transceiver (20) via the first and
second
antennas (11, 21). In this way a wireless link is established between the end
of the
downhole cable (9) and the wellbore instrument (22).
[0057] In an embodiment the downhole cable (9) is arranged to transfer power
from the
downhole control system (70) to the first E-field transmitter (10). Further,
the first E-
field transmitter (10) is arranged to transfer electric power to the second E-
field
transceiver (20) via the first and second antennas (11, 21). In this
embodiment the
second E-field transceiver (20) is arranged for power harvesting of the E-
field picked up
by the second antenna (21) and for distributing electric power to local
electric
components and circuits. Standard power circuit components may be used for
power
harvesting and power stabilizing before distributing the power to other
components.
[0058] The transfer of electric power and communication signals may be
performed
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simultaneously.
[0059] In a configuration the frequency of the E-fleld determined by the size
of the
antenna and the characteristics of the first and second transceivers (10,20)
where
electric power is harvested directly from the E-field, while the communication
signal is
modulated on top of the E-field. The communication signal may be amplitude or
frequency modulated.
[0060] In an embodiment a digital communication signal is converted to a
frequency
modulated signal where the bandwidth is different for a digital "0" and a
digital "1". On
the receiver side the bandwidth can be continuously measured to demodulate the
signal
back to the original digital signal. Further any known transmission protocol
may be
applied to this wireless link, such as e.g. error correction.
[0061] Due to the frequency characteristics of the E-field, a much higher
bandwidth is
possible with the system according to the invention than for prior art
downhole
communication systems. This means that more information can be transferred
between
the wellbore instrument (22) and the downhole control system (70).
[0062] As described previously, wireless power may be supplied to the second
transceiver (20). The second transceiver (20) may contain local electronic
circuits both
for processing signals from the wellbore instrument (22), and for calculating
a signal to
the wellbore instrument. If the wellbore instrument (22) is a sensor device,
the second
transceiver (20) may contain signal processing circuits for processing raw
sensor data
and communicating the processed data from the second transceiver (20) to the
first
transceiver (10). If the wellbore instrument (20) is an actuator device, the
second
transceiver (20) may contain signal processing circuits for converting an
incoming
command to an actuator signal by e.g. triggering a high current switch
supplied with
power from the harvested power of the second transceiver (20). The second
transceiver
may also comprise power storage means such as capacitors or batteries to store
energy
for being able to provide sufficient current for actuation, or as a local back
up.
[0063] The wellbore instrument (22) may also be a combination of sensor and
actuator
means, where e.g. actuation is performed based on sensor signal values. In
this case the
second transceiver (20) or the wellbore instrument (22) may comprise
electronic circuits
for processing sensor signal values and comparing them with threshold values
before
operating the actuator.
[0064] The invention further comprises inventive features related to the
establishment
of wireless communication by using the E-field between the first and second
antennas
(11, 21).
[0065] In an embodiment the first antenna (11) comprises a first dipole
antenna (11d)
as illustrated in Fig. 5. In this case the first dipole antenna is may work as
a two way
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feeding antenna, i.e. power transfer and transfer of communication signals.
The first
dipole antenna (11d) may be directly connected to a downhole cable (9)
connected to a
downhole control system (70) with a first transceiver (10) close to the
downhole control
system (70), or the first transceiver (10) may be arranged between the cable
(9) and the
dipole antenna (11d) in the wellbore (2).
[0066] In an embodiment of the invention one leg of the dipole antenna (11d)
is the
tubing, liner or casing (101) as illustrated in Fig. 6, such that the tubing,
liner or casing
(101) is an active element of the dipole antenna. A layer of de-electric
insulation (12) is
also shown to isolate the two legs of the antenna from each other to provide
optimum
impedance for the antenna.
[0067] Another type of antenna that can be used is a toroidal inductor. In an
embodiment the first antenna (11) is a toroidal inductor as can be seen on
Fig. 1. A
toroidal antenna has the effect that the net current inside the major radius
of the toroid
is zero, which means that the magnetic field remains inside the toroid
inductor itself, and
only an electric field is radiated from the toroid inductor.
