Abstract:
A method and apparatus useful to determine the integrity of a cement bond log disposed in the annular space between a casing and a wellbore. The method and apparatus produce a transversely polarized shear wave and emit the wave through the casing and into the wellbore. The transversely polarized shear wave attenuates upon passage through the cement bond log. The integrity of the cement bond log can be determined through an analysis and evaluation of the attenuation results.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to the field of production of hydrocarbons from wellbores. More specifically, the present invention relates to a method and apparatus to evaluate the integrity of bonds that adhere wellbore casing to a wellbore. 
   2. Description of Related Art 
   Hydrocarbon producing wellbores  2  are drilled from the surface  16  into a subterranean formation  17  containing hydrocarbons entrained therein. Set within the wellbore  2  is casing  4  bonded to the inner surface of the wellbore  2 . The casing is bonded within the wellbore  2  by adding cement  6  within the annulus formed between the outer diameter of the casing  4  and the inner diameter of the wellbore  2 . The resulting cement bond not only adheres the casing  4  within the wellbore  2 , but also serves to isolate adjacent zones (Z 1  and Z 2 ) within the formation  17  from one another. Isolating adjacent zones can be important when one of the zones contains oil or gas and the other zone includes a non-hydrocarbon fluid such as water. Should the cement  6  surrounding the casing  4  be defective and fail to provide isolation of the adjacent zones, water or other undesirable fluid can migrate into the hydrocarbon producing zone thus diluting or contaminating the hydrocarbons within the producing zone. 
   To detect possible defective cement bonds, downhole tools  8  have been developed for analyzing the integrity of the cement  6  bonding the casing  4  to the wellbore  2 . These downhole tools  8  can be disposed within the wellbore  2  on a wireline  10  that is connected to a surface truck  14  via a pulley system  12 . Typically, transducers  18  are disposed on the outer surface of the tool  8  capable of emitting acoustic waves into the casing  4  and recording the attenuation of the acoustic waves as they travel, or propagate, across the surface of the casing  4 . The transducers  18  can either only transmit and receive, or can include those capable of transmitting acoustic signals and receiving a corresponding acoustic signal propagating along the casing. By analyzing the propagation velocity and attenuation of the received acoustic wave, the efficacy and integrity of the cement bond can be evaluated. As is known, pads  19  can be attached to the outer surface of the downhole tool  8  that provide a pedestal on which the transducers  18  can be mounted. 
   The amount of attenuation however can depend on how the acoustic wave is polarized and coupling condition between the casing  4  and the cement  6  bonding the casing  4  to the wellbore  2 . Typical downhole tools  6  having acoustic wave transducers  18  generate acoustic waves that are polarized perpendicular to the surface of the casing  4 . Such waves are referred to as compression/shear or P-SV waves since the particle motion direction of either compressional (P) or shear (S) component of the acoustic wave is in a vertical (V) plane perpendicular to the casing  4 . The attenuation of the acoustic wave as it propagates along the surface of the casing  4  varies in response to the condition of the cement bond and also in response to the type of cement  6  disposed between the casing  4  and the formation  17 . More specifically, as the acoustic wave propagates along the length of the casing  4 , the wave loses, or leaks, energy into the formation  17  through the cement bond—it is this energy loss that produces the attenuation of the acoustic wave. 
   Conversely, when the casing  4  is not bonded, a condition also referred to as “free pipe”, fluid from the formation  17  surrounds the casing  4  instead of cement  6 . The fluid behind the casing  4  does not provide for shear coupling between the casing  4  and the formation  17 . Loss of shear coupling significantly reduces the compressional coupling between the casing  4  and the formation  17 . This result occurs since fluid has no shear modulus as well as a much lower bulk modulus in relation to cement. Because of these physical characteristics of fluid, the entire SV component of the P-SV wave and a large portion of the P component of the P-SV wave do not propagate outside of the casing  4  and thus experience a much reduced attenuation. 
