Tunable Acoustic Transmitter for Downhole Use
TECHNICAL FIELD
This invention relates to acoustic transmitters, and more particularly to tunable
variable frequency acoustic transmitters for use in downhole applications.
BACKGROUND
Wells are commonly used to access regions below the earth's surface and to
acquire materials from these regions. For instance, wells are commonly used to locate
and extract hydrocarbons from underground locations. The construction of wells
typically includes drilling a wellbore and constructing a pipe structure, often called
"casing," within the wellbore. Upon completion, the pipe structure provides access to the
underground locations and allows for the transport of materials to the surface.
Before, during, and after construction of a well, a variety of tools are
conventionally used to monitor various properties of the downhole environment. For
example, underground logging systems may be used to inspect a pipe casing, the
surrounding cement support structure, and/or the surrounding subterranean formations.
These systems may be positioned independently within a wellbore, or may be placed on a
drill string and positioned within the wellbore in conjunction with other downhole
equipment.
In order to provide feedback to control systems and operators on the surface, these
tools can transmit telemetry data to the surface for analysis. For instance, telemetry data
can be transmitted via acoustic transmission. As such, there is a need for improved
acoustic transmitters to optimize the transfer of telemetry data.
DESCRIPTION OF DRAWINGS
FIG. IA is a diagram of an example well system.
FIG. IBis a diagram of an example well system that includes an NMR logging
tool in a wireline logging environment.
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FIG. 1C is a diagram of an example well system that includes an NMR logging
tool in a logging while drilling (LWD) environment.
FIG. 2 is a diagram of an example piezoelectric transducer.
FIG. 3 is a diagram of an example physical model for a transducer.
FIG. 4 is a plot of an example channel transfer function for a drill string.
FIG. 5 is a diagram of a piezoelectric transducer and an example tuning module.
FIG. 6A is a diagram of a magneto-rheological fluid in the absence of an applied
magnetic field.
FIG. 6B is a diagram of a magneto-rheological fluid in the presence of an applied
magnetic field.
FIG. 7 is a diagram of a piezoelectric transducer and another example tuning
module.
FIG. 8 is a plot of example channel transfer functions for a drill string.
Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
FIG. 1A is a diagram of an example well system 1 OOa. The example well system
100a includes a logging system 108 and a subterranean region 120 beneath the ground
surface 106. A well system can include additional or different features that are not shown
in FIG. 1A. For example, the well system 100a may include additional drilling system
components, wireline logging system components, etc.
The subterranean region 120 can include all or part of one or more subterranean
formations or zones. The example subterranean region 120 shown in FIG. 1A includes
multiple subsurface layers 122 and a wellbore 104 penetrating through the subsurface
layers 122. The subsurface layers 122 can include sedimentary layers, rock layers, sand
layers, or combinations of these other types of subsurface layers. One or more of the
subsurface layers can contain fluids, such as brine, oil, gas, etc. Although the example
wellbore 104 shown in FIG. 1A is a vertical wellbore, the logging system 108 can be
implemented in other wellbore orientations. For example, the logging system 108 may
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be adapted for horizontal wellbores, slant wellbores, curved wellbores, vertical wellbores,
or combinations of these.
The example logging system 108 includes a logging tool 102, surface equipment
112, and a computing subsystem 110. In the example shown in FIG. 1A, the logging tool
102 is a downhole logging tool that operates while disposed in the well bore 104. The
example surface equipment 112 shown in FIG. 1A operates at or above the surface 106,
for example, near the well head 105, to control the logging tool102 and possibly other
downhole equipment or other components ofthe well system 100. The example
computing subsystem 110 can receive and analyze logging data from the logging tool
102. A logging system can include additional or different features, and the features of an
logging system can be arranged and operated as represented in FIG. 1A or in another
manner.
In some instances, all or part of the computing subsystem 110 can be
implemented as a component of, or can be integrated with one or more components of,
the surface equipment 112, the logging tool1 02 or both. In some cases, the computing
subsystem 110 can be implemented as one or more discrete computing system structures
separate from the surface equipment 112 and the logging tool102.
In some implementations, the computing subsystem 110 is embedded in the
logging tool 102, and the computing subsystem 110 and the logging tool1 02 can operate
concurrently while disposed in the wellbore 104. For example, although the computing
subsystem 110 is shown above the surface 106 in the example shown in FIG. 1A, all or
part of the computing subsystem 110 may reside below the surface 106, for example, at
or near the location of the logging tool102.
