Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND SYSTEM FOR EVALUATING THE
CHARACTERISTIC PROPERTIES OF TWO CONTACTING MEDIA
AND OF THE INTERFACE BETWEEN THEM BASED ON MIXED
SURFACE WAVES PROPAGATING ALONG THE INTERFACE
Field of the invention
The invention relates in general to evaluating the characteristic
properties of at least one of two contacting media such as a subsurface
formation surrounding a borehole and the borehole. In particular, the
invention relates to the measurement and analysis of mixed surface
waveforms for such a purpose.
Background art
Acoustic techniques for formation and borehole characterization are
well known. All of these techniques involve transmitting an acoustic signal
from a source to a receiver via the formation of interest. Normally, borehole
acoustic measurements rely on bulk or borehole modal waves. It is usually
assumed that the only components of wavefield and waveforms are
compressional, shear, Stoneley, dipole flexural and, possibly, quadrupole and
leaky waves. Other types of elastic waves are viewed as parasitic and are
disregarded. Ultrasonic measurements deserve special comment because they
usually rely just on measuring traveltime of reflected or refracted bulk wave.
To summarize, at present mixed surface waves (MSWs) are not used in
borehole acoustics.
Thus, various techniques exist to evaluate fractures - imaging borehole
wall with electromagnetic measurements (see, for example, US patent
4567759), using properties of Stoneley wave propagation in presence of
fractures (US 4831600), etc. The existing techniques do not rely on MSWs.
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MSWs (which include whispering gallery waves, creeping waves, etc.)
can appear in case of wave propagation along interface which has non zero
effective curvature. The latter means geometrical curvature, velocity gradient
or any combination of those. First treatments of MSWs in academic literature
date back to 60s. They have been studied and described mathematically
[J.B. Keller, A geometrical theory of diffraction. In Calculus of Variations
and Its Applications. pp.27-52, Ed.: L.M.Graves, New-York, (1958);
V.M. Babich, Propagation of Rayleigh waves on the surface of homogeneous
elastic body of an arbitrary form, Dokl.Akad.Nauk. SSSR, v.137, p.1263
(1961); I.A. Molotkov, P.V. Krauklis, Mixed surface waves on the boundary
of the elastic medium and fluid, Izvestia Acad.Sc.USSR, Phys.Solid Earth, v.9
(1970); V.M. Babich, N,Ya. Kirpichnikova, The boundary-layer method in
diffraction problems, v.3, Springer, Berlin, Heidelberg, (1979); B.J. Botter,
J.van Arkel, Circumferential propagation of acoustic boundary waves in
boreholes, J.Acoustic.Soc.Am., v.71, p.790 (1982); A.F. Siggins, A.N. Stokes,
Circumferential propagation on elastic waves on boreholes and cylindrical
cavities, Geophysics, v.52, p.514 (1987)]. MSWs were observed in laboratory
experiment [V.G. Gratsinskiy, Investigation of elastic waves in model of
borehole. Izv. AN USSR, geophys. series, v.6, p.322 (1964);
V.G. Gratsinskiy, Amplitudes of creeping waves on wellbore surface. Izv. AN
USSR, geophys. series, v.6, p.819 (1964); P.G. Gilbershtein, G.V. Gubanova,
Quasicreep of compressional waves in case of concave refracting boundary,
Izv. AN USSR, physics of earth, p.48 (1973)]. However, MSWs have not
been used in borehole acoustics applications so far. Therefore, the invention
is
the first attempt to devise an apparatus, which can excite and detect MSWs in
borehole environment, and a method, which is able to provide tomographic
characterization of borehole and formation properties.
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Summary of the invention
An aim of the invention is to provide an efficient method for evaluating
the characteristic properties of at least one of two contacting media such as
a
subsurface formation surrounding a borehole and the borehole, and the
interface between them such as the borehole wall.
Accordingly a first aspect of the invention provides a method for
evaluating the characteristic properties of at least one of two contacting
media
having non-zero effective curvature interface between them and of the
interface between them, at least one of the media being solid, the method
comprising the steps of registering acoustic signals generated by passage of
acoustic waves in said media, determining one or more wave characteristics
of mixed surface waves propagating along said interface based on the
registered acoustic signals and calculating the characteristic properties of
at
least one of said media and said interface based on the determined wave
characteristics of mixed surface waves .
