Note: Descriptions are shown in the official language in which they were submitted.
CA 02692754 2010-01-06
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NOISE CANCELLATION IN WELLBORE SYSTEM
Ingolf Wassermann & Jose I. Alonso Ortiz
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to U.S. provisional application,
60/949,595, filed July 13, 2007, the entire contents of which are incorporated
herein
by reference.
BACKGROUND OF THE INVENTION
[0002] In mud pulse telemetry data from down hole is transmitted to surface
using pressure pulses generated by a pulser. Pressure sensors at surface
measure the
pressure changes over time that represent the data.
[0003] The pulser, however, is not the only source of pressure changes in a
well. Other pressure varying sources, such as pumps, for example, which
circulate
mud within the well, also generate pressure changes. These pressure changes
act as
noise for the mud pulse transmission. In fact, pumps are generally also
located at
surface and are therefore closer to the pressure sensors and, as such, are a
dominant
source of noise. Signals received by the sensors that are generated by such
pumps
typically have higher energy than do the received telemetric data signals
since the
telemetric signals get highly attenuated while traveling from down hole to
surface. As
such, accurate detection of the telemetric data signals can be difficult.
Methods to
allow for accurate detection of the telemetric data signals in the presence of
pump
noise would be well received in the art.
BRIEF DESCRIPTION OF THE INVENTION
[0004] Disclosed herein is a method of canceling noise in a wellbore telemetry
system. The method includes, acquiring at least one signal in the system,
predicting
at least one deterministic component of the at least one signal based upon a
change of
at least one deterministic component from past signal values, and subtracting
the at
least one predicted deterministic component from the acquired at least one
signal.
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[0005] Further disclosed herein is a processor readable media having stored
thereon a processor program product that is executed by the processor, the
processor
program product for canceling noise in a wellbore telemetry system in a
processor
environment, the processor program product comprising a storage medium
readable
by a processing circuit and storing instructions for execution by the
processing circuit
the processor readable media being readable by a processing circuit and the
processor
program product having instructions for execution by the processing circuit
for
facilitating a method. The method includes, acquiring at least one signal in
the
system, predicting at least one deterministic component of the at least one
signal
based upon a change of at least one deterministic component from past signal
values,
and subtracting the at least one predicted deterministic component from the
acquired
at least one signal.
[0006] Further disclosed herein is a method of canceling noise in a downhole
telemetry system. The method includes, acquiring at least one signal in the
system,
predicting at least one deterministic component of the at least one signal
based upon
one of at least one deterministic component from a past signal values and a
change of
at least one deterministic component from past signal values, and subtracting
the at
least one predicted deterministic component from the acquired at least one
signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007) The following descriptions should not be considered limiting in any
way. With reference to the accompanying drawings, like elements are numbered
alike:
[0008] FIG. 1 depicts a spectrogram of signals received by a sensor in a mud
pulse telemetry system;
[0009] FIG. 2 depicts a spectrogram that has had received pump signals
canceled by methods disclosed herein; and
[0010] FIG. 3 depicts a linear prediction algorithm disclosed herein.
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DETAILED DESCRIPTION OF THE INVENTION
[0011] A detailed description of one or more embodiments of the disclosed
apparatus and method are presented herein by way of exemplification and not
limitation with reference to the Figures.
[0012] Embodiments of the system disclosed herein are used during downhole
measurement and telemetric communication such as the techniques of logging
while
drilling (LWD) and measurement while drilling (MWD), for example. It should be
understood that although embodiments disclosed herein describe a system with
telemetry from downhole to surface, alternate embodiments could include
telemetry
from surface to downhole as well as bidirectional telemetry. Additionally,
embodiment may include telemetry between any desired positions within a well,
such
as, between a first downhole position and a second downhole position, for
example.
Referring to FIG. 1, a spectrogram 10 of a signal received from a sensor is
illustrated.
The spectrogram 10 is a plot of frequency 14 in hertz on the Y-axis versus
time 18 in
seconds on the X-axis. The spectrogram 10 is made such that the higher the
energy of
the received signal at a given frequency the darker the representation on the
spectrogram 10. The darkest areas, therefore, represent frequencies with high
magnitudes of pressure pulsing. By contrast, the lightest areas represent
frequencies
with the lowest magnitude of pressure pulsing.
[0013] Knowing some specifics about the frequency of the pulses being
generated by the pulser and when the pulser is actually transmitting
telemetric data
helps to identify transmitted telemetric data signal 22 on the spectrogram 10.
In the
example of FIG. 1, the pulser began transmitting telemetric data at the 40-
second
mark 26 and stopped transmitting telemetric data at the 250-second mark 30. In
this
example the majority of the telemetric data is transmitted at a frequency of
between
and 40 hertz. As such the telemetric data signal 22 can be identified by the
dark
area 34 positioned between the 40-second mark 26 and the 250-second mark 30 on
the
X-axis, and between the 30 hertz and the 40 hertz frequencies on the Y-axis.
[00141 The spectrogram 10 also includes several dark horizontal lines 38.
30 These dark horizontal lines 38 represent noise from one or more pumps. The
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frequencies of the pumps' noise correlate with frequencies of operation of the
pumps
themselves. As such the pumps' noise is periodic and thus deterministic in
nature
with the periodicity being proportional to the frequency of operation of the
pumps. It
should be noted that although the periodic noise described in embodiments
herein is
pump noise, in alternate embodiments the periodic noise could be from other
sources.
Such sources include, mud motors, rotating bits, rotating drillstrings,
reciprocating
members and pulsing members, for example. In such alternate embodiment the
periodic noise could be attributed to these alternate sources of periodic
noise.
