Patent Publication Number: US-2023136442-A1

Title: Flexural Wave Measurement for Thick Casings

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 15/973839 filed May 8, 2018, which claims priority to European Patent Application No. 17290062.3 filed May 15, 2017, which are herein incorporated by reference. 
    
    
     BACKGROUND 
     This disclosure relates to processing for flexural wave measurements obtained by an acoustic downhole tool, which may allow flexural measurements to be used even in cased wells with relatively thicker casings. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, these statements are to be read in this light, and not as an admission of any kind. 
     A wellbore drilled into a geological formation may be targeted to produce oil and/or gas from certain zones of the geological formation. To prevent zones from interacting with one another via the wellbore and to prevent fluids from undesired zones entering the wellbore, the wellbore may be completed by placing a cylindrical casing into the wellbore and cementing the annulus between the casing and the wall of the wellbore. During cementing, cement may be injected into the annulus formed between the cylindrical casing and the geological formation. When the cement properly sets, fluids from one zone of the geological formation may not be able to pass through the wellbore to interact with one another. This desirable condition is referred to as “zonal isolation.” Yet well completions may not go as planned. For example, the cement may not set as planned and/or the quality of the cement may be less than expected. In other cases, the cement may unexpectedly fail to set above a certain depth due to natural fissures in the formation. 
     A variety of acoustic tools may be used to verify that cement is properly installed. These acoustic tools may use pulsed acoustic waves as they are lowered through the wellb ore to obtain acoustic cement evaluation measurements. Flexural attenuation is one such cement evaluation measurement, since flexural attenuation is a function of acoustic impedance on both sides of the casing, and therefore depends on the material properties of cement on the other side of the casing. In addition, flexural attenuation has a long history of use in determining whether cement behind a casing in a well has been properly installed. To obtain a flexural attenuation measurement, an ultrasonic acoustic downhole tool may emit pulses in the range of a few hundreds of kilohertz. The cement sheath behind the casing is evaluated by sending a short pressure pulse toward the casing wall that excites elastic waves inside the casing. The propagation of these waves is strongly affected by the casing-cement bond quality and the cement properties. An acoustic beam at oblique incidence onto the casing excites modes of the family of Lamb waves, which are predominantly the zeroth-order antisymmetric (flexural) and symmetric (extensional) modes. Based on the flexural mode response, such as the flexural attenuation, the quality of the cement installation may be estimated and output onto a well log. 
     While the flexural attenuation measurement has a long history of being used accurately and effectively to identify the quality of cement installed in a well, this measurement may not conform to expected models under certain conditions. In particular, flexural attenuation measurements, as may be obtained by downhole acoustic tools and processed under conventional methods, may not conform as expected to well-established cement evaluation models at relatively thicker well casings. Since wells are increasingly being completed using thicker casings, many conventionally obtained and processed flexural attenuation measurements may not accurately predict the quality of cement installed in these wells. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. These aspects are presented merely to provide the reader with a summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     It is believed that, at least for larger casings with thickness of about 16 mm or greater, the extensional mode of certain acoustic waves, such as Lamb waves—as opposed to the flexural mode of the acoustic waves—may form an increasingly larger proportion of the acoustic energy of the acoustic waves that return to the downhole tool. The systems and methods of this disclosure may counteract the detrimental effect of these extensional waves by reducing the contribution of the extensional waves to the measured overall acoustic signal. This may be done by adjusting the acoustic pulse to have a lower center frequency and/or by filtering the acoustic signals that return to the downhole acoustic tool. In one example, since the extensional mode waves tend to have more energy at higher frequencies than the flexural mode waves, an initial acoustic pulse having a lower center frequency than may be generally used with thinner casings be used. For instance, a pulse having a center frequency of less than about 3.2 MHzmm in relation to a casing of about 16 mm or greater (e.g., less than around 200 kHz, or around 125 kHz in certain cases) may be used to excite the casing to produce acoustic waves, such as Lamb waves. Additionally or alternatively, the acoustic waves that return may be filtered using a filter that excludes at least some of the extensional waves. In one example, a low pass filter of the center frequency (e.g., less than about 3.2 MHzmm in relation to a casing of about 16 mm or greater, less than about 200 kHz, or around 125 kHz in certain cases) may be used. The acoustic pulse frequency, filter frequency, and/or filter parameters may vary depending on the expected or known conditions of the well, such as borehole fluid velocity. This may filter out the extensional waves that are produced, which may form an increasingly large proportion of the acoustic response signal for thicker casings. Thus, even for thicker casings, the flexural mode signal may be used to accurately and effectively estimate cement parameters. 