   Reduced attenuation of an acoustic wave is not limited to situations where the casing  4  is surrounded by fluid, but the presence of some cements can also significantly reduce acoustic wave attenuation. For example, light weight cement (LWC), or cement having a density less than approximately 12 lbs/gal can reduce acoustic wave attenuation. Light weight cement has a shear modulus, thus light weight cement can maintain shear coupling between the casing  4  and the formation  17 . However, the density of light weight cement is not substantially greater than the density of many fluids (such as water), thus the attenuation of some acoustic waves, especially P-SV waves, is diminished when encountering casing  4  surrounded by a light weight cement. The result of this reduced attenuation is a decreased ability to detect the difference between fluid and light weight cement as well as a diminished capacity to detect poor cement bonds in light weight cement. 
   In spite of recent advances in the development of casing bond interrogation devices, room for improving the accuracy and preciseness of these devices still exists. 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the present invention includes a method of evaluating a casing bond disposed between a casing and a wellbore comprising, (a) inducing a shear wave into the casing, (b) monitoring the shear wave, (c) inducing a Lamb wave into the casing, (d) monitoring the Lamb wave, and (e) estimating a characteristic of the casing bond based on the monitoring. Additionally, the present method includes determining the casing bond integrity. The step of determining the bond integrity may be accomplished by monitoring the shear wave. The method considered herein may further include determining the presence of a micro-annulus as well as determining the presence of free pipe conditions. The steps of inducing the shear wave and inducing the Lamb wave can occur sequentially or simultaneously. The shear wave can be a horizontal wave (transversely polarized shear wave), a vertically polarized shear wave any other type of polarized shear wave, and combinations thereof. The Lamb wave of this method includes both symmetric, asymmetric, and all wave modes thereof. 
   The step of determining the presence of a micro-annulus can be accomplished by monitoring the Lamb wave. The size of a micro-annulus can be calculated based on the step of monitoring the Lamb wave. The method may also include determining bond properties, wherein the properties can be shear, compressional, density, and combinations thereof. 
   The step of determining the presence of a free pipe conditions may be accomplished by monitoring the Lamb wave. The method can also further include determining attenuations of different Lamb modes. The casing bond to be evaluated can comprise cement and the method can further comprise determining the compressional velocity of the cement as well as a step of estimating the thickness of the casing bond. The type of casing bond includes regular cement, light weight cement, and free pipe. 
   The step of inducing the shear wave and the Lamb wave can be accomplished by a piezoelectric device, an electrically magnetized acoustic transmitter, a pulsed laser device, a flexural resonator, or combinations thereof. The step of monitoring the shear wave and the Lamb wave may be accomplished by a piezoelectric device, an electrically magnetized acoustic transmitter, or combinations thereof. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING. 
       FIG. 1  is a partial cutaway side view illustration of a logging tool within a wellbore. 
       FIG. 2  is a flow chart of a method of determining cement bond properties and characteristics. 
       FIG. 3  shows in perspective view shear wave particle movement on a plate. 
       FIGS. 4   a  and  4   b  are a perspective view of particle movement of a Lamb wave in a plane. 
       FIG. 5  is a cutaway view showing wave movement through a plate thickness. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention includes a method and apparatus useful in determining characteristics of a casing bond disposed between a casing and a wellbore. The characteristics of the casing bond include the quality of the casing bond, the integrity of the cement that comprises the bond, the type of cement, and the thickness of the casing bond. The method generally involves inducing different types and modes of acoustic waves in the casing proximate to the location where the casing bond is to be evaluated. The acoustic waves propagating within the casing can then be monitored to estimate characteristics of the casing bond in an area referred to as the region or zone of interest. It is well within the scope of those skilled in the art to ascertain casing bond characteristics based on monitoring the induced acoustic waves, furthermore, this can be accomplished without undue experimentation. 
   One embodiment of the method of the present disclosure described herein is illustrated in flowchart form in  FIG. 2 . In the embodiment as illustrated, the method is initiated after lowering a downhole tool  8  within a well bore  2 . More specifically, the downhole tool  8  should be in proximate to the portion of the casing in which casing bond information is to be obtained, i.e. the region of interest. Signals, such as acoustic waves are then produced by a transducer  18  and directed from the downhole tool  8  into the surrounding casing  4 . Directing the signals into the casing  4  thereby induces a corresponding wave that propagates through the casing  4 . 