The well system 1 OOa can include communication or telemetry equipment that
allows communication among the computing subsystem 110, the logging tool1 02, and
other components of the logging system 108. For example, the components ofthe
logging system 108 can each include one or more transceivers or similar apparatus for
wired or wireless data communication among the various components. For example, the
logging system 108 can include systems and apparatus for wire line telemetry, wired pipe
telemetry, mud pulse telemetry, acoustic telemetry, electromagnetic telemetry, or a
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combination of these other types of telemetry. In some cases, the logging tool1 02
receives commands, status signals, or other types of information from the computing
subsystem 110 or another source. In some cases, the computing subsystem 110 receives
logging data, status signals, or other types of information from the logging tool 102 or
another source.
Logging operations can be performed in connection with various types of
downhole operations at various stages in the lifetime of a well system. Structural
attributes and components of the surface equipment 112 and logging tool1 02 can be
adapted for various types of logging operations. For example, logging may be performed
during drilling operations, during wireline logging operations, or in other contexts. As
such, the surface equipment 112 and the logging tool102 may include, or may operate in
connection with drilling equipment, wireline logging equipment, or other equipment for
other types of operations.
In some examples, logging operations are performed during wireline logging
operations. FIG. 1B shows an example well system 100b that includes the logging tool
102 in a wire line logging environment. In some example wire line logging operations, a
the surface equipment 112 includes a platform above the surface 106 is equipped with a
derrick 132 that supports a wire line cable 134 that extends into the well bore 104.
Wireline logging operations can be performed, for example, after a drilling string is
removed from the well bore 104, to allow the wire line logging tool1 02 to be lowered by
wire line or logging cable into the well bore 104.
In some examples, logging operations are performed during drilling operations.
FIG. 1C shows an example well system lOOc that includes the logging tool102 in a
logging while drilling (LWD) environment. Drilling is commonly carried out using a
string of drill pipes connected together to form a drill string 140 that is lowered through a
rotary table into the well bore 104. In some cases, a drilling rig 142 at the surface 106
supports the drill string 140, as the drill string 140 is operated to drill the well bore 104 to
penetrate the subterranean region 120. The drill string 140 may include, for example, a
kelly, drill pipe, a bottom hole assembly, and other components. The bottom hole
assembly on the drill string may include drill collars, drill bits, the logging tool 102, and
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other components. The logging tools may include measuring while drilling (MWD)
tools, LWD tools, and others.
As shown, for example, in FIG. 1B, the logging tool102 can be suspended in the
well bore 104 by a coiled tubing, wire line cable, or another structure that connects the tool
to a surface control unit or other components of the surface equipment 112. In some
example implementations, the logging tool1 02 is lowered to the bottom of a region of
interest and subsequently pulled upward (e.g., at a substantially constant speed) through
the region of interest. As shown, for example, in FIG. 1C, the logging tool102 can be
deployed in the wellbore 104 on jointed drill pipe, hard wired drill pipe, or other
deployment hardware. In some example implementations, the logging tool1 02 collects
data during drilling operations as it moves downward through the region of interest
during drilling operations. In some example implementations, the logging tool1 02
collects data while the drilling string 140 is moving, for example, while it is being tripped
in or tripped out of the wellbore 104.
In some example implementations, the logging tool 102 collects data at discrete
logging points in the wellbore 104. For example, the logging tool102 can move upward
or downward incrementally to each logging point at a series of depths in the wellbore
104. At each logging point, instruments in the logging tool1 02 perform measurements
on the subterranean region 120. The measurement data can be communicated to the
computing subsystem 110 for storage, processing, and analysis. Such data may be
gathered and analyzed during drilling operations (e.g., during logging while drilling
(LWD) operations), during wireline logging operations, or during other types of
activities.
The computing subsystem 110 can receive and analyze the measurement data
from the logging tool102 to detect properties of various subsurface layers 122. For
example, the computing subsystem 11 0 can identify the density, material content, or other
properties of the subsurface layers 122 based on the measurements acquired by the
logging tool1 02 in the well bore 104.
In some implementations, for example as shown in FIG. 1A, the well system 100a
includes an acoustic transmitter module 130 that transmits telemetry data to an acoustic
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receiver 131, in order to provide wireless communication capability between logging tool
102 and surface equipment 112. In some implementations, acoustic transmitter module
130 can be mechanically coupled to a component of the well system that extend between
the logging tool1 02 and the surface equipment 131, for example a drill string 140 (see
FIG. 1C). To transmit telemetry data, the acoustic transmitter module 130 induces timedependent
acoustic energy (e.g., in the form of stress waves or acoustic pulses) onto the
drill string 140. These waves or pulses contain information regarding the telemetry data,
and propagate through the drill string up to the surface where they are detected by an
acoustic receiver 131. These telemetry signals are interpreted by the surface equipment
112 and computing subsystem 11 0.