[0068] As for the dipole antenna, the toroidal inductor (11t) may also be
directly
connected to the downhole cable (9) connected to a downhole control system
(70) with a
first transceiver (10) close to the downhole control system (70), or the first
transceiver
(10) may be arranged between the cable (9) and the dipole antenna (11d) in the
wellbore (2) as illustrated in Fig. 1.
[0069] In the embodiment illustrated in this figure the first toroidal
inductor (11t) is
arranged about a tubing, liner or casing (101) of the wellbore (2), such that
the tubing,
liner or casing (101) is acting as a waveguide for the electric field (Ec).
[0070] In an embodiment the first toroidal inductor (11t) is arranged about a
stand-
alone metal core (13) within the annulus (210) to as illustrated in Fig. 3.
The metal core
may be an open tube extending in the direction of the wellbore as illustrated
to allow
passage of annulus fluid through the inner core of the antenna.
[0071]
[0072] On the opposite side of the wireless transmission system, i.e. close to
the
wellbore instrument (22) is the second antenna (21). The second antenna (21)
may be
any dipole antenna or toroidal inductor antenna as described above for the
first antenna
(11).
[0073] Some combinations of first and second antennas (11, 21) will be
described
below.
[0074] In Fig. 1 and Fig. 2 the first and second antennas (11, 12) are
toroidal inductor
antennas (11t, 12t) about a tubing, liner or casing (101). In the embodiment
where the
tubing, liner or casing (101) is metallic, it becomes a waveguide able to
transfer signals
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between the first and second antennas (11t, 12t). Fig. 2 illustrates the
special case
where the two antennas are arranged at the same height.
[0075] In Fig. 3 a the second antenna is similar to the first antenna
described above.
I.e. a second toroidal inductor (21t) about a stand-alone metal core (13).
5 [0076] Fig. 6 illustrates the use of a simple dipole antenna arranged in the
annulus as
the second antenna (21). As for the first dipole antenna (11d), the second
dipole antenna
(21d) may also have the tubing, casing or liner (102) acting as an active
element by
connecting one leg to the tubing, casing or liner (102), i.e. the wall to the
right of the
dipole shown, and insulated the two antenna legs with a di-electric material.
10 [0077] The antenna configurations described above may be combined. E.g. in
Fig. 1 and
2 the second antenna may also be a second toroidal inductor (21t) about a
stand-alone
metal core (13) or a dipole antenna. In Fig. 3 the second antenna may be a
toroidal
inductor (21t) about the tubing, casing or liner (101, 102) or a dipole
antenna. In Fig. 5
and 6 the second antenna may be second toroidal inductor (21t) about a stand-
alone
metal core (13) or about the tubing, casing or liner (101, 102).
[0078] According to an embodiment the wellbore E-field wireless communication
system
(1), comprises a metallic resonator (40) surrounding the first antenna (11)
and the
second antenna (21) as illustrated with the thicker line in Fig. 7. The
metallic resonator
may be tuned to the frequency of the E-field to enable more efficient transfer
of both
zo power and communication signals. The first and second antennas (11, 21)
inside the
resonator may be a combination of any of the types described above.
[0079] In one embodiment the resonator (40) comprises one or more metallic
packers
(41) arranged to delimit the size of the annulus (210).
[0080] According to an embodiment of the invention, the second antenna (21) is
arranged in a lateral wellbore (300) as illustrated in Fig. 3, 4 and 7, to
enable wireless
connectivity with a second antenna (21) arranged in the same annulus (210) as
the first
antenna (11) and connected to a wellbore instrument (22).
[0081] Communication between the first antenna and two or more second antennas
arranged in different lateral wellbores in a multi-lateral well may be set up
in the same
way. A multiplexing scheme or any other suitable protocol for network
communication
can be used for communicating with the different lateral wellbores.
[0082] Fig. 8 shows a wellbore E-field wireless communication system (1),
according to
an embodiment of the invention, in a multi-lateral wellbore comprising a main
bore (100)
and lateral wellbores (200, 300, 400). The first antenna or electric dipole
(11) is
connected to a surface control system as described previously.
