Abstract:
An apparatus for determining dynamic mechanical properties of a cement sample. The apparatus includes an isolator located within a cavity defined by a body. The isolator has an attached end that engages the body proximate a narrow end of the cavity. A space between the body and isolator facilitates lateral movement the isolator. A transducer is located in an interior space defined by the isolator. The transducer has an attached end engaging a distal end of the isolator. A space between the isolator and transducer facilitates lateral movement of the transducer distal end. The transducer generates compressional waves and shear waves that travel into the sample. An ultrasonic receiver receives the waves. The waves are separated with filtering. Sample setting time may be determined from propagation time of shear waves through the sample. Compressive strength of the sample is determined by velocity of compressional waves as the sample sets.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/325,611, which application was filed with the Patent and Trademark Office on Sep. 28, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a transducer for simultaneously producing compressional and shear waves in a cement sample. More particularly, but not by way of limitation, the present invention relates to a transducer which allows the measurement of the shear wave velocity and compressional wave velocity in a universal cement analyzer cell so as to determine dynamic mechanical properties, i.e., Poisson&#39;s ratio and Young&#39;s modulus, as well as other properties of a cement sample. 
   2. Background of the Invention 
   Ultrasonic cement analyzers (“UCA”) are well known in the art. Compressive strength measurements of a cement sample are best taken using such a device. While the UCA provides a number of advantages over alternative methods for measuring, or estimating, the characteristics of a particular cement sample, a compelling advantage is the ability of the UCA to perform nondestructive measurements. Thus, a single sample may be used to perform a series of measurements, whereas, with alternative methods, a series of samples are required for each measurement to be made. Using a single sample reduces cost, provides more consistent testing, reduces record keeping, saves time, etc. 
   The UCA was developed to measure the compressive strength of a cement slurry as it sets when subjected to simulated oil field temperatures and pressures. It consists of a high temperature-high pressure autoclave, a heat jacket capable of heating rates up to 5.6.degree. C. (10.degree. F.) per minute, a pair of 400 kHz ultrasonic transducers for measuring the transit time of an acoustic signal transmitted through the slurry, plus associated hydraulic plumbing. The two transducers make transit time measurements through the cement as it sets. A short pulse on a lower transducer propagates through the cement to an upper transducer. Set time and compressive strength are calculated from measured transit time via empirically developed equations. U.S. Pat. Nos. 4,259,868 and 4,567,765 disclose the UCA in detail and are incorporated herein by reference. 
   It is also known in the art that an acoustic shear wave may be used to determine the setting time of cement. In one method for using a shear wave to determine setting time, a shear wave is generated at a location in a cement slurry. The point in time at which the shear wave propagates through the slurry is indicative of the setting time of the cement. U.S. Pat. No. 5,412,990 issued to D&#39;Angelo et al. discloses such a method for measuring cement thickening time using a shear wave and is hereby incorporated by reference. 
   In addition, if both the shear wave velocity and compressional wave velocity are known, it is possible to calculate various dynamic mechanical properties of the sample such as Poisson&#39;s ratio, Young&#39;s modulus, etc. Poisson&#39;s ratio is defined as the absolute value of the ratio of transverse strain to the corresponding axial strain resulting from uniformly distributed axial stress below the proportional limit of the material. Young&#39;s modulus is the elastic modulus in tension or compression. This information if useful for material development or characterization as well as for quality control purposes. While wave velocities have been employed for the measurement of dynamic mechanical properties in rock specimens, they have not heretofore been used to determine such properties in a cement sample. 
   An additional disadvantage associated with traditional UCA devices is that, historically, all acoustic cement tests have been run with the cell oriented vertically. With some cement samples, a layer of low strength material forms at the top, which can cause a limitation in the transfer of acoustic energy from the cell/transducer, to the cement at the send end. Further, the low strength material can cause a limitation in the transfer of acoustic energy from the cement to the cell/transducer at the receiving end. 
   It is thus an object of the present invention to provide a transducer which may be used to simultaneously produce shear and compressional waves in a cement sample. 
   It is yet a further object of the present invention to provide an ultrasonic cement analyzer which will measure shear wave and compressional wave velocities. 
   It is still a further object of the present invention to provide a method for measuring the velocities of the shear and compressional waves produced in a sample and to indicate the dynamic mechanical properties of the sample. 
   It is a further object of the present invention to provide an apparatus and method for overcoming limitations in a transfer of acoustic energy from the cell/transducer to the cement and vise versa due to low strength material adjacent the transducer. 
   SUMMARY OF THE INVENTION 
   The present invention provides a transducer which will simultaneously produce shear and compressional waves in a sample and a method for using the measured velocities of the shear and compressional waves to determine dynamic mechanical properties of the sample such as Poisson&#39;s ratio. 
   In a preferred embodiment, an ultrasonic transducer is mounted in an end plug for a standard UCA cell. The transducer is in acoustic communication with an acoustic element mounted in an isolator such that acoustic element is free to move laterally for production of the shear wave. When an electrical pulse is applied to the transducer, an acoustic pulse travels axially down the acoustic element and into the cement sample as a compressional wave. In addition, the acoustic element moves laterally in response to the electrical pulse to also produce a shear wave in the sample. The internal portion of the inventive acoustic element is secured to the isolator at the end furthest from the cement sample thereby allowing greater freedom of movement in the lateral direction than has heretofore been possible. 
   In another embodiment, the velocities of the compressional wave and shear wave produced by the inventive transducer are measured to determine the dynamic mechanical properties, for example Poisson&#39;s ratio and Young&#39;s modulus, of the sample under test. 
   In addition, the UCA may also provide all of the features and perform all of the functions of a conventional UCA. Thus, an UCA incorporating the present invention provides a convenient method to measure the change in mechanical properties of a cement sample at elevated temperatures and pressures as the sample transitions from a slurry, through gelation, to a solid, as setting occurs. 
   In one embodiment, the transmission and reception of acoustic energy is improved by orienting the transducers and sample in a horizontal configuration. In a horizontal orientation, the cement sample in contact with the centralized transducer is an average of the cement in the cell. If a weak “punky” layer of cement then forms at or near the top of the cement sample, the weak layer will be adjacent a wall of the sample cylinder rather than adjacent a transducer. 
   Other benefits of a horizontal orientation are observable when testing “foamed” cements that have a meaningful percentage of entrained gas, e.g., 10% to 40% entrained gas, although other percentages are also contemplated to be used with the invention. As the cement is pressurized, the gas bubble become smaller. Therefore, a gap can form between the cement and the cell/transducer. The gap can be large enough that tests cannot be run. However, when placed in a horizontal orientation, the cement sample volume can change substantially before the acoustic analysis is seriously affected. 
   In another embodiment, the transmission and reception of acoustic energy is improved by forcing the transducer into