In preferred embodiments the wave characteristics of mixed surface
waves are at least one of the travel times, the slowness and the attenuation
of
said mixed surface waves. The characteristic properties of at least one of
said
media are at least one of the: elastic moduli of the medium; tensors of
compliances of the medium; velocities of compressional waves or shear bulk
waves or both in the medium; gradient of elastic properties in the medium;
profile of velocities of compressional waves or shear bulk waves or both in
these media; depths of penetration of zones where gradient of elastic
properties is present in the media into these media; anisotropy of these
media;
presence of discontinuities in the properties of the medium. The
characteristic
properties of the interface are at least one of the geometrical curvature
radii of
interface and presence of discontinuities in the properties of the interface.
Another aim of the invention is to provide a method for evaluating
parameters of a borehole and a surrounding formation. The method
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comprises, registering acoustic signals generated by passage of acoustic
waves and mixed surface waves, determining one or more wave
characteristics of said mixed surface waves propagating along the borehole
wall based on the registered acoustic signals and or calculating the
characteristic properties of the borehole fluid and/or the surrounding
formation and/or the borehole wall based on the determined wave
characteristics of mixed surface waves.
In preferred embodiments, the step of determining wave characteristics
of mixed surface waves propagating along the borehole wall based on the
registered acoustic signals includes the steps of extracting the mixed surface
waves from other components of detected acoustic signals, and inversing the
results for at least one of the borehole fluid, the formation and the borehole
wall properties evaluation.
In one preferred embodiment, the method further comprises the step of
exciting acoustic waves in at least one of the borehole, the formation and the
borehole wall so as to generate mixed surface waves propagating along the
borehole wall prior to registering acoustic signals generated by passage of
said acoustic waves and said mixed surface waves.
In another preferred embodiment, MSWs are excited by at least one
acoustic source displaced from the borehole axis.
In another embodiment, MSWs are excited by at least one acoustic
source placed at the axis of the borehole penetrating a formation with
velocity
gradient having component in direction normal to the borehole wall.
In further embodiment of this aspect of the invention the method
further comprises the step of exciting acoustic waves by at least one acoustic
detector which is capable to be used for exciting acoustic waves and the step
of registering acoustic signals generated by passage of said acoustic waves
and said mixed surface waves by at least one acoustic source which is capable
to be used for registering acoustic signals.
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In further embodiment of this aspect of the invention acoustic signals
are registered by at least one acoustic detector.
In further embodiment of this aspect of the invention acoustic signals
are registered by azimuthally distributed detectors array.
In other embodiment of this aspect of the invention acoustic waves are
excited and acoustic signals are registered by the same means.
In other embodiment of this aspect of the invention the wave
characteristics of mixed surface waves are at least one of the travel times,
the
slowness and the attenuation of mixed surface waves.
In other embodiment of this aspect of the invention said characteristic
properties of the formation are at least one of the: elastic moduli of the
formation; tensors of compliances of the formation; velocities of
compressional and/or shear bulk waves in the formation; gradient of elastic
properties in the formation; profile of velocities of compressional and/or
shear
bulk waves in the formation; velocity gradient of compressional waves or
shear bulk waves in the formation or both; depths of penetration of zones
where gradient of elastic properties is present in the formation; formation
anisotropy; presence of discontinuities in properties of the formation.
In other embodiment of this aspect of the invention said characteristic
properties of the borehole fluid are at least one of the: elastic moduli of
the
borehole fluid; tensors of compliances of the borehole fluid; velocities of
compressional waves on shear bulk waves in the borehole fluid or both.
In other embodiment of this aspect of the invention the characteristic
property of the borehole wall is its geometrical curvature radii.
Another aim of the invention is to provide a system for evaluating parameters
of a borehole, the borehole wall and a surrounding formation. The system
comprises means for registering acoustic signals generated by passage of
acoustic waves including mixed surface waves propagating along the borehole
wall, data processing means for determining one or more wave characteristics
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of said mixed surface waves propagating along the borehole wall based on the
registered acoustic signals and calculating the characteristic properties of
the
borehole fluid and/or the surrounding formation and/or the borehole wall
based on the determined wave characteristics of mixed surface waves .
In one preferred embodiment, the system further comprises means for
exciting acoustic waves placed in at least one of the borehole, the formation,
and the borehole wall so as to generate mixed surface waves propagating
along the borehole wall.