Acoustic transmitter module 130 can include electromagnetic transducer that
converts electromagnetic energy into translational motion. For instance, in some
implementations, the acoustic transmitter module 130 includes a transducer that is
capable of providing acoustic energy in a desired frequency range (e.g., 50-500kHz) and
at a sufficiently high amplitude, under the conditions typically encountered in downhole
environments (e.g., at high temperatures, such as temperatures in excess of 170° C, and at
high pressures, such as pressures greater than 20,000 PSI). For example, acoustic
transmitter module 130 can include a piezoelectric transducer, an electromagnetic
acoustic transducer (EMAT), a magnetostrictive transducer, or another type of transducer.
In some implementations, acoustic transmitter module 130 includes a
piezoelectric transducer 200. A schematic representation of an example piezoelectric
transducer 200 is shown in FIG. 2. Transducer 200 includes a piezoelectric stack 202
extending axially from a first end 202a and a second end 202b. Piezoelectric stack 202 is
disposed within a support sleeve 204, which encases the radial periphery of piezoelectric
stack 202 and mechanically couples the transducer 200 to the drill string 140.
Piezoelectric stack 202 is clamped axially within support sleeve 204 between a top nut
206, which is mechanically fixed to the proximal end 202a, and a backing mass 206,
which is mechanically fixed to the distal end 202b. In this configuration, piezoelectric
stack 202 is axially compressed between top nut 206 and backing mass 206.
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During operation of transducer 200, an electric input signal (e.g., an electrical
signal with a time-dependent voltage differential) is applied to the piezoelectric stack
202. In response to the applied signal, the piezoelectric stack 202 reacts by expanding or
contracting in the axial directions. Because the piezoelectric stack 202 is axially
compressed against backing mass 206, the expansion of the piezoelectric stack 202 is
transferred as a compressive stress to the support sleeve 204 via the top nut 206. Thus, in
response to an applied time-dependent excitation signal, the transducer 200 "fires," and
induces a time-dependent acoustic signal that is directed through the support sleeve 204
and into the drill string 140. As the piezoelectric stack 202 contracts and expands
according to the applied input signal, the frequency of the induced acoustic signal can be
adjusted by adjusting the frequency of the input signal. Thus, in some implementations,
transducer 200 can be used to induce a range of frequencies by varying the frequency of
the input signal.
Backing mass 206 acts as an inertial element against which the piezoelectric stack
202 can react or "push." The mass of the backing mass 206 can have a predicable effect
on the resonance behavior of the transducer 200. For example, in some implementations,
the relationship between the mass of the backing mass 206 and the resonant frequency of
the transducer 200 can be represented using a physical model300. Referring to FIG. 3,
the model300 includes a spring 302 with spring constant k, a damper 304 with a
damping coefficient C, and a mass 306 with a mass M, arranged in an ideal mass-springdamper
system. When represented by model300, the transducer 200 resonates at a
frequency f, where:
1 {k
f = 2n~M'
and where M is the mass of the backing mass 206, and k and C are dependent on the
physical properties of the piezoelectric stack 202 and the support sleeve 204. Thus, by
changing the mass, M, of the backing mass 206, the resonant frequency of the transducer
200 can be tuned. In some implementations, the resonant frequency of the transducer 200
can be tuned to coincide with the frequency of the induced acoustic signal in order to
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increase the output efficiency of the transducer 200 and/or to increase the amount of
acoustic energy that is directed into the drill string 140.
In some implementations, the drill string 140 does not perfectly transmit acoustic
energy along its length, and may attenuate the acoustic signals produced by the acoustic
transmitter module 130 as the signals travel its length. This attenuation behavior may be
frequency dependent. For example, acoustic energy of certain frequency ranges (i.e.,
"pass bands") can propagate along the length of the drill string 140, while acoustic energy
of other frequency ranges (i.e., "stop bands") are attenuated by the drill string 140 and
cannot fully propagate along its length. Pass bands and stop bands can be visualized in
the frequency domain by a channel transfer function. Referring to FIG. 4, plot 400 shows
a range of input frequencies f, and a channel transfer function H(f) (i.e., a function
describing the ratio between the input and output signals of the system in the frequency
domain) for an example drill string. Pass bands are represented as a series of peaks 402
in the frequency domain, while stop bands are represented as a series of gaps 404. As
shown in plot 400, when an acoustic transmitter module 102 transmits acoustic signals
within the pass bands, it will have higher output efficiency than if it transmitted acoustic
signals outside of the pass bands. Likewise, when the resonant frequency of acoustic
transmitter module 120 coincides with both the pass band and the frequency of the
induced acoustic signal, the output efficiency of the acoustic transmitter module 102 can
be further enhanced.