[0015] The spectrogram 10 also includes random dark speckles 42 that are
from other undefined sources of noise. Such other noises can occur at various
frequencies, various magnitudes and at various times. The magnitude of signals
from
these other noises may be low enough in the frequency range of the transmitted
telemetric data signa122 such that the data transmitted in the telemetric data
signal 22
is legible above the other noises.
[0016] The strength of the pump noise, being greater than the strength of the
telemetric data signal 22 complicates deciphering the telemetric data signal
22 from
the pump noise. It is, therefore, desirable to attenuate the magnitude of the
pump
noise to levels below that of the data. An embodiment of the invention uses
the
deterministic nature of the pump noise to attenuate the effect of the pump
noise. This
embodiment performs such attenuation even while the pump frequencies are
changing
over time due to changes in the operational frequency of the pump, for
example.
Such changes in pump noise may include changes in periodicity or changes in
the
shape of the periodic signal.
[0017] Embodiments of the invention measure pressure in a well with at least
one sensor. It should be noted, however, that alternate embodiments could use
sensors that measure parameters other than pressure. Sensors that measure
flow,
magnetic forces or gravitational force, for example, could also be used with
the
disclosed system. The measured pressure signal is analyzed and some periodic
signals are attributed to pump noise. The telemetric signal is assumed to be
of non-
periodic (stochastic) nature. If that is not the case for a significant time
frame, data
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scrambling techniques may be applied. The periodic pump signals are analyzed
for
changes that are occurring over time. The changes occurring over time are used
to
predict values of the periodic signals at a future point in time. Then, at
that future
point in time, the predicted values of the periodic signals are subtracted
from the
pressure signals being measured. The remaining pressure signals are then
analyzed to
read the telemetric data transmitted. Algorithms to determine a periodic
signal and
then subtract the predicted signal from a signal in real time are called
"Linear
Prediction" and are described in several textbooks including "Adaptive Filter
Theory"
by Simon Haykin, published by Prentice Hall.
[0018] Referring to FIG. 2, a second spectrogram 50 disclosed herein is
illustrated. The second spectrogram 50 corresponds to a received signal that
has been
modified by subtraction of predicted periodic noise, including noise
attributed to
pumps, as disclosed herein. The dark horizontal lines 38, of the periodic
noise, as
shown in FIG. 1 are substantially eliminated. A dark area 54 representative of
a
telemetric data signal 58 is easily observable.
[0019] Embodiments of the invention utilize mathematical algorithms in the
analysis, prediction and subtraction of the periodic signals. For example, the
received
signal r(k) can be expressed as the superposition of the telemetric signal
sT(k) with the
periodic signal sPN(k) and other noise n(k).
r(k)=s,.(k)+sPr,(k)+n(k)
The periodic noise signal has a periodicity and signal shape that may be
changing
slowly over time. Therefore it can be predicted from its previous values sPN(k-
i)
except its variations. This can be done by a prediction filter hPre,a(k),
which has to be
adaptive to track variations of the periodic signal.
sr>N (k) = hp~ed (k) * sP,v (k - z)
Subtracting the predicted periodic signal sP,(k) from the newest received
signal, we
get the telemetric signal distorted by noise and a pump noise prediction error
ePN(k).
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r(k)-s,N(k)=sr(k)+ePN(k)+n(k)=e(k)
This signal is used to control the adaptation of the prediction filter in a
way that the
mean squared error E{je(k~2} of e(k) is minimal, which is only achievable by
minimization of ePN(k) since the other signals are not predictable. If the
mean squared
error is minimal, e(k)equals the telemetric signal plus minimum remaining
noise.
Since, however, the pump signal spN (k - z) is not directly available for
prediction, the
following may be used:
r(k-z)=sT(k-t)+sPr,(k-z)+n(k-z)
which is the pump signal sPN(k-i) distorted by the weak telemetric signal sT(k-
i) and
noise n(k-i).
[0020] Referring to FIG. 3, a linear prediction algorithm 62 in accordance
with an embodiment of the invention is illustrated.
[0021] As described above, embodiments may be in the form of processor-
implemented processes and apparatuses for practicing those processes. In
exemplary
embodiments, the invention is embodied in processor program code. Embodiments
include processor program code containing instructions embodied in tangible
media,
such as floppy diskettes, CD-ROMs, hard drives, or any other processor-
readable
storage medium, wherein, when the processor program code is loaded into and
executed by a processor, the processor becomes an apparatus for practicing the
invention. Embodiments include processor program code, for example, whether
stored in a storage medium, loaded into and/or executed by a processor, or
transmitted
over some transmission medium, such as over electrical wiring or cabling,
through
fiber optics, or via electromagnetic radiation, wherein, when the processor
program
code is loaded into and executed by a processor, the processor becomes an
apparatus
for practicing the invention. The technical effect of the executable
instructions is to
cancel a pump signal received in a mud pulse telemetry system through analysis
of
data received by a single sensor.
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[00221 While the invention has been described with reference to an exemplary
embodiment or embodiments, it will be understood by those skilled in the art
that
various changes may be made and equivalents may be substituted for elements
thereof
without departing from the scope of the invention. In addition, many
modifications
may be made to adapt a particular situation or material to the teachings of
the
invention without departing from the essential scope thereof. Therefore, it is
intended
that the invention not be limited to the particular embodiment disclosed as
the best
mode contemplated for carrying out this invention, but that the invention will
include
all embodiments falling within the scope of the claim
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