In preferred embodiments, said means for exciting acoustic waves
comprises at least one acoustic source displaced from the borehole axis.
In other preferred embodiments of the invention said means for
registering acoustic waves comprises at least one acoustic source placed at
the
axis of the borehole penetrating a formation with velocity gradient having
component in direction normal to the borehole wall.
In other preferred embodiments, said means for registering acoustic
waves comprises at least one acoustic detector.
In further preferred embodiments, said means for registering acoustic
waves comprises azimuthally distributed detectors array
In further preferred embodiments, the means for exciting acoustic
waves is capable to be used for registering acoustic signals and means for
registering acoustic signals is capable to be used for exciting acoustic
waves.
In other embodiments of the invention the means for exciting acoustic
waves are at the same times means for registering acoustic signals.
Other aspects and advantages of the invention will be apparent from the
following description and the appended claims.
Brief description of the drawings
Fig. 1 shows possible examples of MSW measurement schematics: a) a
single source, a single detector; b) a single source, an array of detectors;
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Fig. 2 shows further possible examples of MSW measurement
schematics: a) an array of sources, a single detector; b) an array of sources,
an
array of detectors;
Fig. 3 shows an example of family of MSWs paths (exemplified by set
up with a single source) forming a grid on the borehole wall (examples of
MSWs paths on borehole wall evolvement are shown);
Fig. 4 shows an example, illustrating possibility to use the sources as
the receivers and vice versa;
Fig. 5 shows an example of an acoustic source displaced from the
borehole axis;
Fig. 6 shows parts of waveforms for detectors placed at different
azimuths. Eccentricities (distance to borehole axis) of the source (in
percentages of borehole radius): 10% (dotted line), 50% (dash-dot line), 90%
(solid line);
Fig. 7 shows a schematic of the example of MSW propagation in case
of centered source and velocity gradient in formation (MSWs and their paths
in this example are shown in the assumption that there is velocity gradient in
formation);
Fig. 8 shows an example of family of MSWs paths on wellbore wall
evolvement for the case depicted on Fig. 7 (under the same assumptions);
Fig.9 shows an example of model (a) and set of waveforms from
detectors 3 placed in half-circle fashion on borehole wall 2 (...) (b), which
indicate MSWs excitation (arrivals 5 and 6 on synthetic waveforms
corresponding to MSWs going along paths like 7 and 8);
Fig. 10 shows an example of schematic of one of possible embodiments
of apparatus: a) side view; b) top-down view;
Fig. 11 shows an example of arranged waveforms from receivers lying
on the path of certain MSW.
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Description of the preferred embodiments of the invention
According to the invention acoustic signals generated by passage of
acoustic waves in at least one of two contacting media having non-zero
effective curvature interface including MSWs (e.g., MSWs propagating along
a borehole wall) are registered. In particular, the acoustic waves can be
excited in advance for subsequent registration of the acoustic signals by
using
an acoustic source (or sources array) of a system and then registered by a
detector (or detectors array). Then, one or more wave characteristics of said
mixed surface waves propagating along the borehole wall are determined on
the basis of the registered acoustic signals and the characteristic properties
of
the borehole fluid and/or the surrounding formation and/or the borehole wall
are calculated based on the determined wave characteristics of mixed surface
waves. The calculations are based on a correspondence between MSWs
propagation characteristics and the properties of the borehole fluid and/or
the
surrounding formation, and/or the borehole wall. The step of determining
wave characteristics of mixed surface waves propagating along the borehole
wall based on the registered acoustic signals can include the steps of
extracting the mixed surface waves from other components of detected
acoustic signals, and inversing the results for borehole fluid and/or
formation
and/or the borehole wall properties evaluation.
According to the invention, one needs to register MSWs with a system,
then extract/separate them in the acoustic signal from its other components
and invert the results for physical properties of the borehole and the
formation. The acoustic waves including MSWs are preferably excited prior
to registration. Essential components of such a system are means for exciting
acoustic waves - an acoustic source (or array of sources) 1, which is placed
in
such a way as to excite MSWs, a detector (or detectors array) 3 (Fig. 1, 2)
(placement can be variable - not necessarily at borehole wall) and data
processing means (not shown). Their combination enables excitation and
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registration of MSW (or a family of MSWs, see Fig. 3) at the interface of
interest, e.g., on borehole wall 2, and Also, when the interface is borehole
wall, the paths of the excited family of MSWs will cover its surface (Fig. 3).