     In one example, a method may include emitting an acoustic signal at a casing in a well. The acoustic signal may excite the casing into generating an acoustic response signal containing acoustic waves, such as Lamb waves. The acoustic waves include flexural waves and extensional waves. The acoustic response signal from the casing in the well may be detected and filtered. Filtering the detected acoustic response signal may reduce a relative contribution of the extensional waves and thereby increase a relative contribution of the flexural waves. The filtered acoustic response signal may be used as a flexural-attenuation measurement, even in wells with relatively larger casings (e.g., casings thicker than  16  mm). 
     In another example, a system may include an acoustic downhole tool and a data processing system. The acoustic downhole tool may be able to be positioned in a cased well and obtain an acoustic measurement of acoustic waves, such as Lamb waves, produced by the casing when excited by an acoustic pulse of less than 200 kHz. The data processing system may filter the acoustic measurement to reduce non-flexural-mode components of the acoustic measurement. 
     In another example, an article of manufacture that includes a tangible, nontransitory, machine-readable media may have instructions to receive and filter an acoustic measurement. The acoustic measurement may have been obtained by a downhole acoustic tool, and may include acoustic waves, such as Lamb waves, produced by an acoustically excited casing having a thickness of 16 mm or greater. The acoustic measurement may be filtered using a filter that at least partially remove frequencies of 200 kHz or greater. 
     Various refinements of the features noted above may be undertaken in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may be made individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The summary presented above is intended to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG.  1    is a schematic diagram of a system for verifying proper cement installation and/or zonal isolation of a well, in accordance with an embodiment; 
         FIG.  2    is a block diagram of an acoustic downhole tool to obtain acoustic cement evaluation data relating to material behind casing of the well, in accordance with an embodiment; 
         FIG.  3    is a flowchart of a method for obtaining and processing a flexural attenuation measurement that may conform to cement evaluation models even for relatively thicker well casings, in accordance with an embodiment; 
         FIG.  4    is a plot comparing measured to modeled flexural attenuation measurements without low-pass-filtering the measured flexural attenuation measurements, in accordance with an embodiment; and 
         FIG.  5    is a plot comparing measured to modeled flexural attenuation measurements when the measured flexural attenuation measurements are low-pass filtered, thereby causing the measured flexural attenuation measurements to better conform to the modeled flexural attenuation measurements, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, some features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions may be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would still be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     When a well is drilled, metal casing may be installed inside the well and cement placed into the annulus between the casing and the wellbore. When the cement sets, fluids from one zone of the geological formation may not be able to pass through the annulus of the wellbore to interact with another zone. This desirable condition is referred to as “zonal isolation.” Proper cement installation may also ensure that the well produces from targeted zones of interest. To verify that the cement has been properly installed, an acoustic downhole tool may use pulsed acoustic waves to obtain acoustic cement evaluation measurements. This disclosure focuses on the flexural attenuation cement evaluation measurement, has a long history of use in determining whether cement behind a casing in a well has been properly installed. To obtain a flexural attenuation measurement, an ultrasonic acoustic downhole tool may emit pulses in the range of a few hundred kilohertz. The cement sheath behind the casing is evaluated by sending a short pressure pulse toward the casing wall that excites elastic waves inside the casing. The propagation of these waves is strongly affected by the casing-cement bond quality and the cement properties. An acoustic beam at oblique incidence onto the casing excites modes of the family of Lamb waves, which are predominantly the zeroth-order antisymmetric (flexural) and symmetric (extensional) modes. Based on the flexural mode response, the quality of the cement installation may be estimated and output onto a well log. 