The shape of a channel transfer function can vary based on several factors,
including the physical composition of the drill string (e.g., the material of the drill string
and its components), the physical dimensions and arrangement of the drill string and its
components, the physical properties of the surrounding environment (e.g., the
composition ofthe surrounding environment, the ambient temperature, and so forth), and
other factors. Accordingly, the number, height, location, and width of a drill string's pass
bands and stop bands can differ depending on the specific implementation or application.
For instance, in some implementations, there can be one or more pass bands (e.g., one,
two, three, four, five, and so forth). In some implementations, the height of each pass
band can differ. For example, in some implementations, the height of a pass band can be
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between approximately 1 to 1x103
, 1x103 to 1x106
, and so forth. In some
implementations, the pass bands can regularly or irregularly spaced from each other in
the frequency domain. As an example, a channel transfer function can have several
regularly spaced pass bands with centers around about 225Hz, 450Hz, 675Hz, 900Hz,
and so forth. In some implementations, the width of each pass band can vary. For
instance, in some implementations, each pass band can have a width of about 10-20 Hz,
20-30Hz, 40-50Hz, 60-70Hz, and so forth.
In addition, the transfer function and its pass bands can also change due to
dynamically changing conditions as the drill string is passed through a subterranean
formation. As an example, the center of a pass band can shift in approximately 1-10 Hz,
10-20 Hz, 20 to 1OOHz, and so forth. In another example, the number of pass bands can
increase or decrease. In another example, the height of a pass band can increase or
decrease (e.g., increase or decrease by 10%, 20%, 30%, 40%, and so forth). These
changes to the transfer function can occur gradually, or in discretely, depending on the
nature of the changing conditions.
Accordingly, in order to increase the efficiency of the acoustic transmitter module
130, the input signal can be adjusted such that the transducer induces an acoustic signal
within a pass band of the drill string. Likewise, the transducer 200 can be tuned such that
its resonant frequency is also within the pass band, and continues to be within the pass
band even as the pass band shifts under dynamic conditions.
In order to tune the resonant frequency of the transducer 200, the mass of the
backing mass 206 can be adjusted until the resonant frequency of the transducer 200
coincides with a pass band of the drill string. For example, in some implementations,
backing mass 206 can be replaced with a backing mass of differing mass in order to alter
the resonant behavior of the transducer 200. However, in some implementations,
replacing the backing mass 206 can be difficult to accomplish dynamically. For instance,
in some implementations, in order to adjust the resonant behavior of the transducer 200,
the logging tool1 02 must be withdrawn from the well bore 104, disassembled,
reassembled using a new backing mass, and reintroduced into the well bore 104. Though
feasible, in some circumstances, such a procedure may be impractical or uneconomical.
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Referring to FIG. 5, in some implementations, the resonant frequency of the
transducer 200 can be adjusted by changing the effective mass of the backing mass 206
using a tuning module 500. Tuning module 500 includes an electrical source 502, a
switch 504, and an electromagnetic coil 506 connected in series as an electrical circuit.
Electromagnetic coil 508 is mechanically attached to the backing mass 206, and is
disposed within a reservoir of a magneto-rheological fluid 508 enclosed within a casing
510.
Magneto-rheological (MR) fluids are a type of "smart" fluid whose mechanical
properties can be altered in a controlled fashion by an external magnetic field. Referring
to FIGS. 6A-B, MR fluids can be made of ferrous particles 602 suspended in a lower
density carrier fluid 604. Referring to FIG. 6A, in the absence of a magnetic field, an
MR fluid has a low viscosity, and exhibits continuous deformation properties typical of a
fluidic material. Referring to FIG. 6B, when subject to a magnetic field, the ferrous
particles 602 form chains in the direction of the magnetic flux, and causes the MR fluid
to increase in apparent viscosity. When its magnetic flux density is sufficiently high, the
MR fluid is said to be in an activated (i.e., "on") state, and assumes properties
comparable to a viscoelastic solid, up until a point of yield (i.e., the shear stress above
which shearing occurs). This yield stress is dependent on the magnetic field applied to
the fluid, up to a point of magnetic saturation, after which increases in magnetic flux
density have no further effect. Thus, an MR fluid can be varied between liquid and
viscoelastic quasi-solid states using an applied external magnetic field.
Examples of MR fluid particles include iron-based micrometer or nanometerscale
spheres or ellipsoids. Examples of carrier fluid include water and various types of
oil, such as hydrocarbon oils and silicon oils, with surfactant added to alleviate settling of
magnetic particles. For example, iron-based MR fluids at 40-50% volume fraction can
have yield stress of about 100 kPa (see, e.g., U.S. Pat. Nos. 5,277,282and 5,284,330).
