U.S. patent application number 13/836386 was filed with the patent office on 2014-02-27 for acoustic detector.
The applicant listed for this patent is Cristian PANTEA, Dipen N. SINHA, Cung Khac VU. Invention is credited to Cristian PANTEA, Dipen N. SINHA, Cung Khac VU.
Application Number | 20140056111 13/836386 |
Document ID | / |
Family ID | 50147908 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140056111 |
Kind Code |
A1 |
VU; Cung Khac ; et
al. |
February 27, 2014 |
ACOUSTIC DETECTOR
Abstract
An acoustic detector includes a cylindrical support member and a
plurality of receiver elements that are disposed on a surface of
the cylindrical support member. The plurality of receiver elements
are configured to detect acoustic waves in a plurality of azimuthal
angular directions.
Inventors: |
VU; Cung Khac; (Houston,
TX) ; SINHA; Dipen N.; (Los Alamos, NM) ;
PANTEA; Cristian; (Los Alamos, NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VU; Cung Khac
SINHA; Dipen N.
PANTEA; Cristian |
Houston
Los Alamos
Los Alamos |
TX
NM
NM |
US
US
US |
|
|
Family ID: |
50147908 |
Appl. No.: |
13/836386 |
Filed: |
March 15, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61691602 |
Aug 21, 2012 |
|
|
|
Current U.S.
Class: |
367/180 |
Current CPC
Class: |
G01V 1/40 20130101; E21B
47/005 20200501; G01V 1/42 20130101; B06B 1/0688 20130101; H04R
17/00 20130101; G01V 1/50 20130101 |
Class at
Publication: |
367/180 |
International
Class: |
H04R 17/00 20060101
H04R017/00 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with Government support under
Cooperative Research and Development Agreement (CRADA) Contract
Number DE-AC52-06NA25396 awarded by the United States Department of
Energy. The Government may have certain rights in this invention.
Claims
1. An acoustic detector, comprising: a cylindrical support member;
and a plurality of receiver elements disposed on a surface of the
cylindrical support member, the plurality of receiver elements
being configured to detect acoustic waves in a plurality of
azimuthal angular directions.
2. The acoustic detector according to claim 1, wherein the
plurality of receiver elements are configured as one-dimensional or
a two-dimensional array of receiver elements disposed on a surface
of a cylindrical support member.
3. The acoustic detector according to claim 1, the plurality of
receiver elements comprise a piezo-electric film.
4. The acoustic detector according to claim 3, wherein the
piezo-electric film comprises a PVDF film.
5. The acoustic detector according to claim 3, further comprising a
plurality of electrodes disposed on both sides of the
piezo-electric film, the electrodes being spaced apart from each
other so as to leave a gap between neighboring receiver
elements.
6. The acoustic detector according to claim 1, wherein the
plurality of electrodes on the piezo-electric film are produced by
etching, by vacuum depositing, or by coating a conductor material
on both sides of the piezo-electric film.
7. The acoustic detector according to claim 1, wherein the
plurality of receiver elements are configured as a one-dimensional
array or a two-dimensional array of receiver elements.
8. The acoustic detector according to claim 7, wherein the
one-dimensional array or the two-dimensional array of receivers is
wrapped around the surface of the cylindrical support member so as
to cover a 360 deg. detection angle.
9. The acoustic detector according to claim 8, wherein a row of
one-dimensional array of receivers within the two-dimensional array
of receivers are electronically selectable to detect the acoustic
waves.
10. The acoustic detector according to claim 1, wherein the
cylindrical substrate comprises an acoustic absorbing material.
11. The acoustic detector according to claim 10, wherein the
acoustic absorbing material comprises an acoustic absorbing foam or
silicon rubber.
12. The acoustic detector according to claim 1, wherein a size of
the receiver elements is selected to achieve a desired azimuthal
angular resolution of the acoustic wave between approximately 5
deg. and approximately 15 deg.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is based on and claims priority to
U.S. Provisional Application No. 61/691,602, filed on Aug. 21,
2012, the entire content of which is incorporated herein by
reference.
FIELD
[0003] The present invention relates generally to acoustic
interrogation of rock formations around a borehole, and more
particularly to using the combination of an acoustic source
including a single or an array of transducers in the wellbore
coupled to a linear or non-linear material for producing an
acoustic beam as a probing tool from a borehole to interrogate the
properties of rock formations and materials surrounding the
borehole.
BACKGROUND
[0004] Acoustic interrogation of subsurface features tends to be
limited by the frequency bandwidth of practical sources. High
frequency signals have a relatively short penetration distance,
while low frequency signals do not have collimation and generate
unwanted signals within the well bore. It is difficult to generate
a collimated acoustic beam signal in the sonic frequency range
between about 15 kHz and about 120 kHz from the borehole to probe
the rock formation surrounding a borehole with conventional
transducers. Conventional sonic acoustic sources have large beam
spread, such that as the frequency decreases, the beam spread
increases. The beam spread also depends on the diameter of the
transducer, which is limited by the borehole dimension. Sharp
directivity steering for a particular frequency requires a number
of conditions to be satisfied, including a long source array,
uniform coupling of all the transducers to the rock formation
around the borehole and knowledge of the acoustic velocities of the
rock formation. In the borehole environment, these conditions are
not often achievable because of underlying physics constraints,
engineering feasibility or operating conditions, especially when
the source signal has broad frequency bandwidth.
[0005] Traditional monopole and dipole borehole acoustic logs have
been used to measure sonic velocity near the borehole using
frequency range less than about 8 kHz. However, at this relatively
low frequency, azimuthal resolution is relatively low. There are a
number of patents that attempted to overcome this deficiency by
using additional receivers to detect the direction of the signals
returning to the receivers (see, for example, U.S. Pat. No.
5,544,127 and references cited within)). Applications for borehole
sonic for reflection imaging, refraction imaging, fractures
detection and permeability determination have also been proposed
(see, for example, U.S. Pat. No. 5,081,611, U.S. Pat. No.
4,831,600, U.S. Pat. No. 4,817,059, and U.S. Pat. No. 4,797,859).
All of these conventional techniques have operational and azimuthal
resolution deficiency as the source lacks or has insufficient
azimuthal directivity and desired frequency bandwidth.
[0006] For cement evaluation, ultrasonic waves in the frequency
range of hundreds of kilohertz (e.g., low ultrasonic frequency
range between 80 kHz and about 120 kHz and ultrasonic frequency
range around about 200 kHz) have been used to detect a cement gap
behind the casing. Even though frequencies around 200 kHz allow for
good azimuth resolution, the distance range for detection at around
this frequency is very limited, i.e., the depth of penetration to
investigate behind the formation and channels between cement and
rock formation is limited for ultrasonic source at frequency around
200 kHz. Conventional cement evaluation logs use a frequency of 30
kHz and can investigate deeper. However, these conventional cement
evaluation logs lack azimuthal resolution because the wavelength is
around the borehole radius and, consequently, the borehole modes
would excite the entire borehole. As a result it is difficult to
extract detailed azimuthal information of the cement bonding. In
order to overcome this deficiency, multiple sources (emitting in
the frequency range between 70 kHz and 120 kHz) and multiple
receivers are used in a Sector Bond Tool (SBT) system. However,
even with the use of multiple sources and multiple receivers, the
conventional SBT system was not able to cure the deficiencies of
the prior conventional cement evaluation logs as the source still
lacked azimuthal directivity to effectively detect the existence of
small channels between the cement and the rock formation.
SUMMARY
[0007] An aspect of the present invention is to provide a method
for investigating cement bonding or rock formation structure near a
borehole. The method includes generating an acoustic wave by an
acoustic source; directing at one or more inclination and azimuthal
angles the acoustic wave towards a target location in a vicinity of
a borehole; receiving at one or more receivers an acoustic signal,
the acoustic signal originating from a reflection or a refraction
or surface wave propagation of the acoustic wave by a material at
the desired location; and analyzing the received acoustic signal to
characterize features of the material around the borehole.
