CA2706297A1 - 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 - Google Patents
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 Download PDFInfo
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Abstract
The invention relates to the methods 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 system for evaluating the properties. The method is based on mixed surface waveforms measurement and analysis. The method for evaluating characteristic properties comprises registering acoustic signals generated by passage of acoustic waves in said media and determining one or more wave characteristics of mixed surface waves propagating along said interface based on the registered acoustic signals. The characteristic properties of at least one of said media and said interface are calculated on the basis of determined wave characteristics of mixed surface waves.
Description
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.
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.
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.
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.
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 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.
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 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;
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.
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 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 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 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 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, 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 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 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 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 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:
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 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.
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 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.
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 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;
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.
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 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 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 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 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, 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 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 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 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 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:
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 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.
Claims (26)
1. A method for evaluating 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, characterized by 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.
2. The method of claim 1, further comprising the step of exciting acoustic waves in at least one of said media prior to registering acoustic signals.
3. The method of claim 1, wherein said wave characteristics of mixed surface waves are at least one of the travel times, the slowness and the attenuation of said mixed surface waves.
4. The method of claim 1, wherein said 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, shear bulk waves or both in the medium;
- gradient of elastic properties in the medium;
- profile of velocities of compressional waves, 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.
- elastic moduli of the medium;
- tensors of compliances of the medium;
- velocities of compressional waves, shear bulk waves or both in the medium;
- gradient of elastic properties in the medium;
- profile of velocities of compressional waves, 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.
5. The method of claim 1 wherein said 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.
6. A method for evaluating parameters of a fluid in a borehole, of the borehole wall and of a surrounding formation, the method comprising the steps of registering acoustic signals generated by passage of acoustic waves in at least one of the borehole, the formation, and the borehole wall and mixed surface waves propagating along the borehole wall, 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.
7. The method of claim 6, wherein 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 the borehole fluid, and/or formation and/or the borehole wall properties evaluation.
8. The method of claim 6, further comprising 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.
9. The method of claim 8, wherein said acoustic waves are excited by at least one acoustic source displaced from the borehole axis.
10. The method of claim 8, wherein said acoustic waves 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.
11. The method of claim 8, wherein said acoustic signals are registered by at least one acoustic detector.
12. The method of claim 6, wherein said acoustic signals are registered by azimuthally distributed detectors array.
13. The method of claim 8 further comprising 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.
14. The method of claim 8, wherein acoustic waves are excited and acoustic signals are registered by the same means.
15. The method of claim 6, wherein said wave characteristics of mixed surface waves are at least one of the travel times, the slowness and the attenuation of mixed surface waves.
16. The method of claim 6, wherein 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 waves, or shear bulk waves or both in the formation;
- velocity gradient of compressional waves, or shear bulk waves or both in the formation;
- 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.
- 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 waves, or shear bulk waves or both in the formation;
- velocity gradient of compressional waves, or shear bulk waves or both in the formation;
- 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.
17. The method of claim 6, wherein 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 or shear bulk waves or both in the borehole fluid.
- elastic moduli of the borehole fluid;
- tensors of compliances of the borehole fluid;
- velocities of compressional waves or shear bulk waves or both in the borehole fluid.
18. The method of claim 6, wherein said characteristic properties of the borehole wall is its geometrical curvature radii.
19. A system for evaluating parameters of a borehole, the borehole wall and a surrounding formation in accordance with the method of claim 6, the system comprising 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 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.
data processing means 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.
20. The system of claim 18 further comprising 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.
21. The system of claim 19, wherein said means for exciting acoustic waves comprises at least one acoustic source displaced from the borehole axis.
22. The system of claim 19, wherein said means for exciting 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.
23. The system of claim 18, characterized in that said means for registering acoustic waves comprises at least one acoustic detector.
24. The system of claim 18, characterized in that said means for registering acoustic waves comprises azimuthally distributed detectors array.
25. The system of claim 19, wherein 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.
26. The system of claim 19, wherein means for exciting acoustic waves are at the same times means for registering acoustic signals.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| RU2007000633 | 2007-11-19 | ||
| RUPCT/RU2007/000633 | 2007-11-19 | ||
| PCT/RU2008/000310 WO2009067041A1 (en) | 2007-11-19 | 2008-05-26 | 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 |
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| Publication Number | Publication Date |
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| CA2706297A1 true CA2706297A1 (en) | 2009-05-28 |
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| CA2706297A Abandoned CA2706297A1 (en) | 2007-11-19 | 2008-05-26 | 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 |
Country Status (2)
| Country | Link |
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| CA (1) | CA2706297A1 (en) |
| WO (1) | WO2009067041A1 (en) |
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|---|---|---|---|---|
| CN108292432A (en) * | 2015-11-13 | 2018-07-17 | 奥卢大学 | Assessment system and method for determining at least one of macro topological, milli topology, micro- topology and nanometer topology |
| CN113095593A (en) * | 2021-04-30 | 2021-07-09 | 中国石油大学(北京) | Method, device and equipment for determining well wall state of drilling well |
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| WO2009067041A1 (en) | 2009-05-28 |
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