Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND SYSTEM FOR ENHANCING THE SPATIAL RESOLUTION OF
A FIBER OPTICAL DISTRIBUTED ACOUSTIC SENSING ASSEMBLY
BACKGROUND OF THE INVENTION
The invention relates to a method and system for
enhancing the spatial resolution of a fiber optical
Distributed Acoustic Sensing (DAS) assembly.
International patent application W02007/049004
discloses a Distributed Acoustic Sensing (DAS)assembly
for sensing and monitoring traffic along several
kilometers of the length of a road by means of an fiber
optical cable buried alongside the road. In the known DAS
assembly a series of light pulses are transmitted through
the fiber optical cable by a light transmission and
receiving assembly arranged at or near one end of the
cable and back reflections of the transmitted light
pulses are received by means of an interrogator assembly
arranged at or near said end.
Utilising an optical fiber as an acoustic or
vibration sensor can be achieved in a number of ways.
One method is to launch a pulse of coherent laser light
into a fiber. As the pulse travels through the fiber
imperfections in the crystal lattice making up the fiber
cause light to be reflected back along the fiber and
dispersed out of the fiber. Under normal conditions, say
for communications purposes, these back reflections are
loss terms. However, the nature of the reflection causing
imperfections are a function of the strain state of the
fiber and as such by measuring the intensity of the back
reflections and with multiple pulses it is possible to
determine the strain state of the fiber as this varies
temporally. Therefore an acoustic or vibration source
which changed the strain state of the fiber could be
measured using the back reflection data.
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The launched laser pulse is precisely timed such that
it's length in the fiber is known (10m is a possible
value for the pulse length). Once the pulse is launched
the back reflections are measured. The measurement is
made with a photodetector, which forms part of a light
pulse transmission and receiving assembly and which
integrates or adds up the number of photons received in a
time period giving a figure relating to the total
intensity of back reflected light. The time period can be
matched to the laser pulse length and by using multiple
contiguous readings will provide a measurement of how the
back reflected light varies over the length of the
optical fiber. Further by launching laser pulses in close
succession and at a fixed rate (for example about 10000
pulses per second) a discretized representation of the
change in strain state of the optical fiber as a function
of both time and space can be achieved.
It is possible to reduce the length of the laser
pulse to 5m in the fiber. This also allows the spatial
resolution to be improved to a 5m channel spacing.
However, the pulse length reduction causes a linear
reduction in the energy (half the length = half the
energy), which in turn reduces the level of back
reflected light and leads to a worsening of the Signal to
Noise Ratio(SNR) and therefore sensitivity of the system.
There is a need to provide an improved Distributed
Acoustic Sensing (DAS) method and assembly with enhanced
spatial resolution, which does not reduce the level of
back reflected light, the Signal to Noise Ratio (SNR)
and/or sensitivity of the DAS method and assembly.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a
method for enhancing the spatial resolution of a fiber
optical distributed acoustic sensing (DAS)assembly, the
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method comprising:
- configuring an optical fiber comprising a series of
contiguous channels in a U-shaped loop such that the
fiber comprises substantially parallel fiber sections
with pairs of channels that are arranged at least
partially side by side;
- inducing a light transmission and receiving assembly to
transmit a series of light pulses through the optical
fiber and to receive back reflections of the transmitted
light pulses reflected by each of the channels; and
- processing the received back reflections such that back
reflections stemming from at least one pair of channels
that are arranged at least partially side by side are
correlated to each other.
When used in this specification and claims the term
"series of contiguous channels" means that these channels
form a succession of fiber segments that are sensitive to
acoustic signals or vibration.
Optionally the light transmission and receiving
assembly:
- transmits a series of light pulses into the fiber,
which pulses have each substantially the same duration,
such that a length span of each pulse along the length of
the fiber is known; and
- measures on the basis of time of flight measurement
back reflections stemming from each of the channels,
which have substantially the same length as the length
span of each of the light pulses.
Preferably the channels are arranged along the length
of the fiber such that a first channel begins at or near
the light transmission and receiving assembly and the U-
shaped loop has a mid-point which is located at a
distance from with an interface between a pair of
adjacent channels and at a distance from a mid-point of a
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channel, thereby causing the pairs of channels to be
partially side by side and to be staggered relative to
each other.