[0008] Another aspect of the present invention is to provide a
system for investigating cement bonding or rock formation structure
near a borehole. The system includes an acoustic source configured
to generate an acoustic wave and to direct the acoustic wave at one
or more azimuthal angles towards a desired location in a vicinity
of a borehole. The system also includes one or more receivers
configured to receive an acoustic signal, the acoustic signal
originating from a reflection or a refraction or surface wave
propagation of the acoustic wave by a material at the desired
location. The system also includes a processor configured to
perform data processing on the received signal to analyze the
received acoustic signal to characterize features of the material
around the borehole.
[0009] Yet another aspect of the present invention is to provide an
acoustic source for generating an acoustic beam. The acoustic
source includes a housing; a plurality of spaced apart
piezo-electric layers disposed with the housing; and a non-linear
medium filling between the plurality of layers. Each of the
plurality of piezoelectric layers is configured to generate an
acoustic wave when excited with an electrical signal. The
non-linear medium and the plurality of piezo-electric material
layers have an acoustic matching impedance so as to enhance a
transmission of the acoustic wave generated by each of plurality of
layers through the remaining plurality of layers.
[0010] Another aspect of the present invention is to provide an
acoustic detector that includes a cylindrical support member and a
plurality of receiver elements that are disposed on a surface of
the cylindrical support member. The plurality of receiver elements
are configured to detect acoustic waves in a plurality of azimuthal
angular directions.
[0011] These and other objects, features, and characteristics of
the present invention, as well as the methods of operation and
functions of the related elements of structure and the combination
of parts and economies of manufacture, will become more apparent
upon consideration of the following description and the appended
claims with reference to the accompanying drawings, all of which
form a part of this specification, wherein like reference numerals
designate corresponding parts in the various Figures. It is to be
expressly understood, however, that the drawings are for the
purpose of illustration and description only and are not intended
as a definition of the limits of the invention. As used in the
specification and in the claims, the singular form of "a", "an",
and "the" include plural referents unless the context clearly
dictates otherwise.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1A and 1B show a schematic diagram of a system for
generating a collimated acoustic beam for characterizing formations
and/or materials near a borehole, according to an embodiment of the
present invention;
[0013] FIGS. 1C and 1D show a schematic diagram of an end-fire
array of polyvinylidene difluoride (PVDF) film acoustic source used
for generating a collimated acoustic beam, according to an
embodiment of the present invention;
[0014] FIGS. 1E and 1F depict the signal output by the end-fire
array of PVDF film acoustic source without applying a delaying to
an excitation electrical signal and when applying an appropriate
delay to the excitation electrical signal;
[0015] FIGS. 2A-2C are schematic representations of a receiver,
according to various embodiments of the present invention;
[0016] FIG. 3 is schematic diagram of an acoustic measurement
system, according to an embodiment of the present invention;
[0017] FIG. 4A illustrates a characteristic of a parametric array
beam pulse signal emitted by an acoustic source, according to an
embodiment of the present invention;
[0018] FIG. 4B is a fast Fourier transform (FFT) of the acoustic
beam signal of FIG. 4A to obtain the signal in the frequency
domain;
[0019] FIG. 5 depicts data collected as a function of propagation
time and azimuthal angle, according to an embodiment of the present
invention;
[0020] FIG. 6 depicts a schematic diagram of an experimental set-up
with receiver having a linear array of receiver elements disposed
on a surface of a cylindrical configuration, according to an
embodiment of the present invention;
[0021] FIG. 7 depicts reflection data obtained in an experiment
similar to the data shown in FIG. 5 but after performing signal
processing to filter out the linear arrivals; according to another
embodiment of the present invention;
[0022] FIG. 8 depicts data collected as a function of propagation
time and receiver number after performing signal processing to
filter out the linear arrivals, according to an embodiment of the
present invention;
[0023] FIG. 9 depicts another experiment in which the orientation
of the receiver 24 is fixed (i.e., the receiver is not rotated) and
the mirror is rotated azimuthally; according to another embodiment
of the present invention;
[0024] FIGS. 10A and 10B depict an experimental acoustic setup,
according to another embodiment of the present invention where FIG.
10A is a longitudinal schematic view of the experimental setup and
FIG. 10B is a top view of the experimental setup;
[0025] FIGS. 11A-11C show plots of the measured data for various
azimuthal orientations or angles, respectively, at about 320 deg.,
at about 90 deg. and at about 165 deg., according to an embodiment
of the present invention;
[0026] FIGS. 12A-12C show plots of synthetic wave forms of acoustic
measurement in the frequency range of 15-120 kHz for various
borehole conditions, according to embodiments of the present
invention;
[0027] FIGS. 13A-13C shows simulated frequency chirp propagation
data along with the time-frequency analysis of the same data,
according to embodiments of the present invention;
[0028] FIG. 14A depicts the acoustic measurement system disposed
within a borehole, according to embodiment of the present
invention;
[0029] FIG. 14B depicts the acoustic measurement system disposed
within a borehole, according to another embodiment of the present
invention;
[0030] FIG. 14C depicts the acoustic measurement system disposed
within a borehole, according to yet another embodiment of the
present invention; and
[0031] FIG. 15 is a schematic diagram representing a computer
system for implementing the method, according to an embodiment of
the present invention.
DETAILED DESCRIPTION
[0032] FIGS. 1A and 1B is a schematic diagram of a system for
generating a collimated acoustic beam for characterizing formations
and/or materials near a borehole, according to an embodiment of the
present invention. The system 10 includes one or more electrical
signal generators 12 configured to generate signals at a first
frequency and a second frequency. The signals are transmitted to a
signal amplifier or amplifiers 14 that are configured to increase
the power of the signals. The signals modified by the amplifier 14
are transmitted to one or more transducers 16 that are configured
to generate acoustic waves at the first and the second frequency.
The acoustic waves are transmitted to a non-linear material 17,
which mixes the waves at the first frequency and the second
frequency by way of wave mixing process to produce a collimated
acoustic beam 18 at a third frequency. In one embodiment, the
collimated acoustic beam 18 can have a frequency in the range
between about 15 kHz and about 120 kHz. This frequency range can be
increased by using, for example, different transducers and primary
frequencies. The collimated acoustic beam 18 can be a continuous
acoustic signal or may also comprise one or more acoustic pulses
(e.g., a train of acoustic pulses).
[0033] The non-linear material 17 can be a liquid, a mixture of
liquids, a solid, a granular material embedded in a solid casing,
embedded microspheres, acoustic meta-materials, or an emulsion. By
way of a non-limiting example of such a non-linear material is
Fluorinert FC-43. Fluorinert is selected for its relatively low
sound velocity (646 m/s) and high acoustic nonlinearity
(.beta..about.7.6). Depending on the operating conditions in the
borehole, other non-linear materials can be used as a non-linear
mixing medium with suitable low sound velocity, high non-linear
coupling, absorption length, shock wave length, temperature and
pressure operating ranges, as well as, other requirements required
by operability specifications. Moreover, the length of the
non-linear material can be very compact and can range from between
5 cm to 2 meters for the frequency range between approximately 15
kHz and approximately 120 kHz depending on the type of materials
being used. The non-linear material can be disposed in a housing,
such as for example a cylindrical container. The axis of the
non-linear material-filled housing can be aligned with a borehole
axis, such that the difference frequency acoustic beam that is
output by the non-linear material propagates along this axis.