TThis approach is not limited to the formation type and can be implemented
both for isotropic formations and for those with intrinsic anisotropy.
Besides,
some implementations may offer the opportunity to use sources as detectors
and vice versa further increasing the range of possible embodiments (e.g.,
hydrophones as sources/detectors) (Fig. 4). Excited and registered MSW(s)
propagate along interface and scan information about physical properties
(e.g., interface curvature, velocities in formation and/or borehole fluid,
velocity gradient in formation, possibly anisotropy information, etc.)
(Figs. 1, 2). Thus, registered MSW(s) contain important information about
borehole and formation.
MSWs propagate along interfaces with effective curvature. This could
be due to geometrical curvature, velocity gradient or both. Examples are
borehole wall, pipes, formation layers boundaries, cement-formation
interface, invaded/altered/damaged zone etc. Since MSWs appear on the
interface between two media, which has effective curvature, the concept is
general. Thus it is possible to apply it in various fields. For instance, in
seismic it could be of interest in case of non-flat boundaries between
formation layers or curved boundaries of geological structures (potential
application is seismic imaging), in seismology it could be utilized for
detection of earthquakes at large distances (waves will travel along curved
surfaces), it could be also used to monitor defects (e.g. pipes in liquid
transport systems), etc. In short, applications of MSWs are numerous and
wider than field of borehole acoustics, which this particular invention
focuses
on.
One of possible ways to generate MSWs on a borehole wall 2 is to use
specially placed acoustic source (or array of sources) 1 (see Fig. 1, 2).
There
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are many options regarding the type of the source(s). The most common one
is a monopole source but other sources, for example, dipole, quadrupole,
direct excitation at the borehole wall (e.g. hammer source), array of sources,
etc. can be also used. Placement of the source(s) 1 is selected on the basis
of
knowledge of physics of MSWs propagation. For example, in the formations
without velocity gradient having component in direction normal to the
borehole wall 2 the source(s) 1 should be displaced from the borehole axis 4
(Fig. 5). This is essential, because the source placed on borehole axis will
not
excite MSWs in this case, and novel, because usually the acoustic sources are
centered. In this case larger eccentricities are advantageous because it
facilitates MSWs excitation and makes their detection more robust and
accurate (Fig. 6). The source 1 will produce an acoustic signal. The single or
set of signals with some delay could be sent (it could be sent at the same
time
by all sources or with some delay by different sources if the array of sources
is used) - again there are many options. The signal could be either the same
or
different signals could be used by different sources. Upon reaching borehole
wall 2 the signal will give rise to family of MSWs. They will propagate along
this wall 2 as depicted on Figs. 1, 2, 3. Another example is the formations
with velocity gradient near the borehole wall 2. Here effective curvature is
non-zero even if the geometrical curvature is absent. Hence, in this case even
the source 1 placed at the borehole axis 4 will excite MSWs on borehole wall
2 (Fig. 7). Of course, MSWs paths will be different from previous example
(Fig. 8). By using an acoustic detector (or detectors array) 3 it is possible
to
detect MSWs together with other components of acoustic signal(s) (Fig. 9b).
Each detected MS W contains information about the interface and formation
properties (e.g., curvature, velocity gradient, etc.) along the path of its
propagation. So, even one detector data carry valuable information about the
borehole and the formation and can be used as input to the steps of the
method. Naturally, the more detectors are used the more information is
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collected. Available information about the properties of the borehole and the
formation can be maximized by employing the detectors array. Also, to invert
for spatial distribution of properties it is necessary to collect data by the
detector array. It is also important that paths of detected MSWs form a grid
on
the interface of interest so that they "scan" the borehole wall 2 for
properties
of borehole and formation (Figs. lb, 2b, 3). Thus, for tomographic
applications it is advantageous to use a detector array with array/matrix
arrangement. It should be stressed that this setup made with specific purpose
to excite and detect MSWs is new and constitutes new measurement.