     As noted above, the flexural attenuation measurement has a long history of being used accurately and effectively to identify the quality of cement installed in a well. As such, the models that have been developed to estimate cement quality based on the flexural attenuation measurement remain particularly valuable. Yet the models may not accurately identify the quality of the cement if the actual flexural attenuation measurement obtained by the acoustic downhole tool does not behave as expected. And as increasingly larger casings are being used to complete wells, the flexural attenuation measurement appears to increasingly exhibit an unexpected behavior with increased casing thickness. Indeed, for larger casings, the flexural attenuation measurements may diverge from the expected modeled behavior. 
     The flexural attenuation measurements are generally obtained by measuring the Lamb waves that return following an acoustic pulse at oblique incidence onto the casing. The angle of incidence of the acoustic signal against the casing may be adapted to the logging fluid velocity to optimize coupling to the flexural mode at the fluid-casing interface, as opposed to other modes of the Lamb waves, such as the Lamb waves of the extensional mode. Since obtaining the flexural attenuation measurements has involved measuring the Lamb waves that return to the downhole tool in response, measuring flexural waves has also involved measuring at least some extensional waves. 
     It is believed that, for larger casings of about  16  mm or greater, the extensional mode of Lamb waves—as opposed to the flexural mode of the Lamb waves—may form an increasingly larger proportion of the acoustic energy of the Lamb waves that return to the downhole tool. In other words, it is believed that as casings increase in thickness, the Lamb waves that are excited on the casing and return to the downhole acoustic tool contain more extensional wave signal and less flexural wave signal. As mentioned above, however, the flexural attenuation measurement models that are used to estimate cement quality depend on flexural wave measurements, not extensional wave measurements. Thus, the increasing proportion of extensional waves in relation to flexural waves among the measured Lamb waves is believed to result in less accurate flexural attenuation measurements for larger casings. 
     It has been discovered that the detrimental effect of the extensional waves may be may be counteracted by carefully filtering the acoustic signals that return to the downhole acoustic tool. Any suitable filter that filters the extensional mode waves from the flexural mode waves may be used. In one example, since the extensional mode waves tend to have higher frequencies than the flexural mode waves, an initial acoustic pulse having a lower center frequency than may be generally used with thinner casings be used. For example, a pulse having a center frequency of 125 kHz may be used to excite the casing to produce Lamb waves. Additionally or alternatively, the Lamb waves that return may be filtered using a filter that excludes at least some of the extensional waves. In one example, a low pass filter of the center frequency (e.g., 125 kHz in certain cases) may be used. The filter parameters may vary depending on the expected or known conditions of the well, such as borehole fluid velocity. This may filter out the extensional waves that are produced, which may form an increasingly large proportion of the acoustic response signal for thicker casings. In some cases, a higher center frequency may be used (e.g., 250 kHz) under conditions where the casing is thin (e.g., less than about 16 mm) or where the mud is not highly attenuative (e.g., is less than or equal to about 16 dB/cm/MHz), while a lower center frequency may be used (e.g., 125 kHz) may be used under conditions where both the casing is thick (e.g., greater than about 16 mm) and the mud is highly attenuative (e.g., is greater than about 16 dB/cm/MHz). Based on the techniques of this disclosure, even for thicker casings, the flexural mode signal may be used to accurately and effectively estimate cement parameters that might otherwise be indiscernible—in other words, the systems and methods of this disclosure provide unconventional techniques to solve a technical problem in industry. 
     With this in mind,  FIG.  1    illustrates a system  10  for evaluating cement behind casing in a well. In particular,  FIG.  1    illustrates surface equipment  12  above a geological formation  14 . In the example of  FIG.  1   , a drilling operation has previously been carried out to drill a wellbore  16 . In addition, an annular fill  18  (e.g., cement) has been used to seal an annulus  20 —the space between the wellbore  16  and casing joints  22  and collars  24 —with cementing operations. 
