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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims the benefits of European Patent Application No. 15290288.8, filed on Nov. 16, 2015, titled “Cement Evaluation Using the Integration of Multiple Modes of Acoustic Measurements,” the entire content of which is hereby incorporated by reference into the current application. 
       BACKGROUND 
       [0002]    This disclosure relates to evaluating cement behind a casing of a wellbore and, or particularly, to integrating multiple modes of acoustic measurements for cement evaluation. 
         [0003]    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, it should be understood that these statements are to be read in this light. 
         [0004]    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. 
         [0005]    A variety of acoustic tools may be used to determine well integrity, including verifying that cement is properly installed. These acoustic tools may use pulsed acoustic waves as they are lowered through the wellbore to obtain acoustic cement evaluation data (e.g., flexural attenuation, acoustic impedance measurements, etc.). Different modes of acoustic measurements may be measured in a range of acoustic frequencies. For example, some acoustic tools may be used to generate and measure sonic waveforms, ultrasonic waveforms, etc. Moreover, different modes of acoustic measurements may be particularly suitable for different conditions of the wellbore having different characteristics of mud, cement, and/or casing. While current techniques and modes of acoustic measurements may be suitable for various wellbore conditions, interpreting or discriminating more detailed cement characteristics may still be challenging. 
       SUMMARY 
       [0006]    A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief 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. 
         [0007]    Embodiments of this disclosure relate to various systems, methods, and devices for evaluating an annular fill material in a well. Thus, the systems, methods, and devices of this disclosure describe various ways of using acoustic cement evaluation data obtained from acoustic downhole tools to evaluate annular integrity. In one example, a method includes receiving acoustic cement evaluation data into a data processing system. The acoustic cement evaluation data derives from one or more acoustic downhole tools used over a depth interval in a well having a casing. The acoustic cement evaluation data includes sonic measurements and ultrasonic measurements. The method includes deriving a sonic-derived acoustic impedance Z(sonic) from the sonic measurements deriving an ultrasonic-derived acoustic impedance Z(ultrasonic) from the ultrasonic measurements comparing the Z(sonic) with respect to the Z(ultrasonic), and determining whether an annular fill behind the casing is well bonded, partially bonded, comprises wet microannulus, or comprises dry microannulus based on the comparison of the Z(sonic) with respect to the Z(ultrasonic). 
         [0008]    In another example, a computer-readable media includes instructions to receive sonic measurements and ultrasonic measurements from one or more acoustic downhole tools used in a depth interval of a well having a casing, determine a sonic acoustic impedance from the sonic measurements, determine an ultrasonic acoustic impedance from the ultrasonic measurement, compare the sonic acoustic impedance with the ultrasonic acoustic impedance, and based at least in part on the comparison of the sonic acoustic impedance and the ultrasonic acoustic impedance, classify an annulus behind the casing. The instructions further comprise instructions to: (a) classify the annulus as comprising well-bonded annular fill when the ultrasonic acoustic impedance is greater than an expected acoustic impedance and when the sonic acoustic impedance is approximately equal to the expected acoustic impedance; (b) classify the annulus as comprising wet microannulus when the ultrasonic acoustic impedance is less than or equal to the expected acoustic impedance and when the ultrasonic acoustic impedance is greater than the sonic acoustic impedance; (c) classify the annulus as comprising dry microannulus when the ultrasonic acoustic impedance is less than or equal to the expected acoustic impedance and when the ultrasonic acoustic impedance is significantly smaller than the sonic acoustic impedance; or (d) classify the annulus as comprising partially bonded annular fill when the ultrasonic acoustic impedance is less than or equal to the expected acoustic impedance and when the ultrasonic acoustic impedance is approximately equal to the sonic acoustic impedance. 
         [0009]    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 determined individually or in any combination. For instance, various features discussed below in relation to the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The brief 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 
         [0010]    Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
           [0011]      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; 
           [0012]      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; 
           [0013]      FIG. 3  is a block diagram of another acoustic downhole tool to obtain acoustic cement evaluation data relating to material behind casing of the well, in accordance with an embodiment; 
           [0014]      FIG. 4  is a workflow for classifying an annulus in a wellbore based on an integration of sonic-derived and ultrasonic-derived acoustic impedances, in accordance with an embodiment; 
           [0015]      FIG. 5  is a plot illustrating a relationship between acoustic impedances determined from ultrasonic measurements and sonic attenuation measurements for evaluating an annulus, in accordance with an embodiment; 
           [0016]      FIG. 6  is a plot illustrating a relationship between acoustic impedances determined from ultrasonic measurements and sonic amplitude measurements for evaluating an annulus, in accordance with an embodiment; and 
           [0017]      FIG. 7  is flowchart for classifying an annulus, in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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. 