Referring back to FIG. 5, when switch 504 is closed, electrical source 502 applies
a voltage V and current I to the electromagnetic coil 506, and induces a localized
magnetic field within the magneto-rheological fluid 508. In response to this localized
magnetic field, the magneto-rheological fluid 508 increases in viscosity, assumes
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properties comparable to a viscoelastic solid, and become affixed to the electromagnetic
coil 506. As electromagnetic coil 506 is mechanically attached to the backing mass 206,
the solidified magneto-rheological fluid 508 increases the effective mass of the backing
mass 206 (i.e., the mass coupled to the piezoelectric stack 204). As a result, the resonant
frequency of the transducer 200 is altered.
When the switch 502 is opened, the electrical source 502 no longer applies the
voltage V and current I to the electromagnetic coil 506, and a localized magnetic field is
removed from the magneto-rheological fluid 508. As a result, the magneto-rheological
fluid 508 decreases in viscosity, loses its viscoelastic solid-like properties, and is released
from the electromagnetic coil506. As the electromagnetic coil 506 is now free to shift
independently ofthe magneto-rheological fluid 508, the effective mass ofthe backing
mass 206 is reduced. As a result, the resonant frequency of the transducer 200 is returned
to its original state.
Therefore, in some implementations, by energizing the electromagnetic coil, the
effective mass of the backing mass 206 is increased, and the resonant frequency of the
transducer 200 is decreased. Conversely, by removing the applied voltage and current
from the electromagnetic coil, the elective mass of the backing mass 206 is decreased,
and the resonant frequency of the transducer 200 is increased. Accordingly, in some
implementations, the resonant frequency of the transducer 200 can be altered between
two different frequencies by applying or removing the voltage V and current I from the
electromagnetic coil506.
The magnetic field response of the magneto-rheological fluid 508 can be
continuous, rather than binary. That is, as the applied current is increased continuously,
the magnetic field induced in the magneto-rheological fluid is also increased
continuously, and the viscosity of the magneto-rheological fluid 508 can also
continuously increase until the fluid solidifies. In some implementations, electrical
source 502 is adjustable, and can be used to adjust the viscosity of the magnetorheological
fluid 508 either continuously or discretely. In an example, electrical source
502 can apply varying currents that causes the magneto-rheological fluid 508 to increase
in viscosity to varying degrees, but not fully solidify. This increase in viscosity can
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increase the friction between the electromagnetic coil 506 and the magneto-rheological
fluid 508, and can impede the motion of the electromagnetic coil506 and piezoelectric
stack 202. This friction can have effects similar to increasing the effective mass of the
backing mass 206, and can be used to influence the resonant frequency of the transducer
200. Thus, in some implementations, by increasing the current applied to the
electromagnetic coil, the friction between the electromagnetic coil 506 and the magnetorheological
fluid 508 is increased, and the resonant frequency of the transducer 200 is
decreased. Conversely, by decreasing the applied current to the electromagnetic coil, the
friction between the electromagnetic coil 506 and the magneto-rheological fluid 508 is
decreased, and the resonant frequency of the transducer 200 is increased. Accordingly, in
some implementations, the resonant frequency of the transducer 200 can be altered
between two or more discrete frequencies by adjusting the applied current between two
or more currents. In some implementations, the resonant frequency of the transducer 200
can be altered in a continuous manner within a range of frequencies by adjusting the
applied current continuously within a range of currents.
In some implementations, a tuning module can selectively apply a current to one
of multiple portions of an electromagnetic coil in order to selectively increase or decrease
the effective mass of the backing mass. Referring to FIG. 7, in some implementations,
the resonant frequency of the transducer 200 can be adjusted by changing the effective
mass ofthe backing mass 206 using a tuning module 700. Tuning module 700 includes a
electrical source 702, a switch 704, and an electromagnetic coil 706 connected in series
as an electrical circuit. Electromagnetic coil 706 is mechanically attached to the backing
mass 206, and is disposed within a reservoir of a magneto-rheological fluid 708 enclosed
within a casing 710.
Switch 704 can be toggled between several different states in order to complete an
electrical circuit with selectable portions of electromagnetic coil 706. For instance, when
switch 704 connects circuit points 712 and 714a, electrical source 702 applies a voltage V
and current I to portion 706a of electromagnetic coil 706, and induces a localized
magnetic field within a portion of magneto-rheological fluid 708a (i.e., the portion of
magneto-rheological fluid 708 within portion 706a). In response to this localized
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magnetic field, portion of magneto-rheological fluid 708a increases in viscosity, assumes
properties comparable to a viscoelastic solid, and become affixed to the electromagnetic
coil 706. As electromagnetic coil 706 is mechanically attached to the backing mass 206,
the solidified magneto-rheological fluid 708 increases the effective mass of the backing
mass 206 (i.e., the mass coupled to the piezoelectric stack 204). As a result, the resonant
frequency of the transducer 200 is altered.