[0034] The non-linear behavior may be characterized through the
analysis of the properties of P-waves resulting from the non-linear
mixing phenomenon in which two incident waves at two different
frequencies, f.sub.1 and f.sub.2, mix to generate third frequency
components at the harmonics and inter-modulation frequencies
f.sub.2-f.sub.1, f.sub.2+f.sub.1, 2f.sub.1 and 2f.sub.2, etc. In an
aspect of the invention, the non-linear collinear mixing phenomenon
is designed to occur in the non-linear material inside the
wellbore. In general, only the resulting third wave of difference
frequency f.sub.2-f.sub.1 is of interest to this application. The
higher frequencies only propagate a short distance and tend to be
absorbed in the non-linear material itself. In some embodiments,
the third wave or collimated beam has a frequency between
approximately 15 kHz and approximately 120 kHz. However, a wider
frequency range and higher frequencies are also within the scope of
the present invention. In one embodiment, the frequency bandwidth
of the third wave is determined by the two primary frequencies
f.sub.1 and f.sub.2 where one frequency (e.g., frequency f.sub.1)
is kept fixed and the other frequency (e.g., frequency f.sub.2) is
swept in time very rapidly (e.g., chirped). Hence, for example, by
mixing a tone-burst of a few cycles of high frequency (e.g.
frequency f.sub.1) with a frequency chirp around that frequency
f.sub.1 one can obtain a broadband signal. However, it is also
possible to mix a variety of signals to create a desired time
response as well as a frequency response. For example, the compact
parametric array source can be programmed to generate Gaussian
pulse with frequency range between approximately 15 kHz and
approximately 120 kHz by mixing two high frequency Gaussian pulses
in the Fluorinert-filled chamber. The resultant generated beam
pulse at frequency f.sub.2-f.sub.1 acts like an acoustic particle
(analogous to phonon in solid state physics) traveling in the
propagating medium. The sharp pulse feature allows measurement with
raw data without any kind of signal processing, such as
cross-correlation and this speeds up the measurement significantly.
The experimental measurement system for the evaluation of this
parametric array source for imaging features around a borehole
casing is described in the next paragraph.
[0035] In one embodiment, the transducer 16 and mixing material 17
can be replaced by an end-fire array of polyvinylidene difluoride
(PVDF) film acoustic source 30 shown in FIG. 1C. The end-fire of
PVDF film acoustic source 30 comprises a plurality of spaced apart
piezo-electric (PZT) layers (e.g., PVDF films) 32. PVDF provides
some immediate benefits over piezoceramics. PVDF has high
mechanical damping and a complex permittivity. Transducers
constructed with PVDF can therefore have very broad bandwidth,
producing a pressure wave of short duration, thus offering good
spatial imaging resolution at lower operating (and hence minimally
attenuating) center frequencies than piezoceramics. Additionally,
the acoustic impedance (Z) of PVDF (Measurement Specialties,
Norristown, Pa.) is approximately 2.7 MRayls relative to the
acoustic impedance of water which is equal to approximately 1.48
MRayls. When using PZT layers, the non-linear mixing material or
medium 17 may be removed and substituted with any fluid that has
good transmission properties at the desired operating frequency
range (e.g., between approximately 1 kHz and approximately 120 kHz)
and low acoustic absorption. The PVDF films 32 can be mounted
inside a housing 34 (e.g., a cylinder). Although the end-fire array
acoustic source 30 is described herein as using PVDF films, as it
can be appreciated, other piezo-electric films can be used.
Although, the housing 34 is depicted in FIG. 1C as having a
cylindrical shape with a circular base, the housing 34 can have a
cylindrical shape or configuration with any base-shape (e.g., a
polygonal base-shape). The acoustic source further includes a
non-linear medium filling between the piezo-electric layers (e.g.,
PVDF films). In one embodiment, the housing 34 is filled a medium
such as a fluid having an acoustic impedance substantially matching
the acoustic impedance of the PVDF film 32. In one embodiment, the
fluid can be, for example, water as the acoustic impedance of the
PVDF film 32 substantially matches the acoustic impedance of water.
In another embodiment, water can be replaced by Fluorinert (e.g.,
FC-43). The impedance mismatch between PVDF and fluorinert changes
just slightly but the sound speed in the liquid becomes
significantly lower, that is 640 m/s in FC-43 as compared to 1480
m/s for water. However, Fluorinert FC-43 decomposes at elevated
temperatures, over 390.degree. F. The use of Fluorinert allows the
size of the source to be decreased by almost one third as compared
to the size when using water because the acoustic speed in
Fluorinert is lower. In one embodiment, the end-fire array source
30 further includes acoustic absorber material 31 disposed at a
first end of the housing 34 and a plate 33 disposed at a second end
of the housing 34 opposite the first end. On one embodiment, the
plate 33 can be made into an acoustic lens to provide manipulation
of the acoustic beam collimation or focusing, etc. The PVDF films
provide a very broadband source of sound from 1 kHz to 100 MHz. In
addition, in one embodiment, a lateral wall of the housing 34 can
be layered with acoustic insulation 35 to prevent the acoustic
waves generated by the PVDF films from reflecting from the lateral
wall.
[0036] The end-fire array based on PVDF film acoustic source 30 is
capable of outputting a more powerful acoustic wave (which can be,
for example, in a form of cone or a collimated or parallel beam)
than a conventional parametric array using a single transducer.
Each of the plurality of piezoelectric layers (e.g., PVDF films) is
configured to generate an acoustic wave. The non-linear medium and
the plurality of piezo-electric layers have a matching impedance so
as to enhance a transmission of the acoustic wave generated by each
of plurality of layers through the remaining plurality of
layers.
[0037] In one embodiment, an electrical generator such as
electrical generator 12 can be provided to electrically excite at
least one piezo-electric layer in the plurality of piezo-electric
layers to generate an acoustic wave pulse, as illustrated in FIG.
1D. For example, the electrical signal generator 12 can be
configured to electrically excite the plurality of piezo-electric
(e.g., PVDF) films 32 to generate a plurality of acoustic wave
pulses that are separated in time to form a train of acoustic wave
pulses. The electrical signal generator 12 can be configured to
generate a wide variety of signal waveforms (tone bursts, frequency
chirps, square waves, triangular waves, and any trigonometric
waveform shape etc.) in addition to a Gaussian pulse, and a cascade
of time delay generators. The time delay .tau. can be adjusted so
that it is equal to the time for an acoustic pulse to propagate
from one layer to the other so that it arrives exactly at the time
when the next layer is excited. The time delay .tau. can be
adjusted that it is substantially equal to a separation distance d
between two consecutive PVDF films 32 divided by the velocity "c"
of sound in the medium between the consecutive PVDF films 32.
Hence, if, for example, the first film 32A is excited at time t
equal to zero to generated a first pulse, the second film 32B can
be excited at time t delayed by delay time .tau. to generate a
second pulse, and the third film 32C can be excited at time t
delayed by delay time 2.tau. to generate a third pulse, etc. In
this way, the first pulse generated by the first film 32A arrives
at the second film 32B at substantially the same time the second
pulse is generated at the second layer 32B. Similarly, the first
pulse and the second pulse arrive at the third film 32C at
substantially the same time the third pulse is generated at the
third layer 32C, etc. Each PVDF film 32 can be fed from these delay
generators with the appropriate delay according to the position of
the PVDF film 32 within the housing 34. Each PVDF film 32 can also
be excited by a delayed electrical signal whose amplitude can also
be properly adjusted and shaped. The purpose of this approach is to
have acoustic pulses from all previous layers or films to arrive at
the last layer when the last layer is excited so that all the waves
add up and produce a strong pulse. If there are N layers then the
signal emanating from the last layer will be approximately N times
the power generated by each layer after subtracting off the loss of
the signal in the layer and in the medium. Although it is simpler
to have all layers positioned at equal intervals in space but that
is not necessary. Indeed, the various layers can be positioned at
any position and the interval between the layers can be different.
The time delay can be appropriately selected to take into account
the separation between the various layers. A linear phased array
approach with fixed frequencies can also be implemented by properly
varying the delay between the PVDF films 32.
[0038] In one embodiment, each PVDF film was excited by a 500 kHz
tone burst. Frequencies from 50 kHz to 1 MHz may also be used if
desired. There is no higher cut off frequency till almost 100 MHz
and is only somewhat limited mainly by the absorption of sound in
the liquid that these films are immersed in. Experimental data is
plotted in FIGS. 1E and 1F. FIG. 1E shows the signal from all 4
PVDF transmitters when no electronic delay is used. In this case,
each signal arrived at the receiver based on its distance from the
receiver. FIG. 1F on the other hand shows when appropriate time
delay was used, all the signal arrived at the last transmitter at
the same time. In this case, the detected signal by the receiver
now shows the large superimposed signal.