         [0019]    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, it should be understood that 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. 
         [0020]    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 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, this disclosure teaches systems and methods for evaluating acoustic cement evaluation data. As used herein, “acoustic cement evaluation data” refers to any combination of acoustic attenuation data, acoustic amplitude data, acoustic impedance data, flexural attenuation data, and/or other types of acoustic data useful for well integrity analysis that may be obtained from one or more acoustic downhole tools, including tools using sonic measurements, ultrasonic measurements, or both. 
         [0021]    The acoustic cement evaluation data that is obtained by the acoustic downhole tools may be parameterized based on initial assumptions on the characteristics of the well and/or the acoustic downhole tools and further processed to determine conditions of the well. For instance, the acoustic cement evaluation data may include an assumed characteristic of the annulus of the well. However, conventional processing techniques may not always use thorough and accurate assumptions on all well characteristics. For example, conventional techniques may not account for details such as characteristics of microannulus in the annulus in cement evaluation. Such microannuli may include gaps in the interface of the annular fill material and the casing, and may have different characteristics, such as being filled with liquid or gas to be characterized as wet or dry, for example. Yet parameterization errors or inaccuracies could incorrectly predict the actual conditions in the well. As a result, the acoustic cement evaluation data may not accurately reflect the true conditions of the well. In addition, log data may have other ambiguities or uncertainties which may also reduce accuracy in processing and predicting conditions of the well. 
         [0022]    This disclosure teaches various ways to improve the investigation of annulus material in a well using an integration of multiple modes of acoustic measurements. One or more embodiments involve a workflow for integrating ultrasonic measurements and sonic measurements to evaluate characteristics of the annular material. In some embodiments, the acoustic impedance of the annular material may be derived from sonic attenuation measurements and/or from sonic amplitude measurements, and the sonic-derived acoustic impedance may be integrated with acoustic impedance derived from ultrasonic measurements and processed to determine further characteristics of the annulus acoustic impedance, such as whether the annulus acoustic impedance is attributable to fluids and/or solids and/or whether the annulus can be classified as having wet or dry microannulus, etc. 
         [0023]    With this in mind,  FIG. 1  schematically 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, resin, or any other material for filling the annulus  20 ) has been used to seal an annulus  20 —the space between the wellbore  16  and casing joints  22  and collars  24 —with cementing operations. 
         [0024]    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. 
         [0025]    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. 
         [0026]    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, a Sonic Scanner, an UltraSonic Imager (USI) tool and/or an Isolation Scanner 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 pulse echo measurement that exploits the thickness mode (e.g., in the manner of an ultrasonic imaging tool) or may perform a pitch-catch measurement that exploits the flexural mode (e.g., in the manner of an imaging-behind-casing (IBC) tool). These measurements may be used as acoustic cement evaluation data 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 . 
         [0027]    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. 
         [0028]      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 acoustic impedance and/or 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. For example, in some embodiments, all or a portion of the data processing system  38  may be coupled to the acoustic tool  26 , and some or all of the cement evaluation data processing may occur in the wellbore  16 . 
         [0029]    The data processing system  38  may collect the acoustic cement evaluation data  36  and determine well integrity based on processing of the data  36 . For example, the acoustic cement evaluation data  36  may be processed to derive certain characteristics of the annular fill  18 , such as to determine an acoustic impedance of the annular fill. Additionally, the data processing system  38  may integrate multiple types of acoustic cement evaluation data  36 , including multiple modes of acoustic data  36  obtained with different types of acoustic tools  26 , such as those suitable for measuring sonic and/or ultrasonic 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 . 
         [0030]    In this way, the acoustic cement evaluation data  36  from the acoustic logging tool  26  may be used to determine whether 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 . 
         [0031]    In some embodiments, processing acoustic cement evaluation data  36  by integrating various types of acoustic data, such as sonic measurements (e.g., sonic attenuation, sonic amplitude, etc.) with ultrasonic measurements (e.g., ultrasonic acoustic impedance, ultrasonic attenuation, etc.) may result in more accurate and/or more precise cement evaluation than processing one set of data alone. The present techniques involve using acoustic cement evaluation data  36  to indicate additional details when the annular fill  18  is partially bonded and/or when microannuli are present in the annular fill  18 . For example, in some embodiments, the data  36  may be processed to further characterize microannuli, such as discriminate between regions having wet microannulus and dry microannulus. 
