U.S. patent application number 13/075567 was filed with the patent office on 2012-10-04 for transmitter and receiver synchronization for wireless telemetry systems.
Invention is credited to Benoit FROELICH, Guillaume MILLOT.
Application Number | 20120250461 13/075567 |
Document ID | / |
Family ID | 45974470 |
Filed Date | 2012-10-04 |
United States Patent
Application |
20120250461 |
Kind Code |
A1 |
MILLOT; Guillaume ; et
al. |
October 4, 2012 |
TRANSMITTER AND RECEIVER SYNCHRONIZATION FOR WIRELESS TELEMETRY
SYSTEMS
Abstract
An acoustic modem for communication in a network of acoustic
modems via a communication channel. The acoustic modem comprises a
transceiver assembly, transceiver electronics, and a power supply.
The transceiver assembly is adapted to convert acoustic messages
into electrical signals. The transceiver electronics is provided
with transmitter electronics and receiver electronics. The
transmitter electronics cause the transceiver assembly to send
acoustic signals into the communication channel. The receiver
electronics comprises at least one microcontroller adapted to
execute instructions to (1) enable the receiver electronics to
receive electrical signals indicative of the acoustic message from
at least one other acoustic modem via the transceiver assembly, (2)
estimate a carrier frequency of the electrical signals by analyzing
an estimation frame of the electrical signals, (3) estimate a
starting time of a data frame of the electrical signals by
synchronizing with a synchronization frame of the acoustic message
in parallel with at least two bit rates, and (4) decode the data
frame. The power supply supplies power to the transceiver assembly
and the transceiver electronics.
Inventors: |
MILLOT; Guillaume;
(Bordeaux, FR) ; FROELICH; Benoit; (Marly Le Roi,
FR) |
Family ID: |
45974470 |
Appl. No.: |
13/075567 |
Filed: |
March 30, 2011 |
Current U.S.
Class: |
367/82 |
Current CPC
Class: |
H04B 11/00 20130101;
E21B 47/16 20130101 |
Class at
Publication: |
367/82 |
International
Class: |
E21B 47/14 20060101
E21B047/14 |
Claims
1. A method of transmitting data along tubing in a borehole, the
method comprising the steps of: (i) transmitting an acoustic
message containing data by a first modem at a first location on the
tubing at a first frequency and a first bit rate selected from a
predetermined group of at least two frequencies and at least two
bit rates; (ii) receiving the acoustic message by a second modem at
a second location on the tubing, and detecting the first frequency
of the acoustic message by receiver electronics of the second
modem; and (iii) synchronizing the acoustic message with the
detected first frequency and in parallel with at least two bit
rates by the receiver electronics of the second modem.
2. The method of claim 1, wherein step (i) further comprises
adjusting a frequency of the acoustic message to one of the at
least two frequencies in the predetermined group of at least two
frequencies and at least two bit rates.
3. The method of claim 2, wherein the predetermined group of at
least two frequencies and at least two bit rates comprises the
first frequency and more than two further frequencies, the method
further comprising iterating steps (i)-(iii) through the group of
frequencies.
4. The method of claim 1, wherein step (i) comprises adjusting the
bit rate of the acoustic message to a lower bit rate.
5. The method of claim 4, wherein the step of adjusting the bit
rate follows adjustment of frequency of the acoustic message.
6. The method as of claim 1, further comprising the step of
re-transmitting the data within the acoustic message received by
the second modem to a third modem.
7. The method of claim 1, further comprising the step of
determining whether the step of synchronization is successful
utilizing a predetermined selection algorithm; and wherein the
predetermined selection algorithm is selected from a group of at
least one signal quality parameter consisting of signal distortion,
signal strength, ambient noise, signal-to-interference noise ratio,
signal-to-noise ratio, channel response time, signal amplitude, and
signal auto-correlation.
8. A method, comprising the steps of: converting an acoustic
message into an electrical signal by a transceiver assembly of an
acoustic modem attached to a tubing within a borehole, the acoustic
message including a synchronization frame and transmitted at a
first frequency and a first bit rate selected from a group of
multiple frequencies and bit rates; receiving the electrical signal
by receiver electronics of the acoustic modem; and synchronizing,
in parallel by the receiver electronics of the acoustic modem, a
synchronization frame of the electrical signal with at least two
frequencies and at least two bit rates of the group of multiple
frequencies and bit rates.
9. The method of claim 8, wherein the multiple frequencies and bit
rates within the group are predetermined.
10. An acoustic modem for communication in a network of acoustic
modems via a communication channel, the acoustic modem comprising:
a transceiver assembly adapted to convert acoustic messages into
electrical signals; transceiver electronics, comprising:
transmitter electronics to cause the transceiver assembly to send
acoustic signals into the communication channel; receiver
electronics comprising at least one microcontroller adapted to
execute instructions to (1) enable the receiver electronics to
receive electrical signals indicative of the acoustic message from
at least one other acoustic modem via the transceiver assembly, (2)
estimate a carrier frequency of the electrical signals by analyzing
an estimation frame of the electrical signals, (3) estimate a
starting time of a data frame of the electrical signals by
synchronizing with a synchronization frame of the acoustic message
in parallel with at least two bit rates, and (4) decode the data
frame; and a power supply supplying power to the transceiver
assembly and the transceiver electronics.
11. The acoustic modem of claim 10, wherein the at least one
microcontroller of the receiver electronics enables the transmitter
electronics to transmit an acknowledgement.
12. The acoustic modem of claim 10, wherein the at least one
microcontroller executes instructions to cause (2) and (3) to
execute sequentially.
13. An acoustic modem for communication in a network of acoustic
modems via a communication channel, the acoustic modem comprising:
a transceiver assembly adapted to convert electrical signals into
acoustic messages; transceiver electronics, comprising: transmitter
electronics coupled to the transceiver assembly; receiver
electronics coupled to the transceiver assembly; at least one
microcontroller executing instructions to (1) enable the
transmitter electronics to enter a training phase where the
transmitter electronics transmit a training message having an
estimation frame, a synchronization frame and a data frame to the
transceiver assembly, and (2) enable the transmitter electronics to
enter a data communication phase where the transmitter electronics
transmits a data message to the transceiver assembly having a
synchronization frame and a data frame without an estimation frame;
and a power supply supplying power to the transceiver assembly and
the transceiver electronics.
14. An acoustic modem for communication in a network of acoustic
modems via a communication channel, the acoustic modem comprising:
a transceiver assembly adapted to receive an acoustic message
having a synchronization frame and a data frame and to convert the
acoustic message into an electrical signal; transceiver
electronics, comprising: transmitter electronics to cause the
transceiver assembly to send acoustic signals into the
communication channel; receiver electronics having at least one
microcontroller executing instructions to (1) receive the
electrical signal indicative of the acoustic message; (2)
synchronize, in parallel, a synchronization frame of the electrical
signal with at least two frequencies and at least two bit rates of
a group of multiple frequencies and bit rates; and (3) decode the
data frame; and a power supply supplying power to the transceiver
assembly and the transceiver electronics.
15. An acoustic modem for communication in a network of acoustic
modems via a communication channel, the acoustic modem comprising:
a transceiver assembly adapted to convert an electrical signal into
an acoustic message; transceiver electronics, comprising: receiver
electronics coupled to the transceiver assembly; transmitter
electronics coupled to the transceiver assembly and including at
least one microcontroller adapted to execute instructions to
generate a sequence of electrical signals directed to the
transceiver assembly with different carrier frequencies and with
the sequence ordered based upon a model of a particular, planned
tubing for a borehole; and a power supply supplying power to the
transceiver assembly and the transceiver electronics.
16. A computer system, comprising: one or more input device; one or
more output device; one or more non-transitory computer readable
medium storing instructions for (1) receiving a priori information
of a particular, planned tubing for a borehole from the one or more
input device, (2) predicting optimal carrier frequencies for
acoustic communication utilizing the particular, planned tubing as
a communication channel between two or more modems, and (3)
outputting information to the output device indicative of the
optimal carrier frequencies; at least one processor in
communication with the one or more non-transitory computer readable
medium for executing the instructions based upon a message from the
one or more input device; and one or more power supply supplying
power to the input device, the output device, the one or more
non-transitory computer readable medium and the one or more
processor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
TECHNICAL FIELD
[0002] The present invention relates to telemetry systems for use
with installations in oil and gas wells or the like. In particular,
the present invention relates to the transmission of data and
control signals between a location down a borehole and the surface,
or between downhole locations themselves.
BACKGROUND ART
[0003] One of the more difficult problems associated with any
borehole is to communicate measured data between one or more
locations down a borehole and the surface, or between downhole
locations themselves. For example, in the oil and gas industry it
is desirable to communicate data generated downhole to the surface
during operations such as drilling, perforating, fracturing, and
drill stem or well testing; and during production operations such
as reservoir evaluation testing, pressure and temperature
monitoring. Communication is also desired to transmit intelligence
from the surface to downhole tools or instruments to effect,
control or modify operations or parameters.