Likewise, when switch 704 connects circuit points 712 and 714b, electrical source
702 applies a voltage V and current I to portion 706b of electromagnetic coil 706, and
induces a localized magnetic field within a larger portion of magneto-rheological fluid
708b (i.e., the portion of magneto-rheological fluid 508 within portion 706b ). This
increase in size of the energized portion of the electromagnetic coil 706 increases the
effective mass of backing mass 206, and results in a decrease in the resonant frequency of
actuator 200.
In a similar manner, when switch 704 connects circuit points 712 and 714c,
electrical source 502 applies a voltage V and current I to an even larger portion 706c of
electromagnetic coil 706, and induces a localized magnetic field within an even larger
portion of magneto-rheological fluid 708c (i.e., the portion of magneto-rheological fluid
508 within portion 706c). This increase in size of the energized portion of the
electromagnetic coil 706 further increases the effective mass ofbacking mass 206, and
results in a further decrease in the resonant frequency of actuator 200.
And when switch 704 connects circuit points 712 and 714d, electrical source 502
applies a voltage V and current I to the largest portion 706d of electromagnetic coil 706,
and induces a localized magnetic field within the largest portion of magneto-rheological
fluid 708d (i.e., the portion of magneto-rheological fluid 508 within portion 706d). This
increase in size of the energized portion of the electromagnetic coil 706 even further
increases the effective mass of backing mass 206, and results in a further decrease in the
resonant frequency of actuator 200.
In this manner, switch 704 can be used to apply a voltage and current to a
selectable portion of the electromagnetic coil 708, to selectively change the effective
mass ofbacking mass 206, and to alter the resonant frequency of the transducer 200.
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While FIG. 7 shows a tuning module 700 that can select from among four
portions 708a-d of electromagnetic coil 708, in some implementations, a tuning module
can select from among a lesser number of portions (e.g., two or three) or from among a
great number of portions (e.g., five, six, seven, and so forth). In some implementations,
electromagnetic coil 708 is not divided into discrete portions, and voltage and current can
be applied to a continuously variable portion of electromagnetic coil 708.
In some implementations, for example as shown in FIG. 7, the portions 708a-d
overlap. In some implementations, the portions 708a-d overlap spatially, either partially
or completely, or are spatially independent from each other.
In some implementations, the electrical source 702 is also adjustable, and can
apply varying currents in order to adjust the viscosity of portions of magneto-rheological
fluid 708a-d, either continuously or discretely.
In some implementations, different portions of the drill string may have different
channel transfer characteristics. That is, a first length of the drill string may have a first
channel transfer function, and one or more other lengths of the drill string may have one
or more other channel transfer functions. Due to these varying channel transfer
characteristics, in some implementations, acoustic energy may have difficulty
propagating along the entire length of the drill string. For example, FIG. 8 shows a plot
800 of an example drill string having two lengths, each length represented by a different
channel transfer function. The channel transfer function of the first length (line 802) and
the channel transfer function of the second length (line 804) are not identical, and each
has pass bands and stop bands that do not fully align. Thus, acoustic signals that have a
frequency within the pass band of one length might not be within the pass band of the
other length. In some implementations, acoustic module 200 can be positioned between
the different lengths of the drill string, and can be used as a "repeater" in order to
propagate the acoustic signal. For instance, in an example implementation, an acoustic
module 200 can include an acoustic receiver that detects acoustic signals propagating in a
first length of a drill string, a signal processing module that converts the signal into a
signal having a frequency within a pass band of the adjacent length of the drill string, and
a transducer 200 that induces the converted acoustic signal into the second length. In
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some implementations, the drill string may have more than two different lengths, and
more than one transducer can be used as repeaters to propagate the signal along each
length of the drill string.
Various aspects of the invention may be summarized as follows.
In general, in an aspect, an acoustic transmitter for generating an acoustic signal
includes an actuator module and a tuning module coupled to the actuator module. The
tuning module includes an electromagnetic coil, a variable electrical source, and a
magnetic-rheological fluid, where the variable electrical source is in electrical
communication with the electromagnetic coil and the electromagnetic coil is at least
partially disposed in a magneto-rheological fluid. The acoustic transmitter is arranged so
that during operation, the actuator module converts an electrical signal into vibration to
generate an acoustic signal, and the variable electrical source applies a current to the
electromagnetic coil such that a resonant frequency of the actuator module varies
depending on the applied current.
Implementations of this aspect may include one or more of the following features:
The acoustic transmitter can be arranged so that during operation, the variable
electrical source applies a current across a section of the electromagnetic coil of variable
s1ze.