         [0032]    With this in mind,  FIG. 2  provides a general example of the operation of the acoustic logging tool  26   a  in the wellbore  16 . Specifically, a transducer  52  in the acoustic logging tool  26  may emit acoustic waves  54  out toward the casing  22 . Reflected waves  56 ,  58 , and  60  may correspond to interfaces at the casing  22 , the annular fill  18 , and the geological formation  14  or an outer casing, respectively. The reflected waves  56 ,  58 , and  60  may vary depending on whether the annular fill  18  is of the generally solid character  48  or the generally liquid or gas character  50 . The acoustic logging tool  26  may use any suitable number of different techniques, including measurements of acoustic impedance from sonic waves, ultrasonic waves and/or flexural attenuation. When one or more of these measurements of acoustic cement evaluation data are obtained, they may be integrated and/or processed to determine characteristics of the annular fill  18 . 
         [0033]      FIG. 3  provides another example embodiment of the acoustic logging tool  26   b  having an emitter  68  and a pair of receiver transducers  70 . The emitter  68  in the acoustic logging tool  26   a  may emit acoustic energy  72  out toward the casing  22  resulting in reflected waves  74 ,  76 , and  78 . In the embodiments shown in  FIG. 2 , the emitted energy excites a predominantly zeroth-order asymmetric mode (also referred to as flexural mode). As in the embodiment described above, the acoustic waves  72  propagate via transmission into both sides of the casing wall  22 . The transmission in the casing annulus depends on the material on the outer side of the casing wall with a different amount of energy leak inside the annulus. The acoustic logging tool embodiment depicted in  FIG. 3  may use measurements of acoustic impedance from flexural attenuation. The different distance from the emitter  68  and the two receiver transducers  70  and the energy leak induce different amplitudes on the measured acoustic pressure. 
         [0034]    In accordance with the present techniques, acoustic cement evaluation data  36  may be obtained from the acoustic logging tool  26   a ,  26   b , and/or any other suitable acoustic logging tool, referred to generally as tool  26 , which may measure sonic, ultrasonic, or a combination of acoustic measurements. Different acoustic measurements may be processed and integrated to determine further characteristics of the annular fill  18 . One or more embodiments involve the integration of sonic measurements with ultrasonic measurements to classify sections of the annulus as having a well cemented section, wet microannulus, dry microannulus, and/or channels with partial bonds. 
         [0035]    Acoustic cement evaluation data may be processed in various ways to result in further details and classifications of the annular fill  18 . For instance, as shown by a workflow  88  of  FIG. 4 , the acoustic cement evaluation data may be obtained by measurements using one or more acoustic tools  26  (block  90 ). These measured acoustic cement evaluation data may include, for example, sonic attenuation measurements  92 , sonic amplitude measurements  98 , and ultrasonic measurements  104 , or some combination of these measurements. For example, in some embodiments, only one type of sonic measurement  92  or  98  may be used in the workflow  88  to determine details and classifications of the annular fill  18 . 
         [0036]    The sonic attenuation measurements  92 , referred to as sonic_att, may be processed to derive (block  94 ) acoustic impedance of the annular fill  18 , referred to as sonic_att-derived acoustic impedance, or Z(sonic_att)  96 . The derivation (block  94 ) of acoustic impedance  96  from sonic attenuation measurements  92  may be based on the casing extensional mode having nearly zero sensitivity to compressional coupling. As the compressional coupling effect on casing extensional mode is negligible, the sonic attenuation measurements may represent a free-pipe reading (i.e., a portion of the casing  22  that is not surrounded by annular fill  18 ) as soon as the annular fill  18  loses its shear coupling to the casing  22 . Once the shear coupling is lost, either due to de-bonding of the annular fill from the casing or due to changes in material (e.g., from solid to fluid or gas) behind the casing at the casing and cement interface, the sonic attenuation measurements may no longer be effective for computing the acoustic impedance of the annular fill. Extending the attenuation formula to relatively low values of acoustic impedance may still allow for the calculation of acoustic impedance if the annular fill  18  maintains the finite shear velocity and shear coupling of the sonic waves. Sonic attenuation measurements may be insensitive to gas or fluid, because the casing extensional mode is insensitive to compressional properties. Therefore, in some embodiments, the processing of sonic attenuation measurements  92  may involve processing sonic_att  92  to first determine sonic amplitude  98 , as attenuation-based measurements alone may not discriminate between liquids and gases. Furthermore, in some embodiments, sonic attenuation  92  may be computed from sonic amplitude  98  at multiple receivers. Therefore, the sonic attenuation  92  may implicitly include information related to sonic amplitude  98 , and such sonic attenuation  92  measurements may also be processed to discriminate between liquids and gases. 