[0004] Accurate and reliable downhole communication is particularly
important when complex data comprising a set of measurements or
instructions is to be communicated, i.e., when more than a single
measurement or a simple trigger signal has to be communicated. For
the transmission of complex data it is often desirable to
communicate encoded digital signals.
[0005] Downhole testing is traditionally performed in a "blind
fashion": downhole tools and sensors are deployed in a well at the
end of a tubing string for several days or weeks after which they
are retrieved at surface. During the downhole testing operations,
the sensors may record measurements that will be used for
interpretation once retrieved at surface. It is only after the
downhole testing tubing string is retrieved that the operators will
know whether the data are sufficient and not corrupted. Similarly
when operating some of the downhole testing tools from surface,
such as tester valves, circulating valves, packers, samplers or
perforating charges, the operators do not obtain a direct feedback
from the downhole tools.
[0006] In this type of downhole testing operations, the operator
can greatly benefit from having a two-way communication between
surface and downhole. However, it can be difficult to provide such
communication using a cable since inside the tubing string it
limits the flow diameter and requires complex structures to pass
the cable from the inside to the outside of the tubing. A cable
inside the tubing is also an additional complexity in case of
emergency disconnect for an offshore platform. Space outside the
tubing is limited and a cable can easily be damaged. Therefore a
wireless telemetry system is preferred.
[0007] A number of proposals have been made for wireless telemetry
systems based on acoustic and/or electromagnetic communications.
Examples of various aspects of such systems can be found in: U.S.
Pat. No. 5,050,132; U.S. Pat. No. 5,056,067; U.S. Pat. No.
5,124,953; U.S. Pat. No. 5,128,901; U.S. Pat. No. 5,128,902; U.S.
Pat. No. 5,148,408; U.S. Pat. No. 5,222,049; U.S. Pat. No.
5,274,606; U.S. Pat. No. 5,293,937; U.S. Pat. No. 5,477,505; U.S.
Pat. No. 5,568,448; U.S. Pat. No. 5,675,325; U.S. Pat. No.
5,703,836; U.S. Pat. No. 5,815,035; U.S. Pat. No. 5,923,937; U.S.
Pat. No. 5,941,307; U.S. Pat. No. 5,995,449; U.S. Pat. No.
6,137,747; U.S. Pat. No. 6,147,932; U.S. Pat. No. 6,188,647; U.S.
Pat. No. 6,192,988; U.S. Pat. No. 6,272,916; U.S. Pat. No.
6,320,820; U.S. Pat. No. 6,321,838; U.S. Pat. No. 6,912,177;
EP0550521; EP0636763; EP0773345; EP1076245; EP1193368; EP1320659;
EP1882811; WO96/024751; WO92/06275; WO05/05724; WO02/27139;
WO01/39412; WO00/77345; WO07/095111.
[0008] Tubing within a downhole environment can be constructed of a
plurality of pipe sections that are connected together using
threaded connections at both ends of the pipe sections. The pipe
sections can have uniform or non-uniform pipe lengths. With respect
to the non-uniform lengths, this is typically caused by the pipe
sections being repaired by cutting part of the connection to
re-machine the threads. The uniformity or non-uniformity of the
pipe lengths can affect the way in which acoustic messages
propagate along the tubing.
[0009] Because of the repetitive structure of the pipe sections,
the characteristic of the acoustic propagation along pipe sections
is such that the frequency response of the channel is complex. FIG.
3 shows an experimental and theoretical frequency response of a
section of a tubing. The spectrum has numerous peaks and troughs.
Given the spectrum and the use of a mono-carrier modulation scheme,
choosing a peak for the carrier frequency of the transmitted
modulated signal where noise is incoherent with the signal is
advantageous in terms of signal to noise ratio. Choosing a carrier
frequency around a locally flat channel response, i.e. no
distortion, is advantageous to maximize the bit rate. In any case,
choosing the carrier frequency in situ may be a requirement, and
the process of choosing the right carrier frequency may take time
and computing resources and should preferably be as simple as
possible.
[0010] US 2006/0187755 by Robert Tingley discloses a method and
system for communicating data through a drill string by
transmitting multiple sets of data simultaneously at different
frequencies. The Tingley reference attempts to optimize the
opportunity of successful receipt despite the acoustic behavior of
the drill string, thereby avoiding the problem of selecting a
single frequency.
[0011] Moreover, U.S. Pat. No. 5,995,449 by Clark Robison et al.
discloses a method and apparatus for communicating in a wellbore
utilizing acoustic signals. However, the Robison et al. disclosure
relates specifically to an apparatus and method for transmitting
acoustic waves through the completion liquid as a transmission
medium, rather than the tubing or pipe string.
[0012] In one method of "calibrating" the carrier frequency, a
wireline probe is installed in a tubular string to communicate with
acoustic modems (e.g., acoustic transceivers) interconnected in the
tubular string. When initiated by an operator at the surface, the
wireline probe prompts one of the acoustic modems to transmit a
sweep of frequencies. See for example U.S. Patent Publication No.
2008/0180273
[0013] The wireline probe is positioned at another acoustic modem
during the transmission of the frequency sweep, in order to detect
characteristics of the received signals. The operator can then
select which carrier frequency is optimum for transmission of
messages between the two acoustic modems.
[0014] It will be appreciated that this method is time-consuming,
requires installation and operation of the wireline probe and
requires the services of a highly skilled operator. This method,
and the method which requires prior knowledge of a particular
carrier frequency, are also not suited for coping with changes in
the well environment over time (which will also change the optimum
carrier frequency), without repeating the expensive and complex
operation of calibrating or changing the carrier frequency.
[0015] Techniques have been proposed for calibrating the carrier
frequency without human intervention and without prior knowledge of
the modulating carrier frequency. See for example U.S. Patent
Publication No. 2008/0180273 which describes a method for detecting
a usable downhole wireless telemetry system carrier frequency where
a first telemetry device transmits one or more message(s) modulated
on a carrier frequency; then checks whether a response to the
message is received at the first telemetry device from a second
telemetry device, and each time the response is not received,
repeating the transmitting and checking steps with an incremented
carrier frequency.
[0016] However, the robustness of the system is not based solely on
the carrier frequency. For example, if the channel is chaotic
within the band pass of the modulated signals, or depending upon
the impulse response, the transmitted modulated signals are
distorted by the channel. The distortion creates Inter Symbol
Interference ("ISI"). When the received signals are decoded, the
ISI causes bit errors. If the bit rate is increased, the ISI
increases and the probability of having bit errors also increase.
The channel may also attenuate the transmitted modulated signals.
This attenuation decreases the Signal to Noise Ratio (i.e., "SNR")
which is defined as the power of the modulated signals over the
noise power. The receiver decodes received modulated signals having
a minimum SNR value. If the channel attenuates the modulated
signals too much, the received signals cannot be properly decoded
by the receiver. All these effects depend on the band pass of the
modulated signals and affect the ability of the receiver to decode
the encoded information. It should be pointed out that for
mono-carrier modulations, the modulated signals are narrow band.
Thus, a given carrier frequency can be usable, but not at a given
bit rate.
[0017] In the downhole environment, it is difficult to increase the
processing power of circuitry within the receiver due to the
harshness of the downhole environment and limited power sources.
For this reason, methodologies that use less processing complexity
are desirable because such methodologies extend the use of the
limited power sources.
[0018] There is a need for an acoustic modem that facilitates
automatic synchronization of carrier frequency and/or bit rate of
acoustic messages transmitted along at least a portion of a tubing
section in a borehole for more reliable data transmission. There is
also a need for enhancing the selection of an appropriate carrier
frequency for the acoustic messages. It is to such an acoustic
modem and method of transmission that the present disclosure is
directed.
BRIEF SUMMARY OF THE DISCLOSURE
[0019] In one aspect, the present disclosure describes a method of
transmitting data along tubing in a borehole. In the method, an
acoustic message is transmitted by a first modem at a first
location on the tubing at a first frequency and a first bit rate
selected from a predetermined group of at least two frequencies and
at least two bit rates. The acoustic message contains data. The
acoustic message is received by a second modem at a second location
on the tubing. The first frequency of the acoustic message is
detected by receiver electronics of the second modem. The acoustic
message is synchronized with the detected first frequency and in
parallel with at least two bit rates by the receiver electronics of
the second modem.
[0020] In one version, the frequency of the acoustic message is
adjusted to one of the at least two frequencies in the
predetermined group of at least two frequencies and at least two
bit rates. The predetermined group of the at least two frequencies
and the at least two bit rates comprises the first frequency and
more than two further frequencies. The method can also further
comprise the steps of iterating through the group of frequencies,
and/or adjusting the bit rate of the acoustic message to a lower
bit rate. The step of adjusting the bit rate can also follow
adjustment of the frequency of the acoustic message.