To evaluate properties of the borehole and the formation first it is
necessary to extract/separate MSWs from other components of acoustic signal
(for example see Fig. 9b) in detector (or detector array) data. One should
proceed from general ideas (for example, arrival time determination, time
picking or other ideas [J.L. Mari, D. Painter, Signal processing for
geologists
and geophysicists. Editions Technip (1999)]) and create techniques taking
into account MSWs physics (physics based extraction/separation). This means
properly incorporating in implementation dependence of MSWs properties on
such parameters as interface curvature, velocities in borehole fluid and
formation, velocity gradient etc. One should also keep in mind that these
parameters and hence MSWs properties can vary along MSWs paths. Also, in
case of eccentered source(s) 1 MSWs travel along curved trajectories on
borehole wall 2 (Figs. 1, 2, 4). As example, to extract/separate MSWs one
could use the following procedure. Knowing expected trajectories of MSWs
one can collect and arrange waveforms from detectors lying along the MS W
path (that, generally speaking, will be curved line on borehole wall). To
evaluate MSWs slownesses and travel times one can perform semblance
analysis (see, for example, C.V. Kimball, T.L. Marzetta, Semblance
processing of borehole acoustic array data, Geophysics, v.49, p.274, 1984) on
these waveforms taking properly into account MSWs physics (e.g., MSWs
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dispersion depending on various parameters like curvature radius, etc.). The
latter is significantly different from common notions (e.g., slownesses are
not
the same as formation slownesses as is the case for head waves; dispersion
laws are quite different from those for borehole modes, etc.). Other novel
techniques can be imagined as well. For example, one can deconvolve
detected signal with the source signal, implement full waveform inversion
based on the knowledge of MSWs propagation, perform some selective
processing of acoustic signal, etc. Once MSWs have been extracted/separated,
it is possible to use the result for inversion step to find properties of
interest.
That is detector (or detector array) data should be inverted for properties of
interest. Again, general ideas (for example, ray tracing tomography, wavefield
inversion or other ideas [A. Tarantola, Inverse Problem Theory and Methods
for Model Parameter Estimation, SIAM (2004)]) should be used to create
techniques taking into account MSWs physics (physics based inversion). In
case of MSWs propagation one of the important factors is that their velocities
are affected by the interface curvature. It calls for construction of
inversion
method which takes into account MSWs physics. This means, for example,
accounting for MSWs dispersion dependence on interface curvature and
velocity gradient (normal to interface) when calculating travel times, wave
paths, etc. Another possible approach is to use MSW eikonal equation
inversion. Possibilities are numerous and here just some of them are
mentioned.
Information necessary for inversion step and obtained as a result can
vary. Examples are as follows. MSWs amplitudes and types will be affected
during propagation through fractures. Therefore, this information from MSWs
measurements (detection and extraction/separation) can be used to estimate
fractures on the borehole wall and invert for their properties. Whispering
gallery wave propagating in the fluid has velocity related to borehole fluid
velocity by a very simple formula [P. Krauklis, N. Kirpichnikova,
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A. Krauklis, D. Pissarenko, T. Zharnikov, "Mixed Surface Waves - Nature,
Modelling and Features", abstracts of 69th EAGE conference EAGE2007].
Therefore, travel time and slowness information from either single detector or
detectors array (detection and extraction/separation) can be used to invert
for
mud slowness. Using MSWs travel times measured by detector (or detector
array) in the formation without velocity gradient normal to borehole one can
invert for spatial distribution of properties of borehole and formation (e.g.,
Vp,
VS map, etc.). Measured MSWs data can be used to invert for spatial
distribution of physical properties (e.g., Vp, VS) in borehole and formation.
Dependence of MSWs slownesses and travel times on interface curvature can
be used to invert this data to map and characterize caverns on borehole or to
describe borehole wall roughness. It should be stressed that whatever
particular implementations of extraction/separation and inversion steps, to be
correct they should be based on knowledge of MSWs physics and therefore
novel. Another possible application is as follows. First acoustic signal is
evaluated at detector(s) assuming base model of interface curvature, velocity
gradient area, etc. Then this estimate is compared to measured signal. The
discrepancy can serve as an indicator of presence of anomalies at interface on
the path of MSWs (this is one of the possible implementations of inversion
step). MSWs propagation is affected by velocity gradient and its spatial
distribution. It allows using MSWs to characterize alteration, invasion,
damaged and other zones demonstrating velocity gradient (depth of
penetration of the zone, velocity gradient and profile). Another option is to
evaluate velocity profile in formation. MSWs also can be used to estimate
intrinsic formation anisotropy. Also properties of MSWs propagation through
discontinuities of properties (like interface curvature, velocities, velocity
gradient, etc.) make MSWs measurements suitable for detection of
layers/beds boundaries and characterization of boundaries and layers/beds
themselves. One can also use MSWs to characterize intrinsic formation
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anisotropy. MSWs propagation depending on interface curvature opens
possibilities to apply MSWs to characterize borehole wall geometry
(roughness, caverns, washouts, ellipticity, non-circular boreholes, etc.). It
is
also possible to apply MSWs measurements to create sonic caliper as MSWs
propagation depends on interface curvature and hence can be used to measure
changes in borehole diameter. These are just some of the examples and we
stress that many applications of the MSWs measurements are possible. Also,
the method provides various information depending on particular
implementation. It is often different or of better quality than what can be
achieved by other methods and thus forms new borehole acoustic application.