   In the embodiment of the method described herein, shear waves are initially induced into the casing by the downhole tool  8  (step  30 ). The corresponding attenuations of the shear waves can then be recorded by a receiver transducer. As previously noted, the waves can be induced within the casing  4  by a transmitting transducer, and the propagation of the waves through the casing  4  can be recorded with a receiver transmitter. Optionally, each transmitting transducer can also function as a receiving transducer thereby inducing waves into the casing  4  as well as receiving and recording induced waves. 
   The types of transducers considered for use with the present method is not confined to a single type of transmitter, but can include any transducer capable of inducing a signal within the associated casing  4  and receiving and recorded the subsequently formed acoustical wave as it travels to the casing  4 . Examples of suitable transducers include piezo-electric devices, electro magnetic acoustic transducers, wedge type transducers, pulsed laser devices, flexural resonators, and any other currently used or later developed device capable of creating such a signal within the casing  4 . 
   As shown in step  30  of the method of  FIG. 2 , the shear waves induced within the casing  4  can be conducted at various modes. Once the attenuated modes are recorded, the magnitudes of the values of those attenuations can then be analyzed (step  32 ). If it is determined that the magnitude of the measured shear modes is at or close to 0, this can be an indication of free pipe surrounding the analyzed region, or a micro annulus  20 . Conversely, if the measured attenuation value of the induced shear waves is greater than 0, this can indicate the presence of a good bond in the area of interest (step  33 ). The present method disclosed herein then involves the subsequent application of acoustic signals having the form of Lamb modes within the casing, irrespective of the results of the shear wave analysis. 
   Once the results of the shear wave interrogation reveals that a poor or no bond exists around the region of interest, additional information can be gathered about the casing by application of Lamb mode waves in this region (step  34 ). Similar to inducing the shear waves within the casing, the Lamb modes are induced within the casing and their subsequent attenuations are recorded and measured with the downhole tool  8 . The Lamb waves can be generated by the same acoustical source used to generate the shear waves. Optionally, different acoustical sources can be used for this application. Knowing the Lamb mode attenuation within the casing  4 , the presence of a micro annulus or free pipe situation (step  34 ) within the zone of interest can then be determined. The determination of a free pipe situation or a micro annulus can be evaluated either by an empirically derived comparison, or by a comparison to experimentally obtained attenuation results. Test data can be acquired by sampling Lamb wave attenuation data on a test stand comprising casing bonded by cement, where the cement has free pipe regions and micro annulus regions. Also, the test stand model should be representative of the casing and cement that is disposed in the wellbore  2  being sampled. Having sampled the downhole data and the test stand data, these results can then be compared for distinguishing between a micro annulus condition or a free pipe condition. It is well within the scope of those skilled in the art to analyze Lamb mode attenuation data and determine the presence of either micro annulus or a free pipe situation based on that data (step  36 ). Optionally, mathematic modeling can be implemented to determine the presence of micro annulus or free pipe in the zone of interest. Should it be determined that free pipe surrounds the area of interest, the method allows for an indication of that situation (step  40 ). 
   If it is determined that the area of interest is surrounded by or includes a micro annulus, the size of the micro annulus as well as the cement properties in that region can be calculated (step  38 ). Some of these properties include the shear, the compressional value, the density, as well as combinations of these properties. The way that these values can be determined is similar or identical to the way that the presence of a free pipe or micro annulus is determined. For example, test data from a test stand of cement having known properties can be determined with the above described procedure, and these values can then be compared with the data recorded from within the wellbore  2 . Typically, the attenuation of Lamb waves through a microannulus is less than the Lamb wave attenuation in a bonded situation but greater than Lamb wave attenuation in free pipe. 
   As previously discussed, upon determination that the induced shear waves are measureable, i.e. have a magnitude of greater than 0, it can then be deduced that the cement bond surrounding an area of interest is adequate (step  33 ). Subsequent to the determination of the adequacy of the bond, the shear wave attenuation can be further evaluated to determine the cement density as well as the shear velocity of the cement (step  35 ). Evaluating cement density and shear wave velocity based upon on shear wave mode attenuation, is done much in the same manner as evaluating free pipe or micro annulus presence and magnitude. That is, the shear wave attenuation can be compared to empirically derived data to obtain quantitative values for cement density and shear wave velocity. 