[0021] In other versions, the method also includes a step of
re-transmitting the data within the acoustic message received by
the second modem to a third modem, and/or determining whether the
step of synchronization is successful utilizing a predetermined
selection algorithm selected from a group of at least one signal
quality parameter consisting of signal distortion, signal strength,
ambient noise, signal-to-interference noise ratio, signal-to-noise
ratio, channel response time, signal amplitude, and signal
auto-correlation.
[0022] In another version, the present disclosure describes a
method in which an acoustic message is converted into an electrical
signal by a transceiver assembly of an acoustic modem attached to a
tubing within a borehole. The acoustic message includes a
synchronization frame and is transmitted at a first frequency and a
first bit rate selected from a group of multiple frequencies and
bit rates. The electrical signal is received by receiver
electronics of the acoustic modem and the receiver electronics
synchronizes, in parallel, a synchronization frame of the
electrical signal with at least two frequencies and at least two
bit rates of the group of multiple frequencies and bit rates. The
multiple frequencies and bit rates within the group can be
predetermined.
[0023] In another version, the present disclosure describes an
acoustic modem for communication in a network of acoustic modems
via a communication channel. The acoustic modem is provided with a
transceiver assembly adapted to convert acoustic messages into
electrical signals. The acoustic modem is also provided with
transceiver electronics and a power supply supplying power to the
transceiver assembly and the transceiver electronics. The
transceiver electronics is provided with transmitter electronics
and receiver electronics. The transmitter electronics cause the
transceiver assembly to send acoustic signals into the
communication channel while the receiver electronics comprising at
least one microcontroller. The least one microcontroller is adapted
to execute instructions to (1) enable the receiver electronics to
receive electrical signals indicative of the acoustic message from
at least one other acoustic modem via the transceiver assembly, (2)
estimate a carrier frequency of the electrical signals by analyzing
an estimation frame of the electrical signals, (3) estimate a
starting time of a data frame of the electrical signals by
synchronizing with a synchronization frame of the acoustic message
in parallel with at least two bit rates, and (4) decode the data
frame.
[0024] In yet other aspects, the at least one microcontroller of
the receiver electronics enables the transmitter electronics to
transmit an acknowledgement, and the at least one microcontroller
executes instructions to cause (2) and (3) to execute
sequentially.
[0025] In yet another version, the present disclosure describes an
acoustic modem for communication in a network of acoustic modems
via a communication channel. The acoustic modem is provided with a
transceiver assembly, transceiver electronics, and a power supply.
The transceiver assembly is adapted to convert electrical signals
into acoustic messages. The transceiver electronics is provided
with transmitter electronics and receiver electronics. The
transmitter electronics and the receiver electronics are coupled to
the transceiver assembly. At least one microcontroller is also
provided to execute instructions to (1) enable the transmitter
electronics to enter a training phase where the transmitter
electronics transmit a training message having an estimation frame,
a synchronization frame and a data frame to the transceiver
assembly, and (2) enable the transmitter electronics to enter a
data communication phase where the transmitter electronics
transmits a data message to the transceiver assembly having a
synchronization frame and a data frame without an estimation frame;
and
[0026] In yet another version, the acoustic modem may include
receiver electronics having at least one microcontroller executing
instructions to (1) receive the electrical signal indicative of the
acoustic message; (2) synchronize, in parallel, a synchronization
frame of the electrical signal with at least two frequencies and at
least two bit rates of a group of multiple frequencies and bit
rates; and (3) decode the data frame.
[0027] In yet another version, the acoustic modem may include,
transmitter electronics coupled to the transceiver assembly and
including at least one microcontroller adapted to execute
instructions to generate a sequence of electrical signals directed
to the transceiver assembly with different carrier frequencies and
with the sequence ordered based upon a model of a particular,
planned tubing for a borehole.
[0028] In another aspect, the present disclosure describes a
computer system having one or more input device, one or more output
device, and one or more non-transitory computer readable medium
storing instructions for (1) receiving a priori information of a
particular, planned tubing for a borehole from the one or more
input device, (2) predicting optimal carrier frequencies for
acoustic communication utilizing the particular, planned tubing as
a communication channel between two or more modems, and (3)
outputting information to the output device indicative of the
optimal carrier frequencies. The computer system is also provided
with at least one processor in communication with the one or more
non-transitory computer readable medium for executing the
instructions based upon a message from the one or more input
device, and one or more power supply supplying power to the input
device, the output device, the one or more non-transitory computer
readable medium and the one or more processor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] Certain embodiments of the present invention will hereafter
be described with reference to the accompanying drawings, wherein
like reference numerals denote like elements, and:
[0030] FIG. 1 shows a schematic view of an acoustic telemetry
system according to an embodiment of the present invention;
[0031] FIG. 2 shows a schematic of an acoustic modem as used in
accordance with the embodiment of FIG. 1;
[0032] FIG. 3 depicts an acoustic frequency response of a tubing
section;
[0033] FIG. 4A illustrates a flow diagram of a method according to
an embodiment of the present invention;
[0034] FIG. 4B illustrates a flow diagram of another method
according to an embodiment of the present invention;
[0035] FIGS. 5A and 5B show flow diagrams of a receiver
architecture for use in an embodiment of the present invention in
which receiver electronics of the acoustic modem attempts to
synchronize on combinations of different bit rates and carrier
frequencies in parallel;
[0036] FIG. 6A shows a training message generated by version(s) of
the presently disclosed and claimed inventive concepts;
[0037] FIG. 6B shows a data message generated by version(s) of the
presently disclosed and claimed inventive concepts;
[0038] FIG. 7 is a flow diagram of a receiver architecture for use
in an embodiment of the presently disclosed and claimed inventive
concepts;
[0039] FIG. 8 is a block diagram of a computer system that can be
used to execute a pipe communication algorithm in accordance with
the present disclosure;
[0040] FIG. 9 is a side elevation view of a section of tubing that
can be modeled by the pipe communication algorithm in accordance
with the present disclosure;
[0041] FIG. 10 is an exemplary simulated transfer function based
upon a priori information for a particular, planned tubing to be
modeled formed of pipe sections having a pipe length of 9.5 m, a
connection length of 0.35 m, a connection outer cross-section of
4.5 inches, inner diameter 2.75 inches, and a pipe outer diameter
of 3.5 inches; and
[0042] FIG. 11 is another simulated transfer function of two
exemplary tubing sections showing a difference in attenuation
between one tubing formed of pipe sections having a uniform pipe
length, and another tubing having pipe lengths of differing sizes
within a standard deviation of 0.15 m.
[0043] FIG. 12 is another exemplary simulated transfer function
based upon a priori for a particular, planned tubing illustrating
locations of band stops within the frequency domain.
DETAILED DESCRIPTION
[0044] The present invention is particularly applicable to testing
installations such as are used in oil and gas wells or the like.
FIG. 1 shows a schematic view of such a system. Once a well 10 has
been drilled through a formation, the drill string can be used to
perform tests, and determine various properties of the formation
through which the well has been drilled. In the example of FIG. 1,
the well 10 has been lined with a steel casing 12 (cased hole) in
the conventional manner, although similar systems can be used in
unlined (open hole) environments. In order to test the formations,
it is preferable to place testing apparatus in the well close to
the regions to be tested, to be able to isolate sections or
intervals of the well, and to convey fluids from the regions of
interest to the surface. This is commonly done using a jointed
tubular drill pipe, drill string, production tubing, sections
thereof, or the like (collectively, tubing 14) which extends from
well-head equipment 16 at the surface down inside the well 10 to a
zone of interest. The well-head equipment 16 can include blow-out
preventers and connections for fluid, power and data
communication.
[0045] A packer 18 is positioned on the tubing 14 and can be
actuated to seal the borehole around the tubing 14 at the region of
interest. Various pieces of downhole test equipment (collectively,
downhole equipment 20) are connected to the tubing 14 above or
below the packer 18. Such downhole equipment 20 may include, but is
not limited to: additional packers; tester valves; circulation
valves; downhole chokes; firing heads; TCP (tubing conveyed
perforator) gun drop subs; samplers; pressure gauges; downhole flow
meters; downhole fluid analyzers; and the like.
[0046] In the embodiment of FIG. 1, a sampler 22 is located above
the packer 18 and a tester valve 24 is located above the packer 18.
The downhole equipment 20 is connected to an acoustic modem 25Mi+1
which can be mounted in a gauge carrier 28 positioned between the
sampler 22 and the tester valve 24. The acoustic modem 25Mi+1,
operates to allow electrical signals from the downhole equipment 20
to be converted into acoustic signals for transmission to the
surface via the tubing 14, and to convert acoustic tool control
signals from the surface into electrical signals for operating the
downhole equipment 20. The term "data," as used herein, is meant to
encompass control signals, tool status, and any variation thereof
whether transmitted via digital or analog signals.