It will be understood that if a U-shaped loop has a
mid-point that is located at a distance from an interface
between a pair of contiguous channels and from a mid-
point of a channel, this implies that the mid-point of
the U-shaped loop does not coincide with said interface
and mid-point such that pairs of channels that are
arranged staggered and only partially side by side. The
percentage of overlap of such pairs of channels may vary
between 1 and 99%.
In accordance with the invention there is further
provided a system for enhancing the spatial resolution of
a fiber optical distributed acoustic sensing (DAS)
assembly, the system comprising:
- an optical fiber comprising a series of contiguous
channels, which fiber is arranged in a U-shaped loop
configuration, such that the fiber comprises
substantially parallel fiber sections with channels that
are arranged at least partially side by side;
- a light transmission and receiving assembly for
transmitting a series of light pulses through the optical
fiber and for receiving back reflections of the
transmitted light pulses reflected by each of the
channels; and
- means for processing the received back reflections such
that back reflections stemming from at least one pair of
channels that are arranged at least partially side by
side are correlated to each other.
These and other features, embodiments and advantages
of the method and/or system according to the invention
are described in the accompanying claims, abstract and
the following detailed description of non-limiting
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embodiments depicted in the accompanying drawings, in
which description reference numerals are used which refer
to corresponding reference numerals that are depicted in
the drawings.
5 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a looped DAS assembly according to the
invention;
Figure 2 shows a prior art DAS assembly with an
optical fiber suspended in a single run within a
wellbore;
Figures 3-5 show various alternative embodiments of
DAS assemblies with a looped optical fiber within a
wellbore according to the invention;
Figures 6A-D shows how optical signal back
reflections obtained from staggered channels are combined
to enhance the resolution of the DAS assembly shown in
Figure 5;
Figures 7-9 show various other embodiments of looped
DAS assemblies according to the invention.
DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS
The method and system according to the present
invention improve the spatial resolution of a fiber
optical Distributed Acoustic Sensing (DAS) assembly
without needing to reduce the length of the launched
laser pulse.
In Figures 1-10 similar features are identified by
similar reference numerals.
Figure 1 shows a DAS assembly according to the invention,
which is based on the insight that one or more loops of
fiber 1 are more effective than the conventional single
fiber 1 arrangement shown in Figure 2.
Figure 2 shows a conventional configuration of a
single optical fiber 1 in a wellbore 2 in which a
production tubing 21 is suspended from a wellhead( not
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shown) at the earth surface 22. The single fiber assembly
shown in Figure 2 is configured in accordance with is
standard practice by using a single optical fiber 1 with
upper and lower end terminations to measure acoustic
signals as disclosed in International patent application
W02007/049004, wherein the fiber 1 is divided into a
series of contiguous 10m channels C1-C7 and an acoustic
signal 3 transmitted by an acoustic source 4 at a certain
location along the length of the fiber 1 are measured by
a single channel, for example channel C4. In the known
DAS assembly a series of light pulses 5A,5B are
transmitted through the optical fiber 1 by a light
transmission and receiving assembly 7 arranged at or near
a first end 10 of the cable 1. Back reflections 6A,6B of
the transmitted light pulses 5A,5B are received by means
of a photodetector in the light transmission and
receiving assembly 7.
Utilising the optical fiber 1 as an acoustic or
vibration sensor can be achieved by launching a series of
pulses 5A, 5B of coherent laser light into a fiber 1. As
the pulses 5A,5B travel through the fiber 1 imperfections
in the crystal lattice making up the fiber 1 cause light
to be reflected back along the fiber and dispersed out of
the fiber. The nature of the back reflection causing
imperfections are a function of the strain state of the
fiber and as such by measuring the intensity of the back
reflections 6A,6B and with multiple pulses 5A,5B it is
possible to determine the strain state of the fiber 1 as
this varies temporally. Therefore an acoustic or
vibration source 4 which changed the strain state of the
fiber could be measured using the back reflection data
6A, 6B.