The frequency of the acoustic signal can vary depending on a size of the section
of the electromagnetic coil and/or a strength of a magnetic field induced by the applied
current.
The acoustic transmitter can be arranged so that during operation, the section of
the electromagnetic coil is selectable from among two or more portions. The two or more
portions can at least partially overlap.
The acoustic transmitter can be arranged so that during operation, the tuning
module varies a viscosity of the magneto-rheological fluid by varying the current applied
to the electromagnetic coil.
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The acoustic transmitter can be arranged so that during operation, the transmitter
varies an effective mass of the tuning module by varying the current applied to the
electromagnetic coil.
The resonant frequency of the actuator module can vary inversely with respect to
the current applied to the electromagnetic coil.
A size of a section of the electromagnetic coil to which the current is applied can
be variable and the frequency of the acoustic signal can vary inversely with respect to the
size of the section.
The actuator module can include a piezoelectric stack. The piezoelectric stack
can be enclosed in a sleeve.
The actuator module can include a magnetostrictive material.
A downhole logging tool can include a logging module for inclusion in a drill
string. The logging module can include an acoustic transmitter as described above, an
acoustic receiver, and a control module. The logging module can be arranged so that
during use, the control module controls the resonant frequency of the actuator module by
adjusting the applied current, and the acoustic receiver detects the acoustic signal.
The resonant frequency can be selected from a range of frequencies that overlaps
a pass-band of the drill string.
In general, in another aspect, a method of adjusting a resonant frequency of an
acoustic transmitter includes applying an electrical signal to an actuator of the acoustic
transmitter to generate an acoustic signal, and selecting a resonant frequency of the
acoustic transmitter by applying a current across an electromagnetic coil of the acoustic
transmitter, the electromagnetic coil being at least partially disposed within a magnetorheological
fluid.
Implementations of this aspect may include one or more of the following features:
The method can include adjusting the resonant frequency of the acoustic
transmitter by adjusting the applied current.
The current can be applied across a section of the electromagnetic coil, and the m
method can include adjusting the resonant frequency of the acoustic transmitter by
adjusting a size of the section of the electromagnetic coil.
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The method can include adjusting the resonant frequency of the acoustic
transmitter by selecting the section from among two or more portions of the
electromagnetic coil.
The resonant frequency of the acoustic transmitter can vary inversely with respect
to the size of the section of the electromagnetic coil. The resonant frequency of the
acoustic transmitter can vary inversely with respect to the applied current.
In general, in another aspect, a method of communicating between two
components of a well can include applying an electrical signal to an actuator of the
acoustic transmitter to generate an acoustic signal, and controlling a resonant frequency
of the acoustic transmitter by applying a current across an electromagnetic coil of the
acoustic transmitter, the electromagnetic coil being at least partially disposed within a
magneto-rheological fluid, directing the acoustic signal into a structure of the well, and
obtaining a communications signal by detecting an acoustic signal propagating along the
structure.
The method can include adjusting a resonant frequency of the acoustic signal to
correspond to a pass-band of a drill string.
A number of implementations have been described. Nevertheless, it will be
understood that various modifications may be made without departing from the spirit and
scope of the invention.
For example, while a variety of magneto-rheological fluid-based tuning modules
have been described in the context of transducers used to transfer telemetry data in a drill
string, tuning modules can also be used with transducers that are used to transfer
telemetry data in other media. For instance, in some implementations, magnetorheological
fluid-based tuning modules can be used to tune the resonant frequency of
transducers used to transmit data in coiled tube acoustic telemetry channels or frac string
acoustic telemetry channels (e.g, strings used during hydraulic fracturing operations).
In some implementations, tuning modules can also be used with transducers that
are used for functions other than acoustic telemetry. For example, in some
implementations, magneto-rheological fluid-based tuning modules can be used to tune
the resonant frequency of transducers used in ultrasonic logging tools (e.g., ultrasonic
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logging tools used in wireline, slickline, LWD, and MWD applications), geophones,
speakers, hydrophones, sonar transponders, and other such devices.
In addition, magneto-rheological fluid-based tuning modules can be used to tune
the resonant frequency of various types of transducers, and is not limited to piezoelectric
transducers. For example, magneto-rheological fluid-based tuning modules can be used
in conjunction with electromagnetic acoustic transducers, magnetostrictive transducers,
or other types of transducer that can be tuned by adjusting the effective mass of one or
more of its components. As an example, in some implementations, a transducer can
contain a magnetostrictive material (e.g., terfenol-D), where a changing magnetic field
induces mechanical strain on the magnetostrictive material and causes translational
motion. A magneto-rheological fluid can be used with this transducer in order to change
the effective mass of one or more of its moving components in order to tune the resonant
frequency of the transducer.