[0047] FIG. 2 shows a schematic of the acoustic modem 25Mi+1 in
more detail. The acoustic modem 25Mi+1 comprises a housing 30
supporting a transceiver assembly 32 which can be a piezo electric
actuator or stack, and/or a magnetorestrictive element which can be
driven to create an acoustic signal in the tubing 14 when the
acoustic modem 25Mi+1 is mounted in the gauge carrier 28. The
acoustic modem 25Mi+1 can also include an accelerometer 34 and/or
an additional transceiver assembly 35 for receiving acoustic
signals. Where the acoustic modem 25Mi+1 is only required to
receive acoustic messages, the transceiver assembly 32 may be
omitted. The acoustic modem 25Mi+1 also includes transmitter
electronics 36 and receiver electronics 38 located in the housing
30 and power is provided by means of a battery, such as a lithium
battery 40. Other types of power supply may also be used.
[0048] The transmitter electronics 36 are arranged to initially
receive an electrical output signal from a sensor 42, for example
from the downhole equipment 20 provided from an electrical or
electro/mechanical interface. Such signals are typically digital
signals which can be provided to a micro-controller 43 which
modulates the signal in one of a number of known ways PSK, QPSK,
QAM, and the like. The micro-controller 43 can be implemented as a
single micro-controller or two or more micro-controllers working
together. In any event, the resulting modulated signal is amplified
by either a linear, or non-linear, amplifier 44 and transmitted to
the transceiver assembly 32 so as to generate an acoustic signal
(which is also referred to herein as an acoustic message) in the
material of the tubing 14.
[0049] The acoustic signal passes along the tubing 14 as a
longitudinal and/or flexural wave comprises a carrier signal with
an applied modulation of the data received from the sensors 42. The
acoustic signal typically has, but is not limited to, a frequency
in the range 1-10 kHz, preferably in the range 2-5 kHz, and is
configured to pass data at a rate of, but is not limited to, about
1 bps to about 200 bps, preferably from about 5 to about 100 bps,
and more preferably about 50 bps. The data rate is dependent upon
conditions such as the noise level, carrier frequency, Inter Symbol
Interference and the distance between the acoustic modems 25Mi-2,
25Mi-1, 25M and 25Mi+1. A preferred embodiment of the present
disclosure is directed to a combination of a short hop acoustic
modems 25Mi-1, 25M and 25Mi+1 for transmitting data between the
surface and the downhole equipment 20, which may be located above
and/or below the packer 18. The acoustic modems 25Mi-1 and 25M can
be configured as repeaters of the acoustic signals. The system may
be designed to transmit data as high as 200 bps. Other advantages
of the present system exist.
[0050] The receiver electronics 38 of the acoustic modem 25Mi+1 are
arranged to receive the acoustic signal passing along the tubing 14
produced by the transmitter electronics 36 of the acoustic modem
25M. The receiver electronics 38 are capable of converting the
acoustic signal into an electric signal. In a preferred embodiment,
the acoustic signal passing along the tubing 14 excites the
transceiver assembly 32 so as to generate an electric output signal
(voltage); however, it is contemplated that the acoustic signal may
excite the accelerometer 34 or the additional transceiver assembly
35 so as to generate an electric output signal (voltage). This
signal is essentially an analog signal carrying digital
information. The analog signal is applied to a signal conditioner
48, which operates to filter/condition the analog signal to be
digitalized by an A/D (analog-to-digital) converter 50. The A/D
converter 50 provides a digitalized signal which can be applied to
a microcontroller 52. The microcontroller 52 is preferably adapted
to demodulate the digital signal in order to recover the data
provided by the sensor 42, or provided by the surface. The type of
signal processing depends on the applied modulation (i.e. PSK,
QPSK, QAM, and the like).
[0051] The acoustic modem 25Mi+1 can therefore operate to transmit
acoustic data signals from sensors 42 in the downhole equipment 20
along the tubing 14. In this case, the electrical signals from the
downhole equipment 20 are applied to the transmitter electronics 36
(described above) which operate to generate the acoustic signal.
The acoustic modem 25Mi+1 can also operate to receive acoustic
control signals to be applied to the downhole equipment 20. In this
case, the acoustic signals are demodulated by the receiver
electronics 38 (described above), which operate to generate the
electric control signal that can be applied to the downhole
equipment 20.
[0052] Returning to FIG. 1, in order to support acoustic signal
transmission along the tubing 14 between the downhole location and
the surface, a series of the acoustic modems 25Mi-1 and 25M, etc.
may be positioned along the tubing 14. The acoustic modem 25M, for
example, operates to receive an acoustic signal generated in the
tubing 14 by the acoustic modem 25Mi-1 and to amplify and
retransmit the signal for further propagation along the tubing 14.
The number and spacing of the acoustic modems 25Mi-1 and 25M will
depend on the particular installation selected, for example on the
distance that the signal must travel. A typical spacing between the
acoustic modems 25Mi-1, 25M, and 25Mi+1 is around 1,000 ft, but may
be much more or much less in order to accommodate all possible
testing tool configurations. When acting as a repeater, the
acoustic signal is received and processed by the receiver
electronics 38 and the output signal is provided to the
microcontroller 52 of the transmitter electronics 36 and used to
drive the transceiver assembly 32 in the manner described above.
Thus an acoustic signal can be passed between the surface and the
downhole location in a series of short hops.
[0053] The role of a repeater is to detect an incoming signal, to
decode it, to interpret it and to subsequently rebroadcast it if
required. In some implementations, the repeater does not decode the
signal but merely amplifies the signal (and the noise). In this
case the repeater is acting as a simple signal booster. However,
this is not the preferred implementation selected for wireless
telemetry systems of the present invention.
[0054] The acoustic modems 25M, 25Mi-1, 25Mi-2, and 25Mi+2 will
either listen continuously for any incoming signal or may listen
from time to time.
[0055] The acoustic wireless signals, conveying commands or
messages, propagate in the transmission medium (the tubing 14) in
an omni-directional fashion, that is to say up and down. It is not
necessary for the acoustic modem 25Mi+1 to know whether the
acoustic signal is coming from the acoustic modem 25M above or an
acoustic modem 25Mi+2 (not shown) below. The direction of the
acoustic message is preferably embedded in the acoustic message
itself. Each acoustic message contains several network addresses:
the address of the acoustic modem 25Mi-1, 25M or 25Mi+1 originating
the acoustic message and the address of the acoustic modem 25Mi-1,
25M or 25Mi+1 that is the destination. Based on the addresses
embedded in the acoustic messages, the acoustic modem 25Mi-1 or 25M
functioning as a repeater will interpret the acoustic message and
construct a new message with updated information regarding the
acoustic modem 25Mi-1, 25M or 25Mi+1 that originated the acoustic
message and the destination addresses. Acoustic messages will be
transmitted from acoustic modem 25Mi-1 to 25M and may be slightly
modified to include new network addresses.
[0056] Referring again to FIG. 1, the acoustic modem 25Mi-2 is
provided at surface, such as at or near the well-head equipment 16
which provides a connection between the tubing 14 and a data cable
or wireless connection 62 to a control system 64 that can receive
data from the downhole equipment 20 and provide control signals for
its operation.
[0057] In the embodiment of FIG. 1, the acoustic telemetry system
is used to provide communication between the surface and a section
of the tubing 14 located downhole.
Full-Parallel Synchronization
[0058] A preferred embodiment of the present disclosure is based on
a protocol in which the transmitter electronics 36 of one of the
acoustic modems 25Mi-2, 25Mi-1, 25M or 25Mi+1 transmits a message
(i.e., a control signal or data signal) belonging to a
predetermined set S.sub.f of N frequencies (F.sub.1, F.sub.2,
F.sub.3, F.sub.4, . . . F.sub.n), and the receiver electronics 38
of another one of the acoustic modems 25Mi-2, 25Mi-1, 25M or 25Mi+1
synchronizes in parallel on the predetermined set S.sub.f of N
frequencies until the communication succeeds. The receiver
electronics 38 of the acoustic modem 25Mi+1, for example,
simultaneously tries to demodulate the incoming signals transmitted
by the acoustic modem 25M on the predetermined frequencies S.sub.f.
The protocol is illustrated in FIG. 4A, in which S.sub.f is shown
to comprise four frequencies F.sub.1-F.sub.4, however, the
predetermined set of frequencies may include much more or less. A
scheme of the parallel receiver is shown in FIGS. 5A and 5B and
provides the advantage of not requiring circuitry for frequency
detection.