         [0037]    The sonic amplitude measurements  98 , referred to as sonic_amp, may also be processed to derive (block  100 ) acoustic impedance of the annular fill  18 , referred to as sonic_amp-derived acoustic impedance, or Z(sonic_amp)  102 . The derivation (block  100 ) of acoustic impedance  102  from sonic amplitude measurements  98  may be based on the sonic amplitude measurements  98  having sensitivity to both casing mode attenuation and coupling attenuation. Coupling attenuation may be largely sensitive to acoustic impedance and compressional coupling of solids. Different sonic amplitude measurements  98  (also known as cement bond logs, or “CBL measurements”) may be expected when the casing annulus is air or water due to the different and known acoustic impedances of air and water. The coupling attenuation may be sensitive not only to acoustic coupling but also to the geometries within the annulus (e.g., microannulus gap size). For example, sonic amplitude measurements  98  may change gradually (e.g., from approximately 38 mV to 62 mV in free-pipe) when the annular gap is increased from approximately 0 to 10 mm. An acoustic impedance derived from sonic_amp  98  may also be affected by fluid properties due to the sensitivity of the sonic_amp  98  to fluid, and some embodiments involve compensating for this characteristic. The sonic_amp  98  may be increased by approximately 30-40% when gas or air is present behind casing, at least in part due to significantly low coupling attenuation. 
         [0038]    The acoustic impedances Z(sonic_att)  96  and Z(sonic_amp)  102  derived from sonic_att  92  and sonic_amp  98 , respectively, may in some circumstances be interpreted or applied differently in characterizing the annular fill  18 . Both acoustic impedances  96 ,  102  may be representative of the planned acoustic impedance of the annular material  18  in a region where the annular fill  18  is well bonded (e.g., in the absence of free pipe or microannuluses in the annular fill  18 ). In some circumstances, the Z(sonic_att)  96  may represent free-pipe values not only when there is actually free-pipe, but also when the annular fill includes microannulus (dry or wet), highly contaminated and poorly set (or unset) cement with only compressional coupling and no shear coupling. Z(sonic_att) values  96  may also be varying or inconclusive in the presence of azimuthal heterogeneity, partial cement bonding, or presence of channels in the annular fill  18 . 
         [0039]    Z(sonic_amp) values  102  may represent liquids (e.g., mud or gas) when the annulus is filled with liquid. However, the Z(sonic_amp)  102  may vary between acoustic impedances of liquid and cement in the presence of microannuli, depending on the size of the microannuli and the material filling them (e.g., wet or dry microannuli). The Z(sonic_amp)  102  may also return varying or inconclusive in the presence of azimuthal heterogeneity, partial cement bonding, or presence of channels in the annular fill  18 . Z(sonic_amp) values  102  may also represent air if the annular gap is larger than debonding or in the presence of very small microannuli. 
         [0040]    While Z(sonic_att)  96  and Z(sonic_amp)  102  may be derived from different computations and/or from different measurements and may be interpreted differently, Z(sonic_att)  96  and Z(sonic_amp)  102  may also be referred to as simply Z(sonic), representing the acoustic impedance of annular fill  18  that is derived from a sonic measurement, including sonic amplitude measurements  98  or sonic attenuation measurements  92  or both. 
         [0041]    The ultrasonic measurements  104  may include, for example, pulse-echo measurements, flexural attenuation, or any other suitable ultrasonic measurement. The workflow  88  may involve deriving (block  106 ) acoustic impedance  108  from ultrasonic measurements  104 , resulting in ultrasonic-derived acoustic impedance, referred to as Z(ultrasonic)  108 . In some embodiments, Z(ultrasonic)  108  may be further processed. For example, ultrasonic measurements  104  and/or Z(ultrasonic)  108  may be filtered, averaged, and/or different modalities of ultrasonic measurements  104  (e.g., pulse echo, flexural attenuation, etc.) may be combined or processed to obtain a Z(ultrasonic)  108  to be used in the workflow  88 . 