Naturally, more information can be gained in case of detector array but it is
possible to use method and extract valuable information even for single
detector data. Depending on particular implementation of the methods steps
one can make different applications of MSWs.
General structure of the invention presented above can be exemplified
by describing one of the possible embodiments, which provides one with
distribution of acoustic velocities on borehole wall.
According to the concept, invention embodiment consists of the system
and the method. The target is a tomographic characterization of a borehole
wall 2 and two essential components (see Fig. 9) are acoustic source(s) 1
placed in such a way as to excite MSWs, and detector array 3. The system can
be made of just an acoustic source displaced with respect to borehole axis 4
(which is a new way to place the source) and azimuthally distributed detectors
array 3 attached to some frame 9. Its schematic is depicted on Fig. 10.
Examples of possible acoustic sources are numerous. It can be monopole
piezoelectric type of transmitter, dipole source, hammer source (which
directly excites MSWs at borehole wall) etc. For detectors one can use, for
example, 3C geophones or accelerometers touching borehole wall. This is just
one of possibilities and all above comments about vast variability in possible
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options for embodiments apply. It is also worth mentioning that according to
the concept depending on the implementation it may be possible to use
sources as receivers and vice versa or to have the same element act as both.
That will allow one to increase amount of data without increasing hardware
configuration. For example, if hydrophones are employed for source(s) and
receivers then one may switch them. For example, let all new sources (former
receivers) emit acoustic signal separately. This will cause receiver (former
source) to detect MSWs propagating in the opposite direction (Fig. 10).
Working of the apparatus can be schematically represented as follows.
First, an eccentered acoustic source 1 emits acoustic signal in the borehole
fluid and excites propagating acoustic wavefield (Fig. Ib). When propagating,
this wavefield will encounter the borehole wall 2. Because of source
eccentricity, this will give rise to propagation of surface waves along paths
defined by the rules of ray approximation [V.M. Babich, V.S. Buldyrev,
Short-wavelength diffraction theory (asymptotic methods). Springer-Verlag
(1990)]. They are schematically depicted on Fig. 3. Due to the natural
curvature of the borehole wall these paths will also have geometrical
curvature (Fig. 1b). Thus, MSWs will be generated. They will start
propagating along the borehole wall 2 along these paths. Then acoustic
wavefield can be detected with detectors array 3. Example of pressure
waveforms is presented on Fig. 9b (examples of MSWs arrivals are indicated
as 5 and 6). It can be seen that besides MSWs other components of the
wavefield are registered as well. To illustrate that eccentered source is
essential component on Fig. 6 pressure waveforms for different source
eccentricities are presented. It is easily seen that MSWs amplitudes decrease
and accuracy of MSWs arrivals detection deteriorates when the eccentricity
decreases. Also, according to the invention concept detectors should be placed
in such a way that paths of MSWs form a regular grid on borehole wall to
enable inverse problem solution. Detectors array of described system satisfies
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this requirement (Figs. 1 b, 3, 9, 10). It is easily seen from Fig. 3, which
presents detectors' positions on the borehole wall evolvement together with
MSWs paths.
The data processing means (not shown) for determining one or more
wave characteristics of said mixed surface waves propagating along the
borehole wall based on the registered acoustic signals and calculating the
characteristic properties of the borehole fluid and/or the surrounding
formation and/or the borehole wall based on the determined wave
characteristics of mixed surface waves can represent any data processing
means enabling to perform the steps coded as computer-executable
instructions. For example, the data processing means can be a personal
computer, a server or the like.