[0059] In the example illustrated in FIG. 4A, the transmitter
electronics 36 of the acoustic modem 25M, for example, initially
transmits a signal at frequency F.sub.1. The receiver electronics
38 of the acoustic modem 25Mi+1 attempts to synchronize at multiple
frequencies, F.sub.1-F.sub.4, but due to attenuation or distortion
of the signal at this frequency, is unable to synchronize with this
signal on F.sub.1 and so does not send any acknowledgement signal
back to the transmitter electronics 36 of the acoustic modem 25M.
When starting to transmit at a given frequency, the transmitter
electronics 36 of the acoustic modem 25M starts a timing routine.
If no acknowledgement is received from the receiver electronics 38
of the acoustic modem 25Mi+1 within a predetermined time interval,
the transmitter electronics 36 of the acoustic modem 25M times out
and switches to the next frequency F.sub.2. This process is
preferably repeated until an acknowledgement signal is received
from the receiver electronics 38 of the acoustic modem 25Mi+1 on
the same frequency, at which time the transmitter electronics 36 of
the acoustic modem 25M begins data transmission. One advantage of
the parallel synchronization illustrated in the example of FIG. 4A
is the robustness of the process, and the removal of the need for
frequency detection. In the example of FIG. 4A, synchronization
occurs at frequency F.sub.3. It is contemplated that while one
carrier frequency may be chosen for transmission from one of the
acoustic modems 25M to another one of the acoustic modems 25Mi+1, a
same or different second carrier frequency may be chosen for
transmission from the acoustic modem 25Mi-1 to the acoustic modem
25M, for example.
[0060] The selection of an initial transmission frequency is
preferably chosen from a set of frequencies based on past
experience, but may also include an automatic mechanism at the
beginning of the communication. This mechanism could consist in
having all the transmitter electronics 36 of the acoustic modems
25Mi-2, 25Mi-1, 25M or 25Mi+1 transmitting frequency sweeps at a
predetermined time and all the receiver electronics 38 in the
tubing 14 recording the incoming frequency sweeps, then determining
the N best frequencies based on quality indicators such as
amplitude, signal-to-noise ratio and spectrum flatness. The
determination of the initial and subsequent transmission
frequencies can also be determined based upon a priori knowledge of
the tubing 14, as will be discussed in more detail below.
[0061] Based on the spectral estimate of the communication channel
in various cases and assuming the set S.sub.f is well chosen, it is
likely that there is at least one carrier frequency out of N (N
being small, such as 4 or 5, but may be much more) with limited
attenuation and distortion.
[0062] FIGS. 5A and 5B shows schematically a receiver architecture
of the receiver electronics 38 used for parallel synchronization.
The receiver architecture corresponds to the signal processing,
preferably implemented in the microcontroller 52 of the receiver
electronics 38 depicted in FIG. 2. After the analog signal is
digitalized by the A/D converter 50, the resulting digitalized
signal can be simultaneously demodulated by the microcontroller 52
on the predetermined set of frequencies belonging to S.sub.f. The
demodulation process preferably comprises a synchronization process
140 and a decoding process 142.
[0063] In the synchronization process 140 as depicted in FIGS. 5A
and 5B, the microcontroller 52 simultaneously attempts to
synchronize on the predetermined set of frequencies S.sub.f and a
predetermined set S.sub.b of K bit rates (B.sub.1, B.sub.2, . . .
,B.sub.k). Where the incoming signal only has one frequency, the
microcontroller 52 attempts to synchronize on multiple frequencies,
but may only succeed to synchronize on this signal frequency. The
synchronization process can be based on correlation, where parallel
synchronization consists of multiple, simultaneous correlations. If
the synchronization is successful on the synchronized frequency,
the beginning of the received signal is well known as well as its
frequency. In the decoding process 142, the modulated signal is
decoded and the data recovered. Where the incoming signal is
transmitted on multiple frequencies, the microcontroller 52 selects
the best frequency based on the highest correlation ratio and
proceeds to decode the data on the best frequency.
[0064] In the example of FIG. 4A, the acoustic messages are all
transmitted at the same bit rate and the receiver electronics 38
tries to synchronize on different frequencies at a single given bit
rate. In another embodiment of the present disclosure as depicted
in FIG. 4B, the bit rate can be varied. If the signal channel is
unusually very noisy and none of the transmitted signals is
recovered by the receiver electronics 38, the system of FIG. 4A
will not work. In order to avoid this, the receiver electronics 38
can also synchronize at a lower bit rate for each of the
frequencies belonging to S.sub.f. In other words, the receiver
electronics 38 can be configured to simultaneously synchronize on
multiple different carrier frequencies S.sub.f as well as on a
predetermined set of K bit rates (B.sub.1, B.sub.2, . . .
,B.sub.k). Different combinations of carrier frequencies and bit
rates are attempted by the transmitter electronics 36 until an
acknowledgement is received during a time-out period. Once an
acknowledgement is received, a preferred combination of carrier
frequency and bit rate is selected.
[0065] As shown in FIG. 4B, the transmitter electronics 36 will
preferably first try to transmit acoustic messages at a high bit
rate. In case of failure, the transmitter electronics 36 will
transmit the acoustic messages at successively lower bit rates,
which is shown in FIG. 4B as a "low bit rate". Since the energy per
bit becomes higher as the bit rate decreases, the bit
energy-to-noise ratio (Eb/N.sub.0) is increased. In addition, since
the signal bandwidth is reduced, then there is less ISI and the
received acoustic signal is less distorted by the channel. Though
this adds more complexity to the receiver electronics 38 and
decreases the data rate, the communication becomes more robust.
[0066] In the example depicted in FIG. 4B, the number of available
carrier frequencies and available bit rates can vary so long as the
available carrier frequencies and available bit rates are greater
than one. This is represented in certain parts of this document as
N.sub.f carrier frequencies and N.sub.br bit rates where N.sub.f
and N.sub.br are greater than 1.
[0067] The time out period can vary and depends on the duration in
which the acknowledgement is expected to be received if the
acoustic message was properly decoded. Various factors can be used
to determine the time out period, such as the time duration of the
acoustic message and/or the distance between the acoustic modems
25Mi-2, 25Mi-1, 25M and 25Mi+1. The time out period can be the
length of time that it takes for the receiver electronics 38 to
transmit the acknowledgement to the transmitter electronics 36. For
example, if the receiver electronics 38 is transmitting bits at 50
bits/second, and the acknowledgement includes 150 bits, then the
time out period can be approximately 3 seconds. Further, the
acknowledgement should be as short as possible to optimize the
performance of the system.
[0068] In general, the communication channel between a pair of the
acoustic modems 25Mi-2, 25Mi-1, 25M or 25Mi+1 is assumed to be
symmetric. As such, the acoustic signal used for the
acknowledgement(s) may be transmitted using the same carrier
frequency and/or bit rate as that of the training message(s). In
this instance, after a carrier frequency and/or bit rate have been
selected, the pair of acoustic modems 25Mi-2, 25Mi-1, 25M or 25Mi+1
may store the values for the selected bit rate and/or carrier
frequency for communicating with each other. Thus, for future
transmissions, in this embodiment data will be transmitted between
a particular pair of the acoustic modems 25Mi-2, 25Mi-1, 25M and
25Mi+1 using the selected bit rate and carrier frequency.
[0069] In the example shown in FIG. 4A, the transmitter electronics
36 does not receive an acknowledgement within the time out period
for carrier frequencies F.sub.1 and F.sub.2 which means the
receiver electronics 38 either did not demodulate the acoustic
signal, or the acoustic signal was demodulated but the signal
quality was insufficient. The sufficiency of the signal quality can
be determined by setting a minimum threshold for the signal quality
and comparing the minimum threshold to a quantitative evaluation of
the quality, such as a bit error rate.
[0070] In the example shown in FIG. 4B, once all of the available
carrier frequencies have been tried for a particular bit rate,
another bit rate is selected and then the transmitter electronics
36 of the acoustic modem 25M, for example, cycles through the
available carrier frequencies.
[0071] Higher bit rates are typically preferred over lower bit
rates. If all the available carrier frequencies have been tested
and no demodulation has been successful at a high bit rate or the
quality of the received signals is insufficient, the transmitter
electronics 36 successively transmits low bit-rate signals at
different carrier frequencies until the receiver electronics 38
demodulates the acoustic signal at a sufficient quality and
provides the acknowledgement.
[0072] For mono-carrier modulations, the quality of the received
modulated signals, which is often measured by the Signal to
Interference and Noise Ratio (SINR), is directly related to the
characteristics of the channel within the band pass of the
modulated signal. The SINR is defined as the signal power over the
interference and noise power.
[0073] If the quality measure of a received signal is higher than a
quality threshold, the signal quality is sufficient, e.g., the
signal being decoded with a bit error rate BER which is below a BER
target. The BER target can be specified by the user of the
communication system. It is for example equal to 10.sup.-3 or
10.sup.-4. The quality threshold can be derived from the BER
target.
[0074] The methods set forth in FIGS. 4A and 4B are only examples.