         [0042]    In accordance with the present techniques, the workflow  88  may involve integrating Z(sonic)  96 ,  102  with Z(ultrasonic)  108  to determine more information about the annular fill  18 . In some embodiments, the Z(sonic)  96 ,  102  and Z(ultrasonic)  108  may be parametrically corrected (block  110 ) for processing, though in some embodiments, this correction may not be performed. For example, in some embodiments, the Z(sonic)  96 ,  102  and Z(ultrasonic)  108  may already be sufficiently accurate for the workflow  88 . 
         [0043]    The correction (block  110 ), when performed, may involve providing correction and/or quality control of the impedance values by manual and/or automatic correction of the Z(sonic)  96 ,  102  and Z(ultrasonic)  108  to result in optimum parameters based on the distribution of Z(sonic)  96 ,  102  and Z(ultrasonic)  108  that will lead to reliable estimation of annulus acoustic impedance attributable to fluids and solids in the annulus. 
         [0044]    In some embodiments, the parametric correction (block  110 ) may involve free-pipe analysis. The acoustic impedances  96 ,  102 , and  108  derived from acoustic measurements measured at a free-pipe region should be comparable to the acoustic impedances of the liquids or gases, depending on the fluid in the annulus. Processing parameters for the both sonic and ultrasonic acoustic impedances  96 ,  102 , and  108  may be corrected using a weighted scheme or a user-selected scheme depending on the reliability of a particular measurement in a particular situation. For example, if a free-pipe measurement is obtained and a sonic measurement is known to have very low uncertainty, then the acoustic impedance estimated from the ultrasonic measurements Z(ultrasonic)  108  may be corrected with respect to the relatively low-uncertainty Z(sonic)  96 ,  102 . Similarly, if a free-pipe measurement is obtained and an ultrasonic measurement is known to have very low uncertainty, then the acoustic impedance estimated from the sonic measurements Z(sonic)  96 ,  102  may be corrected with respect to the relatively low-uncertainty Z(ultrasonic)  108 . 
         [0045]    Whether or not the acoustic cement evaluation data is parametrically corrected, the data may be used for classification (block  112 ) of the annulus. As discussed, the acoustic impedances  96 ,  102  derived from sonic attenuation  92  or sonic amplitude  98  may represent different sensitivities to the annulus characteristics (e.g., different sensitivity to microannuli). As such, in some embodiments, different analyses may be used in the integration of sonic and ultrasonic measurements, depending on the type of sonic measurements used. The charts in  FIGS. 5 and 6  represent the different behavior of acoustic impedance depending on characteristic of the sonic measurement  92  or  98 . 
         [0046]    The chart  120  in  FIG. 5  represents an integration of acoustic impedance derived from ultrasonic measurements Z(ultrasonic)  108  plotted with respect to acoustic impedance derived from sonic attenuation measurements Z(sonic_att)  96 . The zones  122 ,  124 ,  126 ,  128 ,  130 ,  132  represent different classifications of the annulus based on the intersection and/or integration of Z(ultrasonic)  108  and Z(sonic_att)  96  at a region where the corresponding ultrasonic and sonic measurements were measured in the wellbore  16 . For example, depending on the integration of Z(ultrasonic)  108  and Z(sonic_att)  96  and using the chart  120 , a region may be characterized as having a gas zone  122 , a liquid zone  124 , or a zone of well-bonded cement  126 . Additionally, due to the integration of Z(ultrasonic)  108  and Z(sonic_att)  96 , additional features of the cemented region may be determined. In some embodiments, based on the integration of Z(ultrasonic)  108  and Z(sonic_att)  96 , the workflow  88  may determine whether a region includes a zone of partial bonding or channels  128 , a zone annular fill having wet microannulus  130 , and/or a zone of annular fill having dry microannulus  132 . 
         [0047]    The chart  140  of  FIG. 6  represents an integration of acoustic impedance derived from ultrasonic measurements Z(ultrasonic)  108  plotted with respect to acoustic impedance derived from sonic amplitude measurements Z(sonic_amp)  102 . As with the chart  120  of  FIG. 5 , the chart  140  of  FIG. 6  also includes zones  122 ,  124 ,  126 ,  128 ,  130 ,  132  representing different classifications of the annulus based on the intersection and/or integration of Z(ultrasonic)  108  and Z(sonic_amp)  102  at a region where the corresponding ultrasonic and sonic measurements were measured in the wellbore  16 . For example, depending on the integration of Z(ultrasonic)  108  and Z(sonic_amp)  102  and using the chart  140 , a region may be characterized as having a gas zone  122 , a liquid zone  124 , or a zone of well-bonded cement  126 . Additionally, due to the integration of Z(ultrasonic)  108  and Z(sonic_amp)  102 , additional features of the cemented region may be determined. In some embodiments, based on the integration of Z(ultrasonic)  108  and Z(sonic_amp)  102 , the workflow  88  may determine whether a region includes a zone of partial bonding or channels  128 , a zone annular fill having wet microannulus  130 , and/or a zone of annular fill having dry microannulus  132 . 