Figure 1 depicts an U-shaped looped fiber 1 with two
substantially parallel fiber sections 1A and 1B, also
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referred to as upward and downward fiber runs or legs 1A
and 1B, that are connected near the bottom of the well 2
by a single U-bend U1. Light pulses 5A,5B are transmitted
into the fiber 1 by a light pulse transmission and
receiving assembly 7, which also monitors back
reflections 6A,6B of the light pulses 5A,5B that are
reflected back when the light pulses travel along the
length of the fiber 1. The U-shaped looped fiber
configuration shown in Figure 1 allows the same acoustic
signals 3 transmitted by the underground sound source 4
to be measured on two channels C3,C13 at the same time.
In this way an average of the signals can be taken and
the effective Signal to Noise Ratio (SNR) of the DAS
assembly improved. The improved Signal to Noise Ratio
(SNR) will also improve the spatial resolution of the DAS
assembly.
It is observed that for the purposes of the
measurement of acoustic signals 3, all channels C1-C14
can be considered to be sampled simultaneously as the
propagation time of the laser pulse 5A, which travels at
the speed of light, is much higher than the frequencies
of interest in the acoustic signals 3, which travel at
the speed of sound.
Figure 3 shows a DAS assembly comprising a single U-
shaped loop U1, which is located at an interface between
a pair of adjacent channels C7 and C8. The looped DAS
assembly with a pair of substantially parallel downward
and upward legs 1A,1B shown in FIG.3 is substantially
similar to that of FIG.1 and has a U-shaped loop U1
arranged in the well 2 at a depth of about 70 meters
below the earth surface.
Figure 4 shows a DAS assembly comprising a single U-
shaped loop U1, which is located at a quarter of the 10m
channel length of channel C8, so that the channels C9-C15
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on the downward leg 1A of the fiber 1 will be offset from
the channels C1-C7 on the upward leg 1B of the fiber 1.
In this embodiment the resolution of the DAS assembly is
increased by creating virtual channels C11, C21, C31,
etc., which are formed by partial overlaps Cl1p=>(C1+C14),
C2ih=>(C1+C13), C3ih=>(C2+C13),... etc. of adjacent
channels Cl and C14, Cl and C13, C3 and C13,... etc.,
centred at 5m intervals, even though the channel
measurement length remains 10m.
Figures 5 and 6A-D show that a 50% overlap of 10 m
long channels C1-C22 will improve the ability of the DAS
assembly according to the invention to provide spatial
discrimination to detect acoustic waves 3 transmitted by
an underground sound source 4 at 5 m intervals by
creating virtual channels C11, C21, C31, ...,etc.,
without requiring modifications to the lasers of the
light pulse transmission and receiving assembly 7 or
reductions in system performance through SNR
considerations.
The left hand diagram in Figure 6A depicts a pulse input
5A which has a natural energy distribution resulting from
acoustic waves 3 emitted by sound source 4 shown in
Figure 5.
The middle diagrams in Figure 6Band C show the detection
of the pulse input 5A in the whole-spaced and staggered
channels C1-C22 of Fig.5.
The right hand diagram in Figure 6D shows the detection
in the virtual half-spaced channels C11 , C21 , C31 ,
etc, created by the overlapping portions of the staggered
channels Cl and C22, C2 and C22, etc. in accordance with
the method according to the invention.
It can be seen in Figure 6D that by combining the
results from the whole and virtual half-spaced channels
that additional information regarding the input signal 3
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can be obtained, because the virtual half spaced channels
Clip=>(C1+C22), C2ih=>(C2+C22), C3ih=>(C2+C21),... etc., are
centred at 5m intervals, even though the channel
measurement length of each of the whole channels Cl to
C22 remains 10m.
Figure 7 shows that it is also possible to use the
method according to the invention to further increase
spatial resolution, such that the spatial resolution is
improved from 10m to 2.5m by installing the fiber 1 in a
zig-zag pattern with three loops U1-U3 which divide the
fiber in two downward fiber runs 1A, 1C and two upward
fiber runs 1B, 1D.
The length of each loop U1-U3 is equal to 1/4 of the
channel length. The length of the fiber 1 is also
determined to be a whole number (n) of channels C1-Cn.