One embodiment of the present disclosure corresponds to the
acoustic modems 25Mi-2, 25Mi-1, 25M and 25Mi+1 being able to
communicate at several frequencies and several bit rates and the
frequencies and the bit rates can be tested in any order. The
quality criteria can be derived from the SINR or any other measure,
such as the SNR, which indicate the communication performance of
the acoustic modems 25Mi-2, 25Mi-1, 25M and 25Mi+1. Several quality
criteria can even be taken into consideration.
[0075] In the example discussed above, it is assumed that the
communication channel between the transmitter electronics 36 and
the receiver electronics 38 is symmetric. However, it should be
understood that the training phase can be adapted to non-symmetric
channels. In the case of a non-symmetric channel, a carrier
frequency and/or a bit rate that works from the transmitter
electronics 36 to the receiver electronics 38 does not necessarily
work from the receiver electronics 38 to the transmitter
electronics 36. Therefore, if the receiver electronics 38
successfully demodulates an acoustic message transmitted by the
transmitter electronics 36, the acknowledgment transmitted from the
receiver electronics 38 to the transmitter electronics 36 will not
necessarily be successfully demodulated by the transmitter
electronics 36 or the quality of the signal might be too poor.
[0076] In reality, the communication channel is partially
symmetric. Locally there might be some slight differences due to
the electronics, and/or the transceiver assembly 32, the
transmitter electronics 36 and the receiver electronics 38 are not
matched. Therefore, the transfer functions from the transmitter
electronics 36 to the receiver electronics 38 and vice-versa are
slightly different and the noise seen by both might also be
different.
[0077] Since however the differences are normally small, in
general, a carrier frequency and bit rate that works when
transmitting from the transmitter electronics 36 to the receiver
electronics 38 also works when transmitting from the receiver
electronics 38 to the transmitter electronics 36. Thus, in one
embodiment, a carrier frequency and bit rate that works in both
directions is selected. However, different carrier frequencies
and/or bit rates for communication in between the transmitter
electronics 36 and the receiver electronics 38 can be selected.
[0078] The modems 25Mi-2, 25Mi-1, 25M and 25Mi+1 have to be able to
communicate at at least one carrier frequency and one bit rate in
accordance with the presently disclosed inventive concepts set
forth in FIG. 4B. The transmitter electronics 36 can generate
mono-carrier modulated signals at different frequencies and/or
different bit rates by preferably using digital and/or analog
circuitry to generate a desired waveform to be provided to the
transceiver assembly 32. However, interpreting and/or decoding the
acoustic signal by the receiver electronics 38 can be far more
complicated since the carrier frequency and the bit rate may be
unknown.
[0079] An exemplary receiver architecture for implementing the
inventive concepts set forth in FIG. 4B is shown in FIGS. 5A and
5B. In the embodiment of FIG. 5B the receiver electronics 38
synchronizes and decodes an acoustic signal having an unknown
carrier frequency and bit rate with the at least one
microcontroller 52 synchronizing with the acoustic signal utilizing
multiple combinations of carrier frequency and bit rate in
parallel. The receiver architecture of FIG. 5B has a full parallel
synchronization state 150 and a decoding state 152. The full
parallel synchronization state 150 estimates the carrier frequency
and the bit rate of the acoustic signal, as well as accurately
estimates the start time of a data frame contained in the acoustic
signal by synchronizing at N.sub.f frequencies and N.sub.br bit
rates in parallel. The microcontroller 52 stores a plurality of
predetermined synchronization frames that are calculated using
different combinations of N.sub.f and N.sub.br. During the full
parallel synchronization state 150, combinations of
N.sub.f*N.sub.br correlation coefficients are calculated in real
time. The normalized correlation coefficients typically have a
range between 0 and 1.
[0080] Assuming that an acoustic signal is received, the receiver
electronics 38 synchronizes at the frequency and bit rate which
give the maximum value for the correlation coefficient to estimate
the carrier frequency and bit rate of the acoustic signal. In
particular, the receiver electronics 38 is directed to monitor the
correlation coefficients and wait for a maximum of the correlation
coefficients. The time instant of this maximum, called
synchronization time, corresponds to the time of the last symbol of
the synchronization frame. The output of the full parallel
synchronization state 150 is preferably (1) a start time of a data
frame of the acoustic signal, (2) an estimated frequency of the
acoustic signal, and (3) an estimated bit rate of the acoustic
signal.
[0081] The start time of the data frame of the acoustic signal, the
estimated frequency of the acoustic signal, and the estimated bit
rate of the acoustic signal are provided to the decoding state 152,
which decodes the data frame of the acoustic signal.
Reduced-Parallel Synchronization
[0082] Shown in FIG. 7 is another receiver architecture which
includes a training phase 200 and a data communication phase 202,
which can be implemented as separate threads, for example. During
the training phase 200, the receiver electronics 38 is adapted to
receive and decode a training message 204 having an unknown carrier
frequency and bit rate, which is shown by way of example in FIG.
6A. During the data communication phase 202, the receiver
electronics 38 is adapted to receive and decode a data message 208
having a known carrier frequency but unknown bit rate, which is
shown by way of example in FIG. 6B.
[0083] The training message 204 utilized in embodiments of the
present disclosure has been modulated using an available carrier
frequency and bit rate and encoded with data using a suitable
encoding scheme as discussed above. In general, the training
message 204 is generally composed of at least three parts. The
first part is called a synchronization frame 210 and is composed of
a number of symbols N.sub.s. The second part is called a data frame
212 and is composed of a number of data symbols N.sub.d.
[0084] The training message 204 also includes an estimation frame
214 to permit the receiver electronics to estimate the carrier
frequency by analyzing the estimation frame 214 of the training
message 204. The estimation frame 214 is prior to the
synchronization frame 210 and preferably includes a sinusoid at the
same frequency as the carrier frequency of the remaining part of
the modulated signal. The length of the estimation frame 214 ranges
typically from a few hundreds of milliseconds to a few seconds
depending upon the targeted performance of the modulation stage.
The estimation frame 214 having a sinusoid at the same frequency as
the carrier frequency may be referred to herein as a "sine
prefix".
[0085] As shown in FIG. 6B, the data message 208 includes the
synchronization frame 210 and the data frame 212, but does not
include the estimation frame 214 since the carrier frequency of the
data message 208 is known. As discussed herein, the estimation
frame 214 can be omitted once the acoustic modems 25Mi-2, 25Mi-1,
25M and 25Mi+1 have entered the data communication phase 202.
[0086] FIG. 7 is a logic flow diagram of the receiver electronics
38 constructed in accordance with the presently disclosed and
claimed embodiments. In this version, the acoustic modems 25Mi-2,
25Mi-1, 25M and 25Mi+1 include the training phase 200 and the data
communication phase 202. The training phase 200 is typically
executed prior to the data communication phase 202 for initializing
the acoustic modems 25Mi-2, 25Mi-1, 25M and 25Mi+1 to communicate
with each other. During the training phase 200, the training
message 204 includes the estimation frame 214 (as shown in FIG. 6A)
to permit the microcontroller 52 to estimate the carrier frequency
by analyzing the estimation frame 214 of the training message 146.
The estimation frame 214 is prior to the synchronization frame 210
and preferably defines the sine prefix, although signals other than
a sinusoid can be used. During the training phase 200 the length of
the estimation frame 214 can be increased to also increase the
robustness of the process.
[0087] In this version, the receiver electronics 38 is programmed
to include at least three states in the training phase 200, i.e., a
frequency estimation state 220, a reduced parallel synchronization
state 222 and a decoding state 224. The frequency estimation state
220 analyzes the estimation frame 214 to detect whether the
frequency of the estimation frame 214 includes a carrier frequency
within a predetermined set of N.sub.f carrier frequencies. The
frequency estimation state 220 can accomplish this in any suitable
manner, such as by using signal processing tools such as fast
Fourier transforms, moving averages and/or filterings and then
comparing the output of the signal processing tools to the
predetermined set of carrier frequencies N.sub.f.
[0088] When an acoustic signal of a predetermined set of N.sub.f
frequencies has been detected; the frequency estimation state 220
branches to the reduced parallel synchronization state 222 at the
detected carrier frequency. The reduced parallel synchronization
state 222 synchronizes in parallel at the detected carrier
frequency and at at least two bit rates. The output of the reduced
parallel synchronization state 222 is the signal bit rate in
addition to the start time of the data frame. The reduced parallel
synchronization state 222 can be implemented by calculating the
correlation coefficients and looking for a maximum as discussed
above.
[0089] After synchronization, the receiver electronics 38 branches
into the decoding state 224 and decodes the data frame 212 using
the detected frequency and the detected bit rate. The decoding
state 224 is similar to the decoding states discussed above. If the
decoding state 224 successfully decodes the data frame 212 to a
particular quality level, then an acknowledgement is transmitted to
the transmitter electronics 36 and the carrier frequency and the
bit rate is stored for future communication during the data
communication phase 202. If the decoding state is unsuccessful,
then an acknowledgement is preferably not sent and the transmitter
electronics 36 sends out another acoustic signal having a different
combination of carrier frequency and bit rate as discussed above.