     As seen in  FIG.  1   , several casing joints  22  (also referred to below as casing  22 ) are coupled together by the casing collars  24  to stabilize the wellbore  16 . The casing joints  22  represent lengths of pipe, which may be formed from steel or similar materials. In one example, the casing joints  22  each may be approximately 13 m or 40 ft long, and may include an externally threaded (male thread form) connection at each end. A corresponding internally threaded (female thread form) connection in the casing collars  24  may connect two nearby casing joints  22 . Coupled in this way, the casing joints  22  may be assembled to form a casing string to a suitable length and specification for the wellbore  16 . The casing joints  22  and/or collars  24  may be made of carbon steel, stainless steel, or other suitable materials to withstand a variety of forces, such as collapse, burst, and tensile failure, as well as chemically aggressive fluid. 
     The surface equipment  12  may carry out various well logging operations to detect conditions of the wellbore  16 . The well logging operations may measure parameters of the geological formation  14  (e.g., resistivity or porosity) and/or the wellbore  16  (e.g., temperature, pressure, fluid type, or fluid flowrate). Other measurements may provide acoustic cement evaluation data (e.g., flexural attenuation and/or acoustic impedance) that may be used to verify the cement installation and the zonal isolation of the wellbore  16 . One or more acoustic logging tools  26  may obtain some of these measurements. 
     The example of  FIG.  1    shows the acoustic logging tool  26  being conveyed through the wellbore  16  by a cable  28 . Such a cable  28  may be a mechanical cable, an electrical cable, or an electro-optical cable that includes a fiber line protected against the harsh environment of the wellbore  16 . In other examples, however, the acoustic logging tool  26  may be conveyed using any other suitable conveyance, such as coiled tubing. The acoustic logging tool  26  may be, for example, an UltraSonic Imager (USI) tool and/or an Isolation Scanner (IS) tool by Schlumberger Technology Corporation. The acoustic logging tool  26  may obtain measurements of acoustic impedance from ultrasonic waves and/or flexural attenuation. For instance, the acoustic logging tool  26  may obtain a measurement of the flexural mode. These measurements may be used as acoustic cement evaluation data using any suitable flexural attenuation measurement models, which may be based on the wealth of historical flexural attenuation measurements and/or computer model data. For example, the flexural attenuation measurements may be used in a solid-liquid-gas (SLG) model map to identify likely locations where solid, liquid, or gas is located in the annulus  20  behind the casing  22 , and/or may be used to generate a well log indicating where the cement in the annulus  20  behind the casing  22  has or has not set. 
     The acoustic logging tool  26  may be deployed inside the wellbore  16  by the surface equipment  12 , which may include a vehicle  30  and a deploying system such as a drilling rig  32 . Data related to the geological formation  14  or the wellbore  16  gathered by the acoustic logging tool  26  may be transmitted to the surface, and/or stored in the acoustic logging tool  26  for later processing and analysis. As will be discussed further below, the vehicle  30  may be fitted with or may communicate with a computer and software to perform data collection and analysis. 
       FIG.  1    also schematically illustrates a magnified view of a portion of the cased wellbore  16 . As mentioned above, the acoustic logging tool  26  may obtain acoustic cement evaluation data relating to the presence of solids, liquids, or gases behind the casing  22 . For instance, the acoustic logging tool  26  may obtain measures of flexural attenuation, which may be used to determine where the material behind the casing  22  is a solid (e.g., properly set cement) or is not solid (e.g., is a liquid or a gas). When the acoustic logging tool  26  provides such measurements to the surface equipment  12  (e.g., through the cable  28 ), the surface equipment  12  may pass the measurements as acoustic cement evaluation data  36  to a data processing system  38  that includes a processor  40 , memory  42 , storage  44 , and/or a display  46 . In other examples, the acoustic cement evaluation data  36  may be processed by a similar data processing system  38  at any other suitable location. The data processing system  38  may collect the acoustic cement evaluation data  36  and determine whether such data  36  represents a solid, liquid, or gas using a solid-liquid-gas (SLG) model map or any other suitable models that are based at least in part on flexural attenuation measurements. To do this, the processor  40  may execute instructions stored in the memory  42  and/or storage  44 . As such, the memory  42  and/or the storage  44  of the data processing system  38  may be any suitable article of manufacture that can store the instructions. The memory  42  and/or the storage  44  may be ROM memory, random-access memory (RAM), flash memory, an optical storage medium, or a hard disk drive, to name a few examples. The display  46  may be any suitable electronic display that can display the logs and/or other information relating to classifying the material in the annulus  20  behind the casing  22 . 