   Lamb waves can then be induced into the cement bond, such as by with transducers as previously described, and the attenuation of these Lamb modes propagating through the casing  4  can be received and recorded with these transducers (step  37 ). After receiving the Lamb modes of step  37 , the compressional velocity of the cement can then be determined as well (step  39 ). As discussed below, the Lamb waves considered for use with the present method include symmetric and asymmetric, and all modes thereof. It should be pointed out that inducement of the Lamb wave may occur subsequent to that of the shear wave or simultaneously with initiation of the shear wave. 
   Referring now to  FIG. 3 , representations of a shear waveform in a horizontal configuration are shown propagating within a wave medium  42 . This waveform can also be referred to as a transversely polarized shear wave (TPSW). A series of arrows  44  are provided to illustrate how the shear wave propagates through the wave medium  42 . Arrows  46  demonstrates how a horizontal shear wave displaces particles within a medium. As shown, the particle displacement is in the horizontal plane of the medium in which it is traveling. While a horizontally polarized shear wave is shown, the use of shear waves with regard to the present method can include those vertically polarized, or any other configuration, orientation, or polarized direction of shear waves. The frequencies and wave lengths of the induced shear waves can be chosen based on the characteristics of the particular transducer creating the waves as well as the wave modes used. It is within the scope of skilled artisans to choose such frequencies and wave lengths. Examples of acoustic sources for creating shear waves include electro magnetic acoustic transducers (EMAT) as well as wedge type transducers. 
     FIGS. 4   a  and  4   b  illustrate examples of Lamb wave motion. In each of these figures the wave motion is illustrated by a series of vertical arrows that demonstrate the amplitude of the wave motion as well as oblique arrows point along the edge of the wave medium illustrating the propagation of the wave passing through the wave medium. It should be pointed out that the wave form of  FIG. 4   a  represents a symmetric Lamb wave  48  and the wave form as shown in  FIG. 4   b  represents an asymmetric Lamb wave  50 . Lamb waves are similar to longitudinal waves, with compression and rarefaction, and they bound together by the sheet or plate surface causing a wave guide effect. Lamb waves can be a complex vibrational wave capable of traveling through the entire thickness of the wave medium  42 . Propagation of the Lamb waves is dependent upon density, elasticity, and material properties of the wave medium. These waves are also influenced a great deal by the frequency and the material thickness. With Lamb waves, many modes of particle vibration are possible, however the two most common are the symmetrical and the asymmetrical modes. The frequency and wave lengths of the induced Lamb waves can be chosen based on the characteristics of the particular transducer creating the waves as well as the wave modes used. It is within the scope of skilled artisans to choose such frequencies and wave lengths. Lamb waves can be induced by piezo-electric devices, electro magnetic acoustic transducers, as well as wedge type transducers. 
   Lamb waves may result from the constructive interference of P (compressional) and S v  (shear vertical) type of waves. When introduced into a well casing, these waves typically propagate around the circumference or axis of the casing. However, such propagation is not limited to circumferential travel, but also includes axial travel, propagation in a helical pattern, and any other pattern of wave propagation through and/or along casing. Very often, the first symmetric mode (S 0 ) of the Lamb wave is called an extensional or dilatational wave. The first symmetric mode may resemble a longitudinal or P-wave in an unbounded solid. At low frequencies, the velocity of this mode is often equivalent to the plate velocity of the extensional wave. When the wave medium is steel, the velocity is very close to the longitudinal velocity of the material of the wave medium, thus rendering the mode its name (P-wave). Accordingly, the “P-wave” in a pipe can be considered as one mode of a Lamb wave. However, it should be pointed out that many Lamb wave modes exist that fall well outside of the accepted definition of a P-wave or compressional mode wave. 
   The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, the present method is applicable in any wellbore  2  having any type of fluid therein, including typical downhole fluids, water, brine, drilling fluids, as well as gas filled boreholes that may have methane, carbon dioxide, or any other downhole gas encountered. Also, the same transducers used in creating the Lamb wave can be used to produce the shear waves, different transducers can be used, or a combination of these can be coupled together. Moreover, these transducers can be stacked on a single mount, or can be disposed at different locations on a downhole tool. Additionally, other permutations of wave modes can be included to attain the advantages above described, such as a horizontal shear wave in association with one of a compressional wave, a Lamb wave, a vertical shear wave, or combinations thereof. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.