[0083] Second antennas, or electric dipoles (21) are arranged in two or more
of the
lateral wellbores (200, 300, 400), each connected to an E-field transmitter
(20) in
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respective lateral wellbores. In turn, each of the E¨field transmitters are
connected to a
wellbore instrument (22). It is also shown a second wellbore instrument (23)
arranged in
the wellbore formation of the wellbore and connected to the E-field
transmitter (20). In
an embodiment the first wellbore instruments (22) are pressure sensors,
measuring a
pressure in the lateral wellbore, and the second wellbore instruments (23) are
sensors
used to measure formation parameters. However, the E-field wireless
communication
system (1), may be used in any application and for the wireless transfer of
any
information from any sensor or actuator within a compartment of a wellbore.
[0084] Fig. 8 illustrates a multi-lateral well with an open hole formation,
but it can be
used in the same way in a wellbore with casings or liners, where the
compartment then
becomes an annulus of the wellbore.
[0085] Figures 1 to 8 above are drafted to illustrate different embodiments of
the
invention. A number of common elements of a wellbore such as packers, valves,
lateral
branching devices etc. are left out as will be understood by a person skilled
in the art.
[0086] Calculations for the comparison of the use of magnetic coil antennas or
toroidal
inductors and electric dipoles as transmitter antennas have been elaborated
and the
results are summarized below. They show that using a coil antenna, i.e.
magnetic dipole
as a transmitter antenna is normally not as good as using an electric dipole
as a
transmitter antenna, in terms of efficiency and the impedance matching.
zo [0087] The power transferring between two antennas can be considered as two
procedures.
- (a) A transmitter antenna generates electromagnetic fields in the space.
The fields
generated are proportional to IL, where I is the current on the Tx antenna,
and L is the
equivalent length of the antenna.
- (b) The receiver antenna picks the fields in the space and generates a
voltage in the
receiver circuit. The received voltage is proportional to the antenna
equivalent length L of
the antenna.
[0088] Therefore it is important to investigate the equivalent lengths of the
electric
dipole and the coil antenna.
[0089] The equivalent length of a coil antenna is:
1 = kS (1)
where
- / is the equivalent antenna length of the coil antenna. For the dipole
case, the
equivalent antenna length is the physical length of the antenna.
- k is the wave number and k = 2a/k (k: wave length)
- S is coil effective area, and
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S=Nit4orira2 (2)
[0090] where N is the number of turns and a is the radius of the coil, and
pcore is the
relative permeability the core material.
[0091] Since at low frequency k is a small number, equation (2) means that the
coil
antenna has low radiation efficiency.
[0092] Equation (1) shows that the equivalent antenna length of a coil is a
function of
the wave length and thus a function of frequency. The following table shows
the number
of turns needed for a coil with diameter 4cm (air core) to reach an equivalent
length lm
for frequency 100kHz, 1MHz, 10 MHz and 2 MHz, for rcore =1.
Table 1: Number of turns for a coil having lm equivalent length
frequency 100kHz 1MHz 10MHz 100MHz
380000 38000 3800 380
[0093] From the table we can see that many turns are needed to realize an
equivalent
length 1m at low frequencies.
[0094] One may increase the coil effective area shown in (2) by introducing a
ferrite
core. However, the saturation of the core stops using high current. That is
why coils are
less applicable as transmitter antennas.
[0095] Here we should comment that for power delivering for the case with
steel casing,
one need to generate magnetic field along the casing direction. For that
application, the
coil antenna may be advantageously used as a Tx antenna.
[0096] For a Tx antenna, it is important to have proper impedance match at the
input
port for increasing the power delivering efficiency. The input impedance of an
electric
dipole is its radiation impedance, which is resistive about 60 Ohm for a
quarter
wavelength antenna. However, the input impedance of a coil antenna is the sum
of its
radiation impedance and the inductance of the coil, which is dominated by the
inductance
part. Hence it is more difficult to make impedance match for the coil antenna
than for the
electric dipole case.
[0097] For the receiver antenna, the current is weak. One can use many turns
on a
ferrite core without saturation. In addition, the impedance matching for the
receiver
antenna is not as important as for the Tx antenna. So the coil antenna can be
used as a
receiver antenna.
[0098] For power delivering without steel casing, using an electric dipole is
better than
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using a coil antenna as a Tx antenna. However, the receiver antenna can use
either the
electric dipole or coil antenna.