This process then repeats for each set of acoustic modems 25Mi-2,
25Mi-1, 25M or 25Mi+1 to be initialized on the tubing.
[0090] Once the acoustic modems 25Mi-2, 25Mi-1, 25M and 25Mi+1 have
been initialized, the data communication phase 202 is actuated as
shown in FIG. 7. The data communication phase 202 is preferably
implemented as the reduced parallel synchronization state 222 and
the decoding state 224. In the data communication phase 202, the
transmitter electronics 36 of the transmitter is adapted to output
data messages 208 as shown in FIG. 6B. The data messages 208
include the synchronization frame 210 and the data frame 212 as
discussed above. In particular, the data message 208 preferably
does not include the estimation frame 214 of the training message
204 since the carrier frequencies are already known. In one
embodiment, the reduced parallel synchronization state 222
calculates multiple correlation coefficients using a same carrier
frequency and multiple bit rates simultaneously to determine the
bit rate of the acoustic signal although other synchronization
methodologies could be used. When an acoustic signal is received at
this frequency, and if the synchronization is successful, the data
communication phase 202 goes into the decoding state 224 at the
same carrier frequency and at the detected bit rate.
[0091] The training phase 200 and the data communication phase 202
are used for different purposes. In particular, assuming that the
transmitter electronics 36 of one of the acoustic modems 25Mi-2,
25Mi-1, 25M and 25Mi+1 is transmitting data to the receiver
electronics 38 of another one of the acoustic modems 25Mi-2,
25Mi-1, 25M and 25Mi+1. The transmitter electronics 36 first
selects the communication frequency and bit rate to communicate
between the transmitter electronics 36 and the receiver electronics
38. Then, the transmitter electronics 36 initiates the training
phase 200 such as the one described above. The receiver electronics
38 uses the training phase 200 to select a proper frequency and bit
rate by estimating the frequency and bit rate of the incoming
training messages 204, demodulating the training messages 204 and
sending an acknowledgement message upon successful demodulation.
After a proper frequency and bit rate have been selected, the
transmitter electronics 36 and/or the receiver electronics 38
initiates the data communication phase 202. Subsequently, the
transmitter electronics 36 then transmits data to the receiver
electronics through data messages 208 at the selected frequency
and/or bit rate. Since the carrier frequency is set, the receiver
electronics 38 uses the data communication phase 202 to demodulate
incoming data signals at the selected frequency.
[0092] Assuming the channel is time-constant, the training phase
200 is preferably only performed once at the beginning of the drill
stem test DST job, for example. Though it cannot be too long, its
time duration may not be critical for the overall performance of
the system. On the contrary, the training phase 200 is preferably
robust. The robustness of the frequency estimation state 220 can be
increased by lengthening the size of the estimation frame 214.
However, in the data communication phase 202, the estimation frame
214 is preferably omitted in the data messages 208 to enhance the
data communication rate between the transmitter electronics 36 and
the receiver electronics 38 and also because there may not be any
requirement for frequency detection. Thus, in a preferred
embodiment, during the training phase 200, the transmitter
electronics 36 generates the training message 204 with the
estimation frame 214, and during the data communication phase 202,
the transmitter electronics generates the data message 208 without
the estimation frame 214.
[0093] The complexity of the receiver electronics 38 is the
complexity of the most complex state and the complexity determines
how much processing power and battery power is needed to optimize
the bit rate by using the highest possible bit rate. As will be
discussed below, the reduced parallel synchronization state 222 is
much less complex than the full parallel synchronization state 150.
In particular, during the reduced parallel synchronization state
222, N.sub.br correlation coefficients are calculated
simultaneously. The complexity of this state is therefore
N*N.sub.br where N stands for the assumed complexity of the
synchronization state. Since this is the most complex state, the
complexity of the transmitter electronics 36 and the receiver
electronics 38 is N*N.sub.br where N stands for the assumed
complexity of the synchronization state at one frequency and bit
rate. Assuming there are 10 possible carrier frequencies, and two
possible bit rates, the reduced parallel synchronization state 222
has a complexity of 2N. The full-parallel synchronization state
150, on the other hand simultaneously attempts to synchronize in
parallel on all possible bit rates and all possible carrier
frequencies resulting in a complexity of N*N.sub.br.=20N. Thus, the
receiver electronics 38 utilizing the reduced parallel
synchronization state 222 is less complex by a factor of 10 thereby
utilizing less processing power and battery power once the receiver
electronics 38 has entered the data communication phase 202.
Prediction of Optimal Communication Frequencies
[0094] In another aspect, the efficiency of the transmitter
electronics 36 in selecting an appropriate carrier frequency can be
improved by using a pipe communication algorithm running on a
computer system 288 shown by way of example in FIG. 8. The pipe
communication algorithm receives and analyzes a priori information
for a particular, planned tubing 14 in order to predict the optimal
communication frequencies to be used by the acoustic modems 25Mi+2,
25Mi+1, 25M and 25Mi-1 once installed on the tubing 14 and during
the DST operations. In general, a priori information, as used
herein, is prior knowledge about the tubing 14, such as an average
pipe section length, a statistical distribution of pipe length, or
planned DST operations which may affect an acoustic channel formed
by the tubing 14 in a predictable way. Using the a priori
information, the presently disclosed concepts produce an acoustic
transfer function 290 (an example of which is shown in FIG. 10) of
the tubing 14 to determine communication frequencies and expected
attenuation within band pass zones of the acoustic transfer
function 290, for example. The identification of communication
frequencies within the band pass zones, as well as the expected
attenuation can be used in selecting optimal carrier frequencies
and their order. The transmitter electronics 36 can then be
programmed with, and/or utilize the optimal carrier frequencies in
selecting the order of carrier frequencies within the training
phases discussed above for full parallel or reduced parallel
synchronization, thereby reducing the time and energy needed for
selecting an appropriate carrier frequency.
[0095] The computer system 288 is provided with one or more
processor 292, one or more non-transitory computer readable medium
294, one or more output device 296, one or more input device 297,
and one or more communication device 298, and one or more power
supply 299.
[0096] The one or more non-transitory computer readable medium 294
can be implemented in a variety of ways, such as a random access
memory, a read-only memory, an EEPROM, an optical disk, a hard
drive, or the like. In general, the non-transitory computer
readable medium 294 stores the pipe communication algorithm and
also stores the a priori information with respect to the tubing 14,
such that the pipe communication algorithm can be executed by the
processor 292 to produce and/or store the acoustic transfer
function 290 as discussed above.
[0097] The one or more processor 292 will be referred to
hereinafter as "the processor 292" for purposes of brevity, however
it should be understood that the processor 292 can be implemented
as a single processor or multiple processors working independently
or together to perform the functions discussed herein. The
processor 292 can be implemented as one or more microcontroller,
central processing unit, digital signal processor, field
programmable gate array, or the like.
[0098] The input device 297 can be used by a person or operator to
input data and/or control the functioning of the processor 292.
Exemplary input devices 297 include a keyboard, mouse, trackball,
touchscreen, USB port, optical drive, or the like. The output
device 296 serves to convey information to the person or operator
(as well as others) regarding the operations of the processor 292.
Exemplary output devices 296 include a monitor, a printer, USB
port, optical drive or the like.
[0099] The communication device 298 serves to permit and establish
communication between the computer system 288 and one or more
devices and/or computer systems, such as the transmitter
electronics 36, which are external to the computer system 288.
Exemplary communication devices 298 include a serial device, an
Ethernet board or network connection, or the like.
[0100] The one or more power supply 299 is connectable to an AC
and/or a DC source and serves to provide suitable power to the
components of the computer system 288, such as the computer
readable medium 294, the output device 296, the processor 292, the
communication device 298 and the input device 297. Of course, some
of the components of the of the computer system 288 may have their
own independent power supply 299, and some may be shared. For
example, if the computer system 288 is implemented as a tablet
computer with an internal monitor (output device 296) and an
external monitor (output device 296), then the internal and
external monitors will typically have different power supplies 299.
The power supplies can be implemented in a variety of manners such
as a switching power supply, a battery or combinations thereof.
[0101] As will be discussed in more detail below, the pipe
communication algorithm is stored on the computer readable medium
294 utilizing the input device 297 or the communication device 298
and installed onto the computer system 288 typically with the aid
of an installation program running on the processor 292. In any
event, once the pipe communication algorithm is installed onto the
computer system 288, a priori information can be received by the
processor 292 through the input device 297 and/or the communication
device 298 and then analyzed with the pipe communication algorithm
to generate the acoustic transfer function 290.