     In this way, the acoustic cement evaluation data  36  from the acoustic logging tool  26  may be used to determine whether cement of the annular fill  18  has been installed as expected. In some cases, the acoustic cement evaluation data  36  may indicate that the cement of the annular fill  18  has a generally solid character (e.g., as indicated at numeral  48 ) and therefore has properly set. In other cases, the acoustic cement evaluation data  36  may indicate the potential absence of cement or that the annular fill  18  has a generally liquid or gas character (e.g., as indicated at numeral  50 ), which may imply that the cement of the annular fill  18  has not properly set. For example, when the indicate the annular fill  18  has the generally liquid character as indicated at numeral  50 , this may imply that the cement is either absent or was of the wrong type or consistency, and/or that fluid channels have formed in the cement of the annular fill  18 . By processing the acoustic cement evaluation data  36 —for example, by filtering away extensional wave measurements to achieve purer flexural wave measurements—the character of the annular fill  18  may be more accurate and/or precise because the flexural attenuation measurements obtained in this way may better conform to established cement evaluation models. Indeed, any suitable cement evaluation models that consider the flexural attenuation measurement may be used. 
       FIG.  2    provides a general example of the operation of the acoustic logging tool  26  in the wellbore  16 . Specifically, an acoustic transmitter  52  in the acoustic logging tool  26  may emit an acoustic signal  54  at oblique incidence onto the casing  22 . This excites modes of the family of Lamb waves  56  in response, which are predominantly the zeroth-order antisymmetric (flexural) and symmetric (extensional) modes. The angle of incidence of the acoustic signal  54  may be adapted to the logging fluid velocity to optimize coupling to the flexural mode at the fluid-casing interface to produce the Lamb waves  56 . The Lamb waves  56  may be detected by a pair of receiver transducers (e.g., a first receiver transducer  58  and a second receiver transducer  60 ) that are disposed different respective distances from the transmitter  52 . 
     In general, Lamb waves exhibit dispersion, which means that the propagation velocity depends on the frequency. Therefore, the envelope of a short-pulsed, and therefore broadband, acoustic Lamb wave pulse propagates with a certain group velocity. The temporal width of the envelope, however, increases with time as different frequency constituents of the wave packet move at different speeds. One specific characteristic of the antisymmetric flexural mode of the Lamb waves may be particularly valuable: namely, that the group velocity of the antisymmetric flexural mode is only weakly frequency-dependent for a large range of casing thicknesses. Thus, the attenuation of the flexural mode of the Lamb waves  56 , as measured by the receiver transducers  58  and/or  60 , thus provides reliable information on the cement, in particular for light cement. The second Lamb wave type, the extensional mode with symmetric particle displacement, is generally also excited alongside the flexural mode in the Lamb waves  58 . But for flexural attenuation cement log interpretations, which are based on the flexural attenuation measurement, the presence of the extensional mode is undesired. In logging thin to medium-thickness casings (e.g., a wall thickness of less than about  16  mm), the flexural wave excitation can be selectively favored over the extensional excitation by a judicious choice of transducer angles. 
     Thus, when logging thicker casings (e.g., above a wall thickness of 16 mm, which is increasingly being used in completing wells), the interpretation may be less robust. And in many situations where the fluid within the casing has a high attenuation (e.g., greater than 16 dB/cm/MHz), pulse frequencies according to the same frequency-thickness product used for thinner casings may prove unacceptable. The systems and methods of this disclosure may overcome some of the challenges of logging thicker casings. 