[0039] The efficiency of the end-fire acoustic source can be
increased by using PVDF films that are slightly curved instead of
being stretched flat. In one embodiment, each PVDF film can be
provided with plastic cross made of thin plastic wire (or a metal
wire) attached to it so as to make the surface of film slightly
curved in a symmetrical manner. Each of the PVDF films has a thin
layer of electrode on opposite sides where electrical connections
are made for the excitation of the film. The array of films 32 is
built into a wire frame and then inserted into the cylinder. The
cables are brought out through an exit hole on the absorber side of
the cylinder.
[0040] For example, in operation, a first PVDF film 32A may be
configured to generate a first acoustic pulse, a second PVDF film
32B may be configured to generate a second acoustic pulse delayed
relative to the first pulse, a third PVDF film 32C may be
configured to generate a third acoustic pulse delayed relative to
the second acoustic pulse, etc. The third PVDF film 32C can be
configured to be transparent to the first and second acoustic
pulses. The second PVDF film 32B can be configured to be
transparent to the first acoustic pulse. The first, second and the
third acoustic pulse are summed and, as a result, the PVDF film
acoustic source 30 can output a signal comprising the first, the
second and the third acoustic pulses. This provides not only the
ability to control the power of the output acoustic beam but also
the energy of the individual pulses and the delay between the
various pulses. In this example, the PVDF film acoustic source 30
is described as having three PVDF films. However, any number of
PVDF films can be used. In one embodiment, the housing 34 is
surrounded by an acoustic absorbing materials (not shown) to
prevent an acoustic energy scattering to the side of the housing
34.
[0041] In another embodiment, the sound waves generated by each of
the PVDF films 32 arrive at the front disc 33 at the same time and
add up in power. Each PVDF film is excited by an electrical pulse
(Gaussian shaped) that has a signal bandwidth between about 15 kHz
and about 120 kHz. However, the PVDF can be excited to generate
acoustic waves at any frequency range within the operating
frequency of the films. In this way, all the sound pulses generated
by each element or PVDF film 32 arrives at the front element 33 at
the same time and sum up to produce a powerful signal that is close
to N times the power output of each element after subtracting the
small transmission loss in the liquid and the film. The loss in the
liquid is minimal at these frequencies.
[0042] In one embodiment, the collimated acoustic beam 18 can be
steered in a particular direction by an acoustic beam guide 20. In
one embodiment, the acoustic source (transducers 16 and non-linear
material 17 or acoustic source 30) and acoustic beam guide or
steering device 20 are disposed within a housing 22. The acoustic
beam guide 20 can be an acoustic reflector or an acoustic lens, or
a combination of both. The acoustic reflector can be a material
with different acoustic impedance from the surrounding medium in
which the beam propagates. One non-limiting example of such an
acoustic reflector is metal plate. In one embodiment, the acoustic
lens may be configured to focus the collimated acoustic beam at a
particular focal point and direction and can have a concave shape.
A Fresnel-type mirror arrangement can also be used for the acoustic
beam guide. The acoustic beam guide 20 can be rotated or tilted
into a particular orientation by using one or more actuators (not
shown) coupled to the acoustic beam guide 20. Alternatively, in
some embodiments, the acoustic beam guide 20 may not be used, and
the collimated beam 18 would propagate along the axis of the
housing 22. For example, the housing 22 can be made of plastic or
other suitable material. In one embodiment, the housing 22 can be
in the form of a cylinder or pipe section with a circular base, as
shown in FIGS. 1A and 1B. However, the housing 22 may have other
configurations such as a cylinder with a polygonal base (e.g.,
square, rectangular, hexagonal, pentagonal, etc.). In one
embodiment, the housing 22 may be filled with a liquid (e.g.,
water).
[0043] FIG. 14A depicts the acoustic measurement system disposed
within a borehole, according to embodiment of the present
invention. The collimated beam 18 can be steered to a particular
direction toward an object or target of interest such as a cement
sheath or rock layers behind casing 19A within a borehole 11 or
object 19B (e.g., crack, fracture, void, etc.) within the rock
formation 13 near the borehole 11, as shown in FIG. 14A.
Inhomogeneities of formations, materials or structures, such as
object 19A, for example, will generate refraction or surface wave
propagation 21A which is scattered as acoustic wave 21B and
detected by receiver 24. Similarly, inhomogeneities within the rock
formation 13 such as crack or fracture 19B creates a reflection or
scattering of the acoustic beam 18 and the reflected acoustic wave
21C can then be detected by receiver 24. Acoustic beam 18 can
generate elastic waves, e.g. refractions and surface propagation
waves, traveling along the boundaries with the rock formation 13
and boundaries between the borehole and rock formation 13. The
reflected, scattered waves or surface waves and other types of
waves are received by receiver 24.
[0044] FIG. 14B depicts the acoustic measurement system disposed
within a borehole, according to another embodiment of the present
invention. In this embodiment, the acoustic beam 18 output by the
acoustic source 16, 30 can be directed using steering device 20
downwardly generally in the direction of axis 15 of borehole 11. In
this case, the acoustic beam or acoustic wave 18 can be used to
investigate rock formation 13 that has not been drilled and thus
investigate ahead of the drill bit. This can be performed, for
example, during drilling operations. When the acoustic beam 18 is
directed generally downwardly towards the rock formation 13,
object(s) 19C (e.g., rock layers within the rock formation) reflect
some of the acoustic beam energy 18 as acoustic wave 21D which can
then be detected by receiver 24. The location or distance of the
object 19C from the acoustic source 16, 30 can then be determined
based on the received acoustic wave 21D.
[0045] Similarly, amount of tilt of the layers 19C can also be
determined based on the inclination of the acoustic beam 18 (e.g.,
relative to borehole axis 15) and received acoustic wave 21D. FIG.
14C shows a situation where the layers 19C are tilted relative to
the borehole 11 or borehole axis 15. In this case, the beam
steering device 20 (e.g., a tiltable acoustic mirror or prism,
etc.) which is disposed in front of the source 30 can be used to
direct the acoustic beam 18 in any direction including a direction
towards the layers 19C. If the steering device 20 is positioned
vertically, then it allows the sound beam from the source to pass
through without any significant amount of blockage. If the layers
19C are tilted at an angle then the acoustic beam 18 will not be
reflected by the layers 19C and as a result no reflected acoustic
signal is detected by the receiver 24. If, on the other hand, the
steering device 20 is rotated or tilted such that the orientation
of the acoustic beam 18 is substantially perpendicular or normal to
layers 19C, an acoustic signal 21D can then be reflected from the
tilted layers 21D and can be detected by the receiver 24. The
steering device can be oriented azimuthally in addition to
inclination or elevation to provide a complete picture of what lies
ahead of the drilling bit.
[0046] As shown in FIG. 1B, the receiver 24 can also be provided
within housing 22. However, the receiver 24 can also be provided
separate from the housing 22 to allow independent movement of the
receiver 24 and source 16. The receiver 24 can be configured so as
to receive the reflected, scattered, diffracted, etc. wave 21. In
one embodiment, an acoustic absorber 23 can be disposed between the
acoustic beam guide 20 and the receiver 24, for example to prevent
acoustic waves that may have not been reflected or otherwise
directed by beam guide 20 from reaching the receiver 24. In one
embodiment, by placing the receiver 24 within the same housing 22,
the receiver 24 is able to receive the reflected or scattered wave
21 while the housing 22 is moved, i.e., the source 16 and the
receiver 24, and the housing 22, etc. are moved as a whole as a
single device 9 along the borehole 11 (as shown in FIG. 14).
However, in another embodiment, the acoustic source (e.g. the
acoustic source 16 with mixing material 17 or the acoustic source
30) and the receiver 24 can be independently moved along the
borehole 13. The reflected acoustic waves 21 are detected by
receiver 24 and are converted into an electrical signal which can
be transmitted to processing electronics 26 for analysis. The
processing electronics 26 can include a computer with appropriate
software for characterizing the rock formation or material or
structure surrounding the borehole, including producing 2D or 3D
images of the formation or the material around the borehole 11.