         [0048]    The flowchart  112  in  FIG. 7  provides a different representation for the classification of annulus as represented in the charts  120  and  140 . The flowchart  112  may be a continuation of the classification (block  112 ) step in the workflow  88  of  FIG. 4 . Parametrically corrected acoustic impedances Z(sonic)  150  and Z(ultrasonic)  152  may be used for the classification, though this may not always be used in some embodiments. Whether or not the acoustic impedances are parametrically corrected, Z(sonic)  150  and Z(ultrasonic)  152  may be evaluated and compared, as represented by decision blocks  154 ,  156 ,  158 , and  160 . In some embodiments, Z(sonic)  150  and/or Z(ultrasonic)  152  may be compared with a known acoustic impedance of the annular fill, referred to as Z(a). Z(a) may be an expected, estimated, or planned acoustic impedance of the annular fill, based on known characteristics of the annular fill. 
         [0049]    In some embodiments, the flowchart  112  for classifying the annulus involves determining whether the ultrasonic-derived acoustic impedance Z(ultrasonic)  152  is greater than or equal to Z(a) and determining whether the sonic-derived acoustic impedance Z(sonic)  150  is approximately the same as Z(a). Z(ultrasonic) may be expected to be greater than Z(a) due to the expected shear coupling in the annular fill  18 . Therefore, if the conditions Z(ultrasonic)  152 ≧Z(a) and Z(sonic)  150 =Z(a) are met, as represented by decision block  154 , then the region may be classified as having well-bonded cement. 
         [0050]    If Z(ultrasonic)  152  is less than or equal to Z(a), then the shear coupling expected in the annular fill  18  may not be present, which may be due to different situations or characteristics in the annular fill  18 . If the Z(ultrasonic)  152  is less than or equal to Z(a), then the flowchart  112  may further compare Z(ultrasonic)  152  with respect to Z(sonic)  150 . The workflow  112  may determine (block  156 ) whether Z(ultrasonic)  152  is greater than Z(sonic)  150 , which may be indicative of the annular fill  18  having wet microannulus. Conversely, if Z(ultrasonic)  152  is significantly smaller than Z(sonic)  150 , the workflow  112  may determine (block  158 ) this to be indicative of the annular fill  18  having dry microannulus. One explanation of this is that ultrasonic measurements are more sensitive to gas and responds earlier to dry microannuli. 
         [0051]    Finally, if Z(ultrasonic)  152  is less than or equal to Z(a), and if Z(ultrasonic)  152  is approximately the same as Z(sonic)  150 , then the workflow  112  may determine (block  160 ) this indicative of the annular fill  18  is partially bonded and/or has channels. The workflow  112  may further make or verify this determination if Z(sonic)  150  is less than or equal to Z(a). 
         [0052]    Further refinements are possible, including using further processing or analyses to make even more appropriate classifications of the annulus based on the integration of ultrasonic and sonic measurements. For instance, further corrections may be made to either or both sets of data, and in some embodiments, more than two types of measurements may be integrated to make annular classifications. 
         [0053]    The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments 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.

Summary:
Systems, methods, and devices for evaluating proper cement installation in a well are provided. In one example, a method includes receiving acoustic cement evaluation data into a data processing system. The acoustic cement evaluation data derives from one or more acoustic downhole tools used over a depth interval in a well having a casing. The acoustic cement evaluation data includes sonic measurements and ultrasonic measurements. The method includes deriving a sonic-derived acoustic impedance Z(sonic) from the sonic measurements deriving an ultrasonic-derived acoustic impedance Z(ultrasonic) from the ultrasonic measurements comparing the Z(sonic) with respect to the Z(ultrasonic), and determining whether an annular fill behind the casing is well bonded, partially bonded, comprises wet microannulus, or comprises dry microannulus based on the comparison of the Z(sonic) with respect to the Z(ultrasonic).