     Indeed, to obtain an improved flexural attenuation measurement, even when logging thicker casings (e.g., above a wall thickness of 16 mm), the impact of extensional wave modes may be reduced by emitting the acoustic signal  54  at a lower center frequency than used for thinner casings. In some cases, where the frequency-thickness product used for thinner casings is about 3.2 MHz·mm (e.g., about 200 kHz for an 16 mm casing or about 400 kHz 8 mm casing), a lower frequency-thickness product (e.g., less than about 3.2 MHz·mm for a 16 mm casing, such as about 2.5 MHz·mm) may be used for thicker casings. This may amount to less than about 200 kHz, between about 100 to 150 kHz, or about 125 kHz in some situations. The pulse center frequency may be used in a filter that excludes some of the extensional modes from the measurement of the Lamb waves  56 . For example, as shown by a flowchart  70  of  FIG.  3   , the acoustic logging tool  26  may be placed into the cased wellbore  16  (block  72 ). The acoustic logging tool  26  may emit an acoustic signal  54  that excites Lamb waves  56  due to an interaction of the acoustic signal with the casing  22  (block  74 ). The acoustic logging tool  26  may emit any suitable acoustic signal  54  of any suitable frequency. In certain examples, for relatively thinner casings, the frequency may be higher (e.g., about 250 kHz), and for relatively thicker casings, the frequency may be lower (e.g., less than about 200 kHz, such as about 125 kHz). In general, however, the acoustic logging tool  26  may emit a pulse having any suitable frequency lower than 500 kHz (e.g., 125 kHz, 150 kHz, 175 kHz, 200 kHz, 225 kHz, 250 kHz, 275 kHz, 300 kHz, 325 kHz, 350 kHz, 375 kHz, 400 kHz, 425 kHz, 450 kHz, 475 kHz, 500 kHz, or the like), but other frequencies higher than 500 kHz or lower than 100 kHz may also be used, provided that acoustic signals of those frequencies excite Lamb waves in the casing  22 . 
     The acoustic logging tool  26  may obtain measurements of the Lamb waves  56  that result as an acoustic response to the originally emitted acoustic signal  54  (block  76 ). Any suitable filter that excludes at least some non-flexural-wave components may be applied to the measurement of the Lamb waves  56  (block  78 ). The resulting filtered acoustic response signal may contain a greater proportion of energy due to flexural attenuation and a lower proportion of energy due to other acoustic components, such as extensional waves. This resulting filtered acoustic response signal may be used to estimate cement quality using any suitable models that are based at least in part on flexural attenuation (block  80 ). Because the filtered acoustic response signal is due more to the desirable flexural waves and less to other less-desirable modes (e.g., extensional waves) than the unfiltered acoustic response signal, the filtered acoustic response signal may provide a more accurate estimate of the cement quality than the unfiltered acoustic response signal, even if the filter does not fully remove the non-flexural-wave components from the unfiltered acoustic response signal. The resulting estimate of cement quality may be output in a well log or used in any other suitable cement evaluation visualization or presentation (block  82 ). 
     Any suitable form(s) of filtering may be applied at block  78  of the flowchart  70 . In one example, the acoustic response signal obtained by the acoustic logging tool  26  may be digitized and provided as acoustic cement evaluation data  36  to the data processing system  38 . The data processing system  38  may process the acoustic response signal, including by applying a filter that filters out non-flexural-mode waves from the acoustic response signal. One example of such a filter may be a low pass filter (or any other suitable filter, such as a band pass filter or band gap filter) that may exclude at least some frequencies that tend to include more extensional components and fewer flexural components. For example, it has been identified that an acoustic signal  54  having a center frequency of about 125 kHz may produce better results for relatively thicker casings (e.g., of  16  mm thickness or greater) than the 250 kHz signal that may produce better results for a thinner casing (e.g., of less than 16 mm thickness), particularly when the attenuation of the fluid in the casing is relatively high (e.g., greater than 16 dB/cm/MHz). 
     Moreover, the parameters of the filter may vary based at least partly on the conditions of the wellbore  16 . For example, a filter length parameter may be selected at least in part on different mud attenuations. When the mud attenuation is higher, the filter aggressiveness (e.g., length) may be lower, and when the mud attenuation is lower, the filter aggressiveness may be higher. In one example, the filter length may have more nonzero coefficients (e.g., 20 or more coefficients, between 20 and 30 coefficients, or about 25 coefficients) for lower-attenuation fluids (e.g., fluids of less than about 16 dB/cm/MHz), and the filter may have fewer nonzero coefficients (e.g., 20 or fewer coefficients, between 10 and 20 coefficients, or about 15 coefficients) for higher-attenuation fluids (e.g., fluids of 16 dB/cm/MHz or more). Any other suitable filter parameters may vary depending on the casing thickness and/or attenuation of casing fluid to provide more aggressive filtering when the mud attenuation is lower and less aggressive when the mud attenuation is higher. Filter parameters that may be adjusted may include, among other things, the way the coefficients are applied in the filter, which may vary, for example, depending on the mud attenuation. 