Regarding the method, in this embodiment example its goal is to find
distribution of sonic velocities (Vp, VS) on the borehole wall. According to
the
invention to do so one should extract/separate MSWs in detected acoustic
signal and invert this data from detector array to sonic velocities. One of
the
simplest implementations of the separation step is to use procedure described
above. That is, to arrange waveforms recorded by detectors lying on the
approximate path of the same MSW (Fig. 11) and apply semblance analysis
(see, for example, C.V. Kimball, T.L. Marzetta, Semblance processing of
borehole acoustic array data, Geophysics, v.49, p.274, 1984) taking into
account MSWs physics. This means correcting for dependence of MSW
trajectory, velocity, dispersion etc. on various parameters like on curvature
radius of the MSW path, signal frequency etc. [I.A. Molotkov, P.V. Krauklis,
Mixed surface waves on the boundary of the elastic medium and fluid,
Izvestia Acad.Sc.USSR, Phys.Solid Earth, v.9 (1970); P. Krauklis,
N. Kirpichnikova, A. Krauklis, D. Pissarenko, T. Zharnikov, "Mixed Surface
Waves - Nature, Modelling and Features", abstracts of 69th EAGE conference
EAGE2007] when calculating semblance. For example, in case velocity
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gradient in formation is absent phase velocity of creeping P wave (1St mode)
depends on the signal frequency, wave velocity in the bulk and effective
curvature radius. In this case approximate formula holds:
2/3
V,,sin28
V = V,, 1- 24/3 2
n
where V,, is compressional velocity in the formation, f denotes signal
frequency, Rb stands for borehole radius, 0 is the angle between MSW
trajectory and generatrix of the borehole wall and , is the first root of Airy
function. Different and/or more complex formulas should be applied in other
cases. It should be stressed that such procedure is new. Failing to account
for
MSWs physics will lead either to failure to obtain MSWs arrival times or to
their erroneous estimates. On the contrary, proper MSWs physics based
extraction/separation will lead to identification of MSWs, picking of MSWs
arrivals and determination of MSWs arrival times (Fig. 9b, 11). For inversion
step the following procedure can be used. Using equations for dependence of
MSWs velocities on formation and mud speeds, curvature radius, frequency
and other factors [I.A. Molotkov, P.V. Krauklis, Mixed surface waves on the
boundary of the elastic medium and fluid, Izvestia Acad.Sc.USSR, Phys.Solid
Earth, v.9 (1970); P. Krauklis, N. Kirpichnikova, A. Krauklis, D. Pissarenko,
T. Zharnikov, "Mixed Surface Waves - Nature, Modelling and Features",
abstracts of 69th EAGE conference EAGE2007] their paths and travel times
can be calculated given velocity model (Figs. 1, 2, 3). Such models can be
anisotropic, e.g., if curvature radius is not constant, there is velocity
gradient
(that may vary in space), intrinsic formation anisotropy, etc. In general case
interface curvature at the same point will depend on the direction of MSW
propagation. In this sense there is additional type of anisotropy present,
which
should be properly taken into account. In turn, the above equations can be
utilized in 2d travel time tomography procedure [A. Tarantola, Inverse
CA 02706297 2010-05-19
WO 2009/067041 PCT/RU2008/000310
18
Problem Theory and Methods for Model Parameter Estimation, SIAM
(2004)]. Applying this novel procedure to MSWs travel times from detector
array (determined during separation step) permits inverting these data to
spatial distribution of sonic velocities on borehole wall.
Finally, the invention not only introduces new system and outlines
many possible measurements but also presents of new borehole acoustic
application as an example of the invention embodiment. That is, tomographic
characterization of borehole and formation properties providing the
information, which no other method is able to give at present.
In fact, MSWs concept is general and numerous other applications are
imaginable. For example, in principle one can use acoustic measurements and
MSWs concept to detect and evaluate fractures, measure mud slowness,
characterize altered/invaded/damaged zones, estimate formation anisotropy,
detect and characterize layers, beds, etc. Using MSWs measurements will
offer new way to perform these tasks and can provide advantages over
existing techniques.
While the invention has been described with respect to a limited
number of embodiments, those skilled in the art will devise other
embodiments of this invention which do not depart from the scope of the
invention as disclosed therein. Accordingly the scope of the invention should
be limited only by the attached claims.