[0102] The computer system 288 can be implemented in a variety of
manners such as a personal computer, a mainframe computer, a
distributed processor computer system, a mainframe computer, a
tablet computer, or the like. The pipe communication algorithm can
be run by the computer system 288 had any suitable location, such
as at a headquarters or research center, or at the location of the
tubing 14.
[0103] Referring now to FIG. 9, shown therein is an exemplary
section of the tubing 14 formed of multiple pipe sections 300a,
300b, 300c, 300d, 300e and 300f. The pipe sections 300a-f are
substantially identical in construction and function and so only
one of the pipe sections 300a will be described herein. In
particular, the pipe section 300a, for example, is provided with a
box end 302 and a pin end 304. The box end 302 includes internal
threads (not shown) and the pin end 304 includes external threads
(not shown). The pipe sections 300a-f are connected together by
threading the external threads of the pin end 304 of one pipe
section 300a-f into the internal threads of the box end 302 of
another pipe section 300a-f. As used herein, a pipe length 306 is
defined as a distance between the box ends 302 of adjacently
disposed and connected pipe sections 300a-f.
[0104] When two of the acoustic modems 25Mi+2 and 25Mi+1, for
example, are located on the tubing 14, the first step to establish
communication is to find a frequency of modulation which enables
such communication. As discussed above, the receiver electronics 38
of the acoustic modems 25Mi+2, 25Mi+1, 25M, or 25Mi-1 can have an
identical set (typically 10) of predefined carrier frequencies
stored in each acoustic modem 25Mi+2, 25Mi+1, 25M, or 25Mi-1 to be
used for synchronizing with the acoustic messages. Acoustic modem
25Mi+2, for example, first broadcasts the acoustic message at
frequency #1. If the receiver electronics 38 of the acoustic modem
25Mi+1 detects such acoustic message, the transmitter electronics
36 of the acoustic modem 25Mi+1 preferably replies to the receiver
electronics 38 of acoustic modem 25Mi+2 at the same frequency which
is then chosen as the carrier frequency between the acoustic modems
25Mi+2 and 25Mi+1. If the acoustic modem 25Mi+1 does not detect the
acoustic message, acoustic modem 25Mi+2 broadcasts another acoustic
message at frequency #2, and so on, until acoustic modem 25Mi+1
answers.
[0105] If the set of predefined frequencies is randomly chosen,
then the process of finding a possible frequency of communication
can be lengthy, even between two acoustic modems 25Mi+1 and 25Mi+2,
for example. In a typical downhole installation, between 10 to 20
acoustic modems 25 will be located on the tubing 14 and spaced
between 1000-2000 ft. Establishing the full network between surface
and bottom can then last up to hours, which can be detrimental to
the efficiency of operations, especially if the network fails to
operate in a given configuration: The full network has then to be
rediscovered, with a noticeable interruption in data
communication.
[0106] To reduce the time to establish the network, it is proposed
to select the set of predefined frequencies, based on a priori
knowledge available at the beginning of the operations. The a
priori knowledge can include the following:
A pipe tally including information indicative of the pipe sections
300 to be used to create the tubing 14 including, for example, an
average pipe length, and thus an estimation of the location of the
band pass. A pipe length standard deviation, and thus an estimation
of the attenuation associated with the band pass (because
attenuation is frequency dependant, it favors the choice of lower
frequencies). Knowledge of operations to be performed during the
DST job: For example, replacing the water/brine originally present
inside the pipes with lighter fluids such as gas or oil slightly
reduces the frequency location of the band pass. Knowledge of an
acoustic transducer response: If the transducer is resonant, then
the frequency of operation should be biased around this resonance
frequency. Experimental knowledge on the expected noise level: Flow
induced noise is frequency dependant, with decreasing amplitude at
higher frequencies. This effect could favor higher frequencies of
operations.
[0107] This a priori information is input into the computer system
288 to produce an expected signal to noise ratio on the acoustic
modem 25Mi+2, 25Mi+1, 25M, or 25Mi-1 versus frequency, and a list
of expected preferred carrier frequencies based on this ratio, and
to be written into each acoustic modem 25Mi+2, 25Mi+1, 25M, or
25Mi-1 before the start of the job. This list should preferably be
ordered such that the first frequency has the highest probability
of successful communication, again to minimize the time of network
discovery.
[0108] The tubing 14 formed of pipe sections 300a-f with equal pipe
lengths 306 is a periodic medium. The pipe sections 300a-f, for
example, are connected together via the pin end 304 and the box end
302. The cross-section of the box end 302 is larger than the
cross-section of the remainder of the pipe section 300a. The
acoustic message travelling along the tubing 14 is partially
reflected at each cross section change and these periodic
reflections generate a transfer function characterized by band pass
zones 320a-d (only four of which are labeled in FIG. 10 for
purposes of clarity), and band stops 324a-d (only four of which are
labeled in FIG. 10 for purposes of clarity) in the frequency domain
as shown in FIG. 10. For purposes of example, it is assumed that
propagation at frequencies within the band pass 320a-d is un
attenuated and desirable for communication, while propagation at
frequencies within a band stop 324a-d is severely attenuated and
must be avoided for successful communication over a significant
distance. One of the main challenges of acoustic communication is
selecting the proper frequency of communication for a reliable
operation.
[0109] The center frequency (.about.270 Hz) of the first band stop
324a is such that the half wavelength .lamda./2 is equal to the
pipe length 306 (.about.10 m), and higher order band stops
frequencies are multiples of this value. The frequency width of the
band pass 320a-d depends upon the order of the band stop 320a-d,
and decreases with the frequency in the range 0-5 kHz. The upper
and lower frequency bounds of each band pass 320a-d can be
predicted from the knowledge of the pipe and connection
characteristics (lengths and cross sections) and the acoustic
propagation velocity in the tubing 14. A strong reflection
coefficient at the pipe/connection boundaries, typical of a drill
pipe, gives a rather narrow band pass.
[0110] The following is an exemplary equation for predicting the
frequency location of the band stops:
g=cos(.omega.L.sub.1/c)cos(.omega.L.sub.2/c)-0.5(S.sub.1/S.sub.2+S.sub.2-
/S.sub.1)sin(.omega.L.sub.1/ c)sin(.omega.L.sub.2/c)>1
[0111] Where: L.sub.1 is the pipe length; L.sub.2 is a length of
the box end 302; S.sub.1 is a cross section area of the pipe
section; and S.sub.2 is a cross-section area of the box end 302, c
is a velocity of the acoustic signals propagating through steel,
i.e., 5210 m/s, and .omega. is 2.pi.f, where f is the
frequency.
[0112] FIG. 12 shows the function g versus frequency, for a
particular case with L.sub.1 equal to 354 inches; L.sub.2 equal to
16 inches, S.sub.1 equal to 3.7 inches.sup.2; and S.sub.2 equal to
8.2 inches.sup.2. Shaded sections within FIG. 12 show locations of
band stops 324a, 324b, 324c, 324d, 324e, 324f, and 324g. For
example, the function g is larger than 1 between 794 and 852 Hz,
which can be identified as the band stop 324c.
[0113] The intended acoustic mode of propagation is the extensional
mode in the tubing 14, and its velocity is predominantly controlled
by the steel material properties (bulk longitudinal and shear
velocities and density), which are reasonably known and constant.
However, the extensional velocity is also slightly affected by the
fluids present inside and outside the tubing 14. The fluids
(especially inside) are expected to change during DST operations:
typically water/brine will be replaced by formation fluids (oil or
gas), or by nitrogen pumped from surface in order to reduce the
downhole pressure prior to open the testing valve. These fluid
substitutions slightly changes the boundaries of the band pass
320a-d, in a predictable way, once approximate values of their
properties (acoustic velocity and density) are known.
[0114] Actual pipe sections 300a-f are not strictly identical in
lengths, and this is especially the case for drill pipe which can
be repaired by cutting part of the connection to re-machine the
threads. When the pipe lengths 306 of the pipe sections 300a-f in
the tubing 14 are not equal, then it can be shown that this
introduces attenuation within the band pass 320a-d as shown in FIG.
11. FIG. 11 show simulations for the attenuation across 30 pipe
sections 300. Shown in the solid lines is the simulation with pipe
sections 300 of identical length, and shown in the dotted lines is
the simulation with pipe sections 300 of non-identical lengths, but
with a standard deviation of 0.15 m. As shown in FIG. 11, the
higher the standard deviation of pipe lengths 306, the stronger the
attenuation as the carrier frequency and the overall effect is to
restrict the range of possible communication frequencies to lower
frequencies.
[0115] Although only a few embodiments of the present invention
have been described in detail above, those of ordinary skill in the
art will readily appreciate that many modifications are possible
without materially departing from the teachings of the present
invention. Accordingly, such modifications are intended to be
included within the scope of the present invention as defined in
the claims.
* * * * *