Accordingly, other embodiments are within the scope of the following claims.20. The method of claim 15, wherein the resonant frequency of the acoustic
transmitter varies inversely with respect to the applied current.
21. A method of communicating between two components of a well, the method
comprising:
applying an electrical signal to an actuator of the acoustic transmitter to generate
an acoustic signal;
controlling a resonant frequency of the acoustic transmitter by applying a current
across an electromagnetic coil of the acoustic transmitter, the electromagnetic coil being
at least partially disposed within a magneto-rheological fluid;
directing the acoustic signal into a structure of the well; and
obtaining a communications signal by detecting an acoustic signal propagating
along the structure.
22. The method of claim 21, further comprising adjusting a resonant frequency of the
acoustic signal to correspond to a pass-band of a drill string.
Dated this

WHAT IS CLAIMED IS:
1. An acoustic transmitter for generating an acoustic signal, comprising:
an actuator module; and
a tuning module coupled to the actuator module, the tuning module comprising an
electromagnetic coil, a variable electrical source, and a magnetic-rheological fluid,
wherein the variable electrical source is in electrical communication with the
electromagnetic coil and the electromagnetic coil is at least partially disposed in a
magneto-rheological fluid;
wherein the acoustic transmitter is arranged so that during operation, the actuator
module converts an electrical signal into vibration to generate an acoustic signal, and the
variable electrical source applies a current to the electromagnetic coil such that a resonant
frequency of the actuator module varies depending on the applied current.
2. The acoustic transmitter of claim 1, wherein the acoustic transmitter is arranged
so that during operation, the variable electrical source applies a current across a section
ofthe electromagnetic coil ofvariable size.
3. The acoustic transmitter of claim 2, wherein the frequency of the acoustic signal
varies depending on a size of the section of the electromagnetic coil and/or a strength of a
magnetic field induced by the applied current.
4. The acoustic transmitter of claim 2, wherein the acoustic transmitter is arranged
so that during operation, the section of the electromagnetic coil is selectable from among
two or more portions.
5. The acoustic transmitter of claim 4, wherein the two or more portions at least
partially overlap.
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6. The acoustic transmitter of claim 1, wherein the acoustic transmitter is arranged
so that during operation, the tuning module varies a viscosity of the magneto-rheological
fluid by varying the current applied to the electromagnetic coil.
7. The acoustic transmitter of claim 1, wherein the acoustic transmitter is arranged
so that during operation, the transmitter varies an effective mass of the tuning module by
varying the current applied to the electromagnetic coil.
8. The acoustic transmitter of claim 1, wherein the resonant frequency of the
actuator module varies inversely with respect to the current applied to the
electromagnetic coil.
9. The acoustic transmitter of claim 1, wherein a size of a section of the
electromagnetic coil to which the current is applied is variable and the frequency of the
acoustic signal varies inversely with respect to the size of the section.
10. The acoustic transmitter of claim 1, wherein the actuator module comprises a
piezoelectric stack.
11. The acoustic transmitter of claim 10, wherein the piezoelectric stack is enclosed
in a sleeve.
12. The acoustic transmitter of claim 1, wherein the actuator module comprises a
magnetostrictive material.
13. A downhole logging tool comprising:
a logging module for inclusion in a drill string, the logging module comprising:
the acoustic transmitter of any one of claims 1 to 12;
an acoustic receiver; and
a control module;
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wherein the logging module is arranged so that during use, the control module
controls the resonant frequency of the actuator module by adjusting the applied current,
and the acoustic receiver detects the acoustic signal.
14. The downhole logging tool of claim 13, wherein the resonant frequency is
selected from a range of frequencies that overlaps a pass-band of the drill string.
15. A method of adjusting a resonant frequency of an acoustic transmitter, the method
compnsmg:
applying an electrical signal to an actuator of the acoustic transmitter to generate
an acoustic signal; and
selecting a resonant frequency of the acoustic transmitter by applying a current
across an electromagnetic coil of the acoustic transmitter, the electromagnetic coil being
at least partially disposed within a magneto-rheological fluid.
16. The method of claim 15, further comprising adjusting the resonant frequency of
the acoustic transmitter by adjusting the applied current.
17. The method of claim 15, wherein the current is applied across a section of the
electromagnetic coil, and wherein the method further comprises adjusting the resonant
frequency of the acoustic transmitter by adjusting a size of the section of the
electromagnetic coil.
18. The method of claim 17, further comprising adjusting the resonant frequency of
the acoustic transmitter by selecting the section from among two or more portions of the
electromagnetic coil.
19. The method of claim 17, wherein the resonant frequency of the acoustic
transmitter varies inversely with respect to the size of the section of the electromagnetic
coil.