In Figure 7 the virtual channel numbers C1.25, C1.5,
C1.75 mean that these channel numbers measure
accumulations of partially overlapping channels Cl +
0.25Cx, C1+0.5Cy, C1+0.75Cz, etc.
The method according to the invention can be further
extended with more fiber runs and different length loops.
This follows the basic formula that the fiber runs should
be whole numbers of channels long and the loops at the
top and bottom should length of the desired overlap of
detection, such that:
1/2 spacing = 1/2 channel length loop back
1/4 spacing = 1/4 channel length loop back
1/8 spacing = 1/8 channel length loop back
1/10 spacing =1/10 channel length loop back
1/20 spacing = 1/20 channel length loop back.
1/x spacing = 1/x channel length loop back.
The ratio 1/x does not need to be a accurately
predetermined ratio.
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The number of increments is only limited by the range
of the optical pulse (up to about 40 or 50km) and the
number of substantially parallel fiber runs 1A-1D that
can be installed downhole in a wellbore 2 (5 pairs of
substantially parallel fiber runs is fairly standard).
Another feature of the method and system according to
the invention is that they can to an extend be
reconfigured from surface.
Figure 8 shows that in the case of a single fiber loop
1A,1B the method is simple and can be achieved
synthetically by shifting the channels 1C-22C using the
gate timing of the photodetector in the optical signal
transmission, receiving and interrogation assembly 7.
There are no requirement for distances from the surface
to the loop back position in this case. In this
situation, the channels are arranged such that they
receive signals from the same spatial location. It would
be necessary to establish that this situation had been
achieved through measurement of the fiber or calibration
with a known source. However, once a calibration of
channel position had been achieved, it becomes trivial to
modify the channel positions as shown in Figure 9.
Figure 9 shows that by offsetting the channel
starting position by 2.5m (through adjustment to the
timing of the photodiode sampling), that the channels Cl-
C18 are switched throughout the fiber 1 from being 100%
overlapped to being 50% overlapped. This demonstrates
that it is possible to exploit the SNR improvements
possible with paired channels C1&C18, C2&C17, etc) when
this is required and then reconfigure to the offset
channel arrangement shown in Fig.9 when this is required
from surface 22 and without modification to the optical
path. It is also possible that noise sources could be
tracked by dynamically varying the starting position.
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This could be used to centre the channels C1-C18 on known
acoustic sources such as gas lift injection points or, in
the case of flowing fluids, to track in real time slugs
of liquid moving through the wellbore 2 and/or production
tubing 21.
Figures 10 and 11 show that with multiple zig-zag fiber
loops U1-U4 it is only necessary that the loops U1,U3 at
the bottom of the well are of equal distance from the
surface, that the fiber runs 1A-1D are a whole number of
channel lengths (which can be adjusted from surface) and
that the loop U2 at surface 22 is equal to length of the
incremental steps (1/4 channel length in the case of a 4
fiber run, 1/4 channel resolution system).
Figures 10 and 11 further show that it is also
possible to configure a system of for example four fiber
runs 1A-1D and three fiber loops U1-U3 to provide two
separate measurements of the same depth with one pair of
fiber runs 1A,1B offset from the other 1C,1D by a half
channel length. This configuration with four fiber run
1A-1D is shown in Figure 10 and allows to increase the
Signal to Noise Ration (SNR) through averaging of the
matched signals as well as doubling the spatial
resolution of the DAS assembly 1.
Figure 11 shows that the DAS assembly 1 shown in Figure
10 can later be reconfigured at surface to provide 1/4
channel spacing simply by reducing the length of the
surface loop U2 and altering the timing of the
photodetector gate of the light pulse transmission and
receiving assembly 7 by a known and predictable amount.
This change can be made (and reversed) from the earth
surface 22. It is observed that redistribution of sensing
channels C1-Cn and/or C11-Cnih can only be achieved when
these channels C1-Cn and/or C11-Cnih are to an extent
virtual as is the case here.
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It will be understood that there are many
alternatives to the embodiments shown in Figures 1-10 to
increase the Signal to Noise Ration (SNR) and the spatial
resolution of a looped DAS assembly 1 according to the
invention.