     Additionally or alternatively, the acoustic response signal may be filtered using analog circuitry or even mechanically through the design of the receiver transducers  58  or  60 . For instance, the electrical signals from the receiver transducers  58  and  60  may be filtered through analog electrical circuitry that filters out extensional waves in favor of flexural waves (e.g., via an analog low pass filter). Additionally or alternatively, the receiver transducers  58  and  60  may be designed so as to mechanically filter out extensional waves in favor of flexural waves. 
       FIGS.  4  and  5    illustrate the effect of filtering out non-flexural-mode waves from the acoustic response signal detected by an acoustic logging tool  26 . In  FIG.  4   , a plot  100  relates flexural attenuation in units of dB/m (ordinate  102 ) to casing thickness in units of mm (abscissa  104 ) in a zero-attenuation fluid. Modal model curves  106 ,  108 , and  110  represent 1D models for values of flexural attenuation that depend on frequency. In this example, the modal model curve  106  represents a modal model of flexural attenuation for 250 kHz frequency, the modal model curve  108  represents a modal model of flexural attenuation for 200 kHz frequency, and the modal model curve  110  represents a modal model of flexural attenuation for 125 kHz frequency (for comparison). A measured flexural attenuation curve  112  represents reprocessed lab data acquired using a 250 kHz firing pulse with zero mud attenuation. While there is an offset at the 8 mm casing thickness, this may be removed by calibration. As can be seen, however, the measured flexural attenuation curve  112  does not follow the same general trajectory as the modal model curves  106 ,  108 , or  110 . Indeed, beyond 16 mm of thickness, the measured flexural attenuation curve  112  begins to trend upward, increasing as the casing thickness increases. The divergence between the measured flexural attenuation curve  112  and the modal model curves  106 ,  108 , and  110  is believed to be due to extensional wave contamination. Note that the 250 kHz model (curve  106 ) is in better agreement with the measured flexural attenuation curve  112 . It is believed that the spectra contain significant energy above 250 kHz. 
     While the plot  100  of  FIG.  4    illustrates the effect of using measured flexural attenuation data that is not filtered, a plot  120  shown in  FIG.  5    illustrates the effect of using measured flexural attenuation data that has been filtered and that is based on an acoustic pulse with center frequency of about 125 kHz. In  FIG.  5   , the plot  120  relates flexural attenuation in units of dB/m (ordinate  102 ) to casing thickness in units of mm (abscissa  104 ) in a zero-attenuation fluid. The modal model curves  106 ,  108 , and  110  are reproduced in  FIG.  5   , representing 1D models for values of flexural attenuation that depend on frequency. Thus, the modal model curve  106  represents a modal model of flexural attenuation for 250 kHz frequency, the modal model curve  108  represents a modal model of flexural attenuation for 200 kHz frequency, and the modal model curve  110  represents a modal model of flexural attenuation for 125 kHz frequency (for comparison). A filtered measured flexural attenuation curve  122  represents reprocessed lab data acquired using a 125 kHz firing pulse with zero mud attenuation, and which has been filtered using a low pass filter of 125 kHz. 
     In contrast to the plot  100  of  FIG.  4   , the measured flexural attenuation curve  122  does follow the same general trajectory as the modal model curves  106 ,  108 , and  110 . Even beyond  16  mm of thickness, the measured flexural attenuation curve  122  tends to track the general shape of the model curves  106 ,  108 , and  110  as the casing thickness increases. The reduced divergence between the measured flexural attenuation curve  122  and the modal model curves  106 ,  108 , and  110  is believed to be due to the reduction in extensional wave contamination by low-pass-filtering away the extensional wave energy at frequencies above 125 kHz. 
     The specific embodiments described above have been shown by way of example, and may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.