[0047] In some embodiments, the entire device 9 including the
transducers 16 (or the end-fire acoustic source 30), the non-linear
material 17, the steering device 20, and receiver 24 can be moved
up and down the length of the borehole 11 to image a particular
formation near the borehole or investigate the structure of the
borehole casing. However, in other embodiments, the acoustic source
(e.g., the acoustic source 16 with mixing material 17 or the
acoustic source 30) and the steering device 20 can also be moved
independently from the receiver 24 (for example while the receiver
is fixed). Moreover, the entire device 9 with or without the
receiver 24 can be rotated around the axis 15 of the borehole 11 to
image rock formations, structures, materials, etc. in any azimuthal
direction around the borehole 11.
[0048] FIG. 2A is a schematic representation of the receiver 24,
according to an embodiment of the present invention. The receiver
24 comprises a plurality of receiver elements 40. The receiver
elements 40 can be an array of PVDF films. In one embodiment, the
array can be produced from a single PVDF sheet with properly
depositing electrodes on both sides of the film (or etching out a
previously metallized electrode over the entire surface) and
leaving a gap between neighboring elements. Each of these
electrodes then behaves as a piezoelectric receiver element. A
typical array element size can be approximately 1 cm.times.1 cm but
it can be almost any size depending on the needed resolution of the
experiment. In one embodiment, electrical lines can be laid down on
the film or the PVDF sheet for electrical connections. The entire
sheet with electrodes can then be covered with a very thin sheet of
material (e.g., Mylar) for protection and electrical shorts.
Therefore, a linear array can be wrapped around, as shown in FIG.
2B, as a circular configuration made around an acoustically
absorbing material (e.g., foam) to create a circular array that
covers 360 degrees. The receiver elements 40 are mounted on surface
42S of an acoustic absorbing material (e.g., acoustic absorbing
foam, sponge or various types of silicon rubber) 42. FIG. 2C
depicts a schematic representation of the receiver 24, according to
another embodiment of the present invention. In this embodiment, a
large sheet of PVDF film can be used to create an array of
2-dimensional arrangement (i.e., a matrix arrangement) of
transducer receiver elements 40. The array of transducers 40 can
then be wrapped around a cylindrical configuration to create an
array that can provide complete 360 degree coverage around the axis
of the cylindrical configuration, in addition to vertical coverage
along the axis of the cylindrical configuration. In this way, the
receiver array does not need to be physically rotated azimuthally
in the borehole. In this case, a different vertical series or rows
of PVDF layers or a one-dimensional array of PVDF films within the
two-dimensional array of PVDF film can be electronically selected
to detect acoustic signals. Typically, in operation, all PVDF
elements can be scanned first at a given location to determine the
direction from which any signal is coming and then the appropriate
vertical arrays can be used to track this signal. Signal
multiplexor electronics can be used to carry out this kind of
electronic scanning and the detected signal can be subsequently
amplified and digitized. As shown in FIGS. 2B and 2C, the acoustic
absorbing material 42 has a cylindrical configuration with a
circular base. However, as it can be appreciated, the absorbing
material 42 can have any desired configuration such as a
cylindrical configuration with a polygonal base or elliptical base,
or other shape. By mounting the receiver elements 40 on the surface
42S of the absorbing material 42, the receiver elements 40 receive
acoustic signals from the front side of the receiver elements 40
and not from the back side of the receiver elements 40.
[0049] FIG. 3 is schematic diagram of a laboratory measurement
system or experimental set up to test the measurement system when
deployed in a borehole environment, according to an embodiment of
the present invention. In the experimental set up, the housing 22
including the acoustic source (e.g., the acoustic source 16 with
mixing material 17 or the acoustic source 30), the beam steering
device 20 and the receiver 24 are positioned within an axial
borehole 11A in a barrel (e.g., a cement barrel) 29 that simulates
the borehole 11 with a cement casing. The acoustic measurement
system 9 includes acoustic source (e.g., the acoustic source 16
with mixing material 17 or the acoustic source 30), mirror system
20 and receiver 24. In one embodiment, acoustic source (e.g., the
acoustic source 16 with mixing material 17 or the acoustic source
30), non-linear medium 17, mirror system 20 and receiver 24 are
disposed inside housing 22. In one embodiment, the receiver 24 is
configured such that it only receives acoustic signals from the
front. The receiver 24 is shielded from other signals such as back
signals (i.e., signals that are incident on the back of the
receiver 24 are absorbed by absorber 42). In one embodiment, the
receiver 24 is configured to move with acoustic source (the
acoustic source 16 with mixing material 17 or the acoustic source
30). In another embodiment, the receiver 24 can be moved
independently of the acoustic source (the acoustic source 16 with
mixing material 17 or the acoustic source 30). In order to test the
efficacy of this measurement system, a groove 25 is provided at an
outer periphery or outer surface of the barrel 29 (e.g., concrete
or cement barrel), as will be explained further in detail in the
following paragraphs.
[0050] In one embodiment, the acoustic source (the acoustic source
16 with mixing material 17 or the acoustic source 30) and the
receiver 24 are configured such that the beam direction from the
acoustic source (the acoustic source 16 with mixing material 17 or
the acoustic source 30), i.e., acoustic beam 18, and the received
signal 21 lie on the same plane. In one embodiment, both the
acoustic source (the acoustic source 16 with mixing material 17 or
the acoustic source 30) and receiver 24 are rotated azimuthally
from 0 to 360 degree. However, in another embodiment, only the
mirror 20 is rotated while the source (the acoustic source 16 with
mixing material 17 or the acoustic source 30) and the receiver 24
are fixed. Indeed, by providing receiver 24 with a cylindrical
configuration where receiver elements 40 are disposed on the
surface of the cylindrical configuration, the receiver 24 is able
to detect an acoustic signal at angle from 0 to 360 deg. without
having to move or rotate the receiver 24. Similarly, the acoustic
source (the acoustic source 16 with mixing material 17 or the
acoustic source 30) need not be reoriented to be able to scan a
desired field azimuthal angle. The azimuthal field angle can be
scanned by simply rotating the steering device (e.g., mirror 20).
The acoustic beam emitted by the acoustic source (the acoustic
source 16 with mixing material 17 or the acoustic source 30) is
reflected by the beam steering device (e.g., mirror) 20 and
directed as acoustic beam 18 towards inner wall of cement barrel
29. The acoustic beam 18 interacts with the material of the barrel
29, the material outside barrel 29, the interface between the
housing 22 and the barrel 29, etc., and generates reflections,
refractions or surface waves, or any combinations of thereof. In a
first scenario, the acoustic beam, after being reflected by the
mirror 20, may undergo a reflection by the material of the barrel
(e.g., cement barrel) 29 or the rock formation, or both. The
reflected acoustic signal may then be detected by receiver 24. This
scenario is generally referred to as a reflective mode. In a second
scenario, the acoustic signal, after being reflected by the mirror
20, may be refracted by the material of the barrel (e.g., cement
barrel) 29 at the interface between the cement barrel 29 and the
rock formation. The refracted acoustic signal may then be detected
by receiver 24. This scenario is generally referred to as a
refractive mode. In yet a third scenario, the acoustic signal,
after being reflected by mirror 20 may generate surface waves at
the interface between a surface of the borehole and the cement in
the barrel 29 (or rock formation in a field deployment) or at
interface boundaries within the cement (or rock formation). The
surface waves will emit returning acoustic signals that can be
detected by the receiver 24. This scenario is generally referred to
as a surface wave mode.
[0051] FIG. 4A illustrates a characteristic of the beam pulse
signal emitted by the parametric acoustic source disposed within
the borehole in a laboratory experimental set up, according to an
embodiment of the present invention. The acoustic beam signal
pattern 45 on the exterior surface of the barrel 29 as a function
of time (time domain) is measured using a laser Doppler vibrometer.
The waveform of the signal 45 is shown in FIG. 4A. FIG. 4B is a
fast Fourier transform (FFT) of the acoustic beam signal 45 to
obtain the signal in the frequency domain. The frequency bandwidth
of the signal 45 can be extracted from the FFT showing a broad
frequency bandwidth between about 15 kHz and about 120 kHz.
[0052] The unique characteristics of the acoustic source (the
acoustic source 16 with mixing material 17 or the acoustic source
30) can be combined with various receiver elements or modules 40
into a measurement system to perform azimuthal borehole sonic
measurements, three-dimensional (3D) reflection imaging from a
borehole, 3D refraction imaging, 3D fracture detection, 3D mapping
of permeability, and 3D mapping of channels between the cement
barrel and rock formation.
[0053] Because the high directivity of the beam pulse, many of the
deficiencies of the existing borehole acoustic measurement systems
cited above can be minimized. As discussed below, the system has
good azimuthal resolution as well as inclination direction control.
In one embodiment, the azimuthal angular resolution is between
about 5 deg. and about 15 deg., for example 10 deg. This new
capability enables the extension of borehole acoustic measurement
to full 3D measurement (the 3.sup.rd dimension being the azimuthal
angle).
[0054] FIG. 5 depicts data collected as a function of propagation
time, distance between receiver elements and acoustic beam source
and azimuthal angle in an experiment using the experimental setup
shown in FIG. 3, according to an embodiment of the present
invention. In this experiment the beam source is directed at the
rock formation at one azimuthal angle and one inclination angle,
and the linear receiver 24 with receiver elements 40 is oriented to
detect the returning signal in the same azimuthal angles as the
source beam, as shown in FIG. 6. The entire assembly of source,
mirror and receiver are rotated azimuthally in incremental steps of
10 degrees and the returning acoustic signals data are recorded for
all receiver elements for each azimuthal increment. FIG. 5 shows
five panels labeled as panel 1 to panel 5 (P1, P2, P3, P4 and P5).
Each panel corresponds to data displayed for one azimuth
measurement (i.e., azimuthal angle). Each 10 deg. azimuthal angle
(i.e., 0 deg., 10 deg., 20 deg., etc.) corresponds to a different
panel (P1 through P5). The y-coordinate in each panel represents
the arrival time of the signal detected at the various receiver
elements 40. The x-coordinate in each panel corresponds to the
distance from vertical receiver element to the source. The gray
scale of the display corresponds to the amplitude of the received
acoustic signal. Within each panel are shown a plurality of data
points 58. Each of these points 58 corresponds to a signal detected
by one of the plurality of the receiver elements 40 of receiver 24.
In this example, receiver 24 is provided with 12 receiver elements
40. Therefore, 12 data points are detected by the receiver 24, each
point corresponding to a signal detected by one of the 12 receiver
elements 40. Each of the 12 data points has a different arrival
time corresponding to the arrival of the signal to each of the 12
receiver elements 40. As shown in FIG. 5, the first linear signal
arrival 50 corresponds to P-wave compression refraction wave
commonly measured in sonic log. The second and third linear signal
arrivals 52 and 54 correspond to surface waves such as Rayleigh,
Stoneley or Lamb waves. Signal arrivals due to reflection from
cement/air interface at barrel perimeter are shown at 56.
[0055] FIG. 7 depicts reflection data obtained in an experiment
similar to the data shown in FIG. 5A but after performing signal
processing to filter out the linear arrivals. There are 36 panels
P1-P36 and each panel corresponds to an azimuthal angle and the 36
panels range from 0 to 180 degrees. For example, panel P1
corresponds to azimuthal angle of 0 deg. The y-coordinate
represents the arrival time at the receiver 24. The y-coordinate in
each panel represents the arrival time of the signal detected at
the various receiver elements 40. The x-coordinate in each panel
corresponds to the vertical distance from receiver element to the
source. The gray scale of the display corresponds to the amplitude
of the received acoustic signal. Within each panel, i.e., within
each azimuthal angle range, hyperbola-like curves 59 can be seen.
Each curve 59 corresponds to data of a signal detected by one
single receiver element 40 in the receiver 24. The series of wave
patterns 60 and 62 correspond to a reflection from a perimeter or
outer periphery of the cement barrel 29 while the wave pattern 64
corresponds to a reflection from a surface of the groove 25 (at an
interface of the cement and air). As it can be noted, the waves
reflected from the surface of the groove 25 arrive to the receiver
24 earlier than waves reflected from the cylindrical surface of the
barrel 29. Furthermore, the position of the groove 25 can be
ascertained by using the azimuthal measurement method and system
described herein. The present method achieves excellent azimuthal
resolution which allows detecting defects within a structure such
within a casing within a borehole or at an interface of the
borehole and the rock formation, etc. For example, as it can be
noted in FIG. 7, the groove 25 can be located at specific azimuthal
angles or within an azimuthal angular range allowing a
determination of a position or location of a structure, such as a
structural defect, a fracture, or the like.
[0056] FIG. 8 depicts a different data display of the same
experiment with a different sorting. There are 12 panels (from P1
to P12) in FIG. 8, Each panel (P1, P2, . . . , P12) corresponds to
data of signals detected by one of the 12 receiver elements 40 in
receiver 24. Within each panel (e.g., panel P1) the x-coordinate
represents the azimuthal angle (in the range from 0 deg. to 360
deg.). The y-coordinate represents arrival time at each of the 12
receiver elements 40 of receiver 24. The gray scale of the display
corresponds to the amplitude of the received acoustic signal. As
can be seen in FIG. 8, the reflection from the groove 25 is
detected by some detector elements 40 (for example, at panels P1
through P4) and not by other detector elements (for example, at
panels P9 through P12). In addition, it can be noted that, for
panel P1 for example, the groove 25 is clearly seen in the middle
of the panel which correspond to an azimuthal angle around 90 deg.
The reason for detecting the groove 25 with specific receiver
elements 40 (panels P1 through P4) and not by other receiver
elements 40 (panels P9 through P12) is due to the fact that the
acoustic beam 18 has a specific angular elevation spread and thus
is reflected selectively to specific detector elements 40. Hence,
detector elements 40 (corresponding to panels P9 through P12)) that
are outside of the scattered, reflected, diffracted acoustic wave
beam from the groove 25 are not able to detect the reflected,
diffracted, scattered beam from groove 25. However, as it can be
appreciated, if the receiver 24 is moved vertically, other receiver
elements 40 within the receiver 24 can then detect the signal
reflected, diffracted or scattered by the groove 25. In this case,
the groove may then be seen in panels P6 through P10 if the
inclination of the groove changes, for example. Hence, the present
measurement system is not only capable to resolve a position of a
structure in azimuthal angle but also in elevation angle as
well.
[0057] Furthermore, the elevation information can be utilized to
determine an orientation of the structure (e.g., groove 25). For
example, in the laboratory experiment described in the above
paragraphs, the groove 25 is parallel to the axis of the borehole
in the cement barrel 29. However, the grove 25 can also be
positioned oblique, i.e., at an angle relative to the borehole
axis, in which case, the angular elevation information which
depends on the orientation of the groove 25 can be different.
Indeed, depending on the angular orientation of the structure (e.g.
groove 25) relative to the borehole axis, the reflected, diffracted
beam by the groove 25 can be directed preferentially to specific
receiver elements 40. As a result, the groove 25 can be seen in the
plotted data or image at different panels (e.g., at panels P7 and
P8). By determining in which panels the groove 25 is detected, it
is possible to infer the angular inclination of the groove 25.
[0058] FIG. 9 depicts another experiment in which the orientation
of the receiver 24 is fixed (i.e., the receiver is not rotated) and
the mirror is rotated azimuthally between 0 and 360 degrees at an
increment of 20 degrees. 19 panels are displayed with each panel
corresponds to signal data recorded with one the azimuthal angle
from 0 deg. to 360 deg. azimuthal angle at 20 degree increment. The
y-coordinate represents arrival time at the receiver elements 40 of
receiver 24. The x-coordinate in each panel corresponds to the
vertical distance between the receiver element and the source. The
gray scale of the display corresponds to the amplitude of the
received acoustic signal. The data clearly shows excellent azimuth
resolution with the maximum energy of the linear arrivals occurring
when the beam orientation and receiver reception orientation are
aligned. This shows that the propagation path that is rather narrow
in extent and does not spread too much azimuthally.
[0059] FIGS. 10A and 10B depict an experimental acoustic setup,
according to another embodiment of the present invention. FIG. 10A
is a longitudinal schematic view of the experimental setup and FIG.
10B is a top view of the experimental setup. The experimental setup
includes is similar in many aspects to the experimental set up
shown schematically in FIG. 3. The cement barrel 22 is lined with a
steel axially arranged inner casing 100. A pipe or tube 102 is
embedded within the cement barrel 22. A groove 25 is also cut or
carved on an exterior surface of the cement barrel 22. A detachment
foil 104 (e.g., Aluminum foil) is also provided within the housing
22. In this embodiment, the detachment foil 104 is disposed in
contact with inner casing 100. Within the casing 100 of the cement
barrel 22 are disposed the acoustic source 16, 30, the non-linear
material 17, the mirror 20 and the receiver 24. As shown in FIG.
10B, axes are drawn to indicate azimuthal angular orientation (the
orientation of the two axes is arbitrary). The groove 25 is located
at azimuthal angle between about 230 deg. and about 280 deg. The
pipe or tube 102 is located at an azimuthal angle between about 80
deg. and about 100 deg. The detachment foil (e.g., aluminum foil)
is located at an azimuthal angle between about 140 deg. and about
190 deg.
[0060] FIGS. 11A-11C show plots of the measured data for various
azimuthal orientations or angles, respectively, at about 320 deg.,
at about 90 deg. and at about 165 deg., according to an embodiment
of the present invention. The azimuthal orientation or angle of
about 320 deg, (FIG. 11A) corresponds to the orientation of the
acoustic beam in a region where there is no inclusion behind the
inner casing 100, i.e., there is only the cement barrel wall. The
azimuthal orientation or angle of about 90 deg. (FIG. 11B)
corresponds to the orientation of the acoustic beam in a region
where the tube (e.g., plastic pipe) 102 is included. The azimuthal
orientation or angle of about 165 deg. (FIG. 11C) corresponds to
the orientation of the acoustic beam in a region where the
detachment foil (e.g., aluminum foil) 104 is provided. In this
plot, the y-coordinate corresponds to the time it takes for the
acoustic wave to be received by receiver 24, the x-coordinate in
each panel corresponds to the vertical distance of receiver element
from the source. The various curves in each plot correspond to the
acoustic signals received by the various receiver elements 40 in
receiver 24. In this example, there is provided 12 receiver
elements 40 in receiver 24. However, any number of receiver
elements can be used. The curve closest to the x-coordinate
corresponds to the signal detected by the first receiver element
and the curve farthest to the x-coordinate corresponds to the
signal detected by the 12.sup.th receiver element. The first
receiver element is the receiver element that is closest to the
acoustic source 16, 30 and the 12.sup.th receiver element is the
receiver element that is farthest from the acoustic source 16,
30.
[0061] As shown in FIG. 11A, with no inclusion behind the inner
casing 100, the surface waves decay with distance along the
borehole, i.e. decay from the first receiver element to the
12.sup.th receiver element. As shown in FIG. 11C, with the
delamination or detachment foil 104 behind the casing 100, the
surface wave amplitude is larger and decays more slowly as expected
because the steel pipe is not dampened by the contact with the
cement (i.e., the aluminum foils carries the acoustic waves farther
along the borehole). As shown in FIG. 11B, at azimuth angles
corresponding to the pipe 102, the surface wave amplitude is larger
and also decays more slowly. In addition to the surface wave, a
fast linear arrival just behind the P-wave first arrival is
recorded indicating additional wave mode traveling along the wall
of the pipe 102. This measurement data clearly show that azimuthal
information of rock formation behind the steel casing can be
gleaned from linear arrivals using a borehole acoustic measurement
system.
[0062] In addition to the ability of changing the azimuthal
orientation of the acoustic source beam by changing the azimuthal
angular direction of the mirror 20, the inclination of mirror 20
can also be changed to send the acoustic source beam along any
vertical direction. This allows the acoustic source beam to be
injected at different inclinations and azimuthal directions to
probe for reflection boundaries, refraction boundaries and
fractures of different orientations in the rock formation. The data
can be subsequently analyzed using various conventional methods.
Analysis of refraction arrivals along with azimuthal resolution can
provide for 3D imaging of velocity by refraction analysis. This can
provide better characterization of near borehole alteration and
characterization of the skin of reservoirs.
[0063] In one embodiment, the measurement data are collected using
broadband beam pulse. In this way, information with broad frequency
bandwidth can be collected relatively quickly. Indeed, in this
case, there is no need to sweep the frequency by chirping.
Furthermore, in one embodiment, the use of multiple acoustic
sources to cover the entire the bandwidth, for example, between
about 15 kHz and about 120 kHz, may not be needed. The acoustic
beam pulse with broad bandwidth, for example between about 15 kHz
and about 120 kHz, can provide measurements that can yield
information on cement bonding between the cement and the rock
formation in a borehole.
[0064] The present measurement system can be used for evaluating a
cement casing or steel casing in a borehole. A simulation of guided
wave propagation through the steel casing when a sound beam pulse
interacts with the steel casing is performed under certain
geometrical conditions. In this simulation, a 25 mm thick layer of
cement is used between the steel casing and Berea sandstone. The
Berea is considered infinite in extent. It is also assumed that the
borehole is filled with water and there is energy sink along the
axis of the borehole. The simulations were carried out using the
DISPERSE software package from the Imperial College, UK.
[0065] FIG. 12A-12C show plots of the acoustic simulation in the
frequency range of 20-120 kHz for various conditions. The data in
these plots are captured in the instance where the borehole is
filled with water. Each data set is generated under different
condition but in each case the receiver is at a distance of 12
inches from the excitation point on the steel casing in the axial
direction. These data are showing the propagation characteristics
of a sound pulse (frequency chirp) of 100 microsecond duration with
a frequency span of 20-120 kHz and with a Gaussian envelope. The
graphs on the left side show the amplitude of the received acoustic
signal as a function of time and the graphs of the right side show
the fast Fourier transform of the acoustic signal to the frequency
domain where the amplitude is plotted as a function of the
frequency. FIG. 12A is a plot of the data captured with
water-steel-concrete-air, where there is an air gap between the
concrete and the Berea sandstone. FIG. 12B is a plot of the data
with water-steel-concrete-water-Berea, where there a water gap
(e.g., a 1 mm gap) between the concrete and Berea. FIG. 12C is a
plot of the data captured with water-steel-concrete-Berea, where
everything is the interfaces between the water, steel, concrete and
Berea are in physical contact.
[0066] The plots depicted in FIGS. 12A-12C show significant
differences among them in terms of the characteristics of the
signal. When the concrete is in good contact with the Berea
sandstone, the energy of the waves through the steel dissipates
into the Berea and the observed amplitude is rather low (as shown
in FIG. 12C). When there is a detachment or gap between concrete
and Berea, the signal level is higher (as shown in FIGS. 12A and
12B).
[0067] The second set of plots on the right which represent the
amplitude of the signal vs. the frequency shows the frequency
content of the received signal. Higher frequencies are damped out
when the concrete and the Berea are in good contact (as shown in
FIG. 12C). In addition, as can be noted in FIG. 12B, the presence
of water between the concrete and Berea confines the energy to
earlier times and the frequency content is also narrowed. As shown
in FIG. 12A, when the concrete is in good contact with Berea, the
signal spreads out in time with the main arrival delayed
significantly. The differences between the various scenarios can be
readily seen in these plots. The simulated data shows that the
described measurement method or system can be used effectively for
cement evaluation around a borehole casing.
[0068] FIGS. 12A-12C above shows the frequency content of the
propagated signal under various conditions of borehole casing
integrity in reference to the concrete and the Berea rock formation
behind it. Therefore, one cannot see in these figures which
frequencies are propagating at what strength at different times.
Another way to view the information presented in FIGS. 12A-12C can
be based on a joint time-frequency analysis of the data using a
short-time Fourier transform (STFT) approach. This provides the
frequency content of the signal as a function of time and thus
allows one to see the frequencies that are prominent at certain
times during the propagation. Hence, the STFT analysis of the data
enhances the information provided by FIGS. 12A-12C and introduces a
powerful analysis approach.
[0069] FIGS. 13A-13C show the original simulated frequency chirp
propagation data along with the time-frequency analysis of the same
data. The plots on the right represent the 3D time-frequency
information for each of the situations discussed above with
reference to FIGS. 12A-12C, respectively. The x-axis corresponds to
the time, the y-axis corresponds to the frequency, and the z-axis
or vertical axis corresponds to the amplitude. In FIG. 13A, as
shown in the 3D plot, where the concrete is detached from the rest
of the system and does not see the rock formation, the energy in
the waves propagate through at three different velocities and this
gives rise to the three peaks at 0.1 second time interval. It also
noted that the wave also arrives relatively quickly, after 0.1
second. In FIG. 13B, as shown in the 3D plot, the situation is that
there is a 1 mm gap filled with water between the cement and the
rock formation. The propagation characteristics of the acoustic
wave are completely different from the propagation characteristics
of the acoustic wave shown in FIG. 13A. Indeed, all the energy
seems to be bunched together and propagates relatively quickly
through the casing and the cement and the propagation is not
influenced by the rock formation as if the two parts are isolated.
FIG. 13C depicts the situation where all the layers are tightly
coupled (steel casing, cement and the rock formation). As shown in
the 3D plot in FIG. 13C, the presence of the rock formation has a
strong loading influence on the wave propagation and the wave
propagation is delayed significantly and the main energy peak
arrives with a delay of almost 0.5 second. These three examples
show how the various detachments or coupling between the layers can
be detected by this type of analysis and measurements.
[0070] In addition, by providing azimuthal resolution in borehole
acoustic measurements, rock characterization can be improved and
thus improve production engineering systems. Furthermore, by
proving azimuthal resolution in borehole acoustic measurement, the
integrity of the borehole can be evaluated and thus improve the
overall drilling safety. In addition, azimuthal resolution in
borehole measurements can allow measure a stress surrounding the
borehole and as a result improve borehole completion
methodology.
[0071] Furthermore, the borehole acoustic measurement system and
method described can also be used for imaging the rock formation,
indeed, the present measurement system and method can fill a
measurement gap between conventional sonic tools that investigate
less than a foot (approximately 33 cm) from the borehole with
relatively a good vertical resolution and conventional long range
sonic image tools such as borehole acoustic reflection survey
(BARS), from Schlumberger corporation, which investigate rock
formation at tens of feet from the borehole but with lower vertical
resolution and limited azimuthal resolution. For example the
present acoustic measurement method and system may be utilized in
various applications including:
[0072] 1. 3D imaging of reservoir layers, stratigraphy, fractures,
faults, vugs (up to few feet such as 10 feet from the borehole)
with full azimuth resolution.
[0073] 2. Measurement of compressional velocity Vp and shear
velocity Vs of the rock formation with full azimuth
determination.
[0074] 3. 3D analysis of geo-mechanical properties around boreholes
from analysis of refraction waves and Lamb waves to improve
characterization of the invasion zone and any borehole damage.
[0075] 4. 3D imaging of velocity of rock formation near the
borehole using refraction analysis.
[0076] 5. 3D mapping of fractures from reflections of linear
arrivals
[0077] 6. 3D mapping of permeability and production skin of
reservoirs.
[0078] 7. Focusing the acoustic beam with a phase-code Gaussian
pulses in the lower frequency range, e.g., between about 10 kHz and
about 30 kHz for deeper penetration into the rock formation while
discriminating against background noise.
[0079] For example, in one embodiment, measurement of the
compressional and/or shear velocity of the rock formation in the
vicinity of the borehole at a plurality of azimuthal angles using
the above described measurement system can provide valuable
information on the stress around the borehole hence allowing
determining or predicting potential fracture position and/or
fracture propagation with the rock formation in the vicinity of the
borehole. It is known that formations having relatively larger
velocity variations are either relatively less consolidated, or the
stress in the formation is large. In both situations, this may
provide an indication as to the likelihood of borehole collapsing.
The acoustic measurement system described in the above paragraphs
can provide information on the velocities as a function of
azimuthal angle and/or elevation angle within the rock formation
around the borehole. Using the velocity as a function of azimuthal
angle and or elevation angle can in turn provide the azimuthal
and/or inclination angle of various stress areas and/or fractures,
faults, etc., and thus can ultimately provide information on the
anisotropy of the earth stress field around the borehole. In
addition, the position of a fracture or fault can be mapped in 3
dimensions (3D mapping) using the data acquired as a function of
azimuthal and elevation angle.
[0080] The above described measurement system and method can also
be used in mapping fluid permeability of subsurface formations such
as sub-surfaces penetrated by a borehole including permeability due
to fractures in the rock formation. For example, this can be
performed by measuring velocities (compression velocity or shear
velocity or surface waves or any combination of the velocities
cited) at various points within the rock formation around the
borehole. Based on the measured velocity, the permeability can be
extracted using various known models.
[0081] In one embodiment, the method or methods described above can
be implemented as a series of instructions which can be executed by
a computer. As it can be appreciated, the term "computer" is used
herein to encompass any type of computing system or device
including a personal computer (e.g., a desktop computer, a laptop
computer, or any other handheld computing device), or a mainframe
computer (e.g., an IBM mainframe), or a supercomputer (e.g., a CRAY
computer), or a plurality of networked computers in a distributed
computing environment.
[0082] For example, the method(s) may be implemented as a software
program application which can be stored in a computer readable
medium such as hard disks, CDROMs, optical disks, DVDs, magnetic
optical disks, RAMs, EPROMs, EEPROMs, magnetic or optical cards,
flash cards (e.g., a USB flash card), PCMCIA memory cards, smart
cards, or other media.
[0083] Alternatively, a portion or the whole software program
product can be downloaded from a remote computer or server via a
network such as the internet, an ATM network, a wide area network
(WAN) or a local area network.
[0084] Alternatively, instead or in addition to implementing the
method as computer program product(s) (e.g., as software products)
embodied in a computer, the method can be implemented as hardware
in which for example an application specific integrated circuit
(ASIC) can be designed to implement the method.
[0085] FIG. 15 is a schematic diagram representing a computer
system 130 for implementing the methods, according to an embodiment
of the present invention. As shown in FIG. 15, computer system 130
comprises a processor (e.g., one or more processors) 132 and a
memory 134 in communication with the processor 132. The computer
system 130 may further include an input device 136 for inputting
data (such as keyboard, a mouse or the like) and an output device
138 such as a display device for displaying results of the
computation. The computer system 130 may be configured to control
various modules including a control module 140 to control the
signal generator 12, a control module 142 to control the steering
of the mirror 20, and acquisition electronics 26 for acquiring the
measurement data. The measurement data can be stored in a storage
device (e.g., a flash drive) for latter visualization or
processing, etc.
[0086] In one embodiment, there is provided a system for
investigating structure near a borehole. The system includes an
acoustic source configured to generate an acoustic wave and to
direct the acoustic wave at one or more azimuthal angles towards a
desired location in a vicinity of a borehole. The system further
includes one or more receivers configured to receive an acoustic
signal, the acoustic signal originating from a reflection or a
refraction of the acoustic wave by a material at the desired
location. The system also includes a processor configured to
perform data processing on the received signal to analyze the
received acoustic signal to characterize features of the material
around the borehole.
[0087] Although the invention has been described in detail for the
purpose of illustration based on what is currently considered to be
the most practical and preferred embodiments, it is to be
understood that such detail is solely for that purpose and that the
invention is not limited to the disclosed embodiments, but, on the
contrary, is intended to cover modifications and equivalent
arrangements that are within the spirit and scope of the appended
claims. For example, it is to be understood that the present
invention contemplates that, to the extent possible, one or more
features of any embodiment can be combined with one or more
features of any other embodiment.
[0088] Furthermore, since numerous modifications and changes will
readily occur to those of skill in the art, it is not desired to
limit the invention to the exact construction and operation
described herein. Accordingly, all suitable modifications and
equivalents should be considered as falling within the spirit and
scope of the invention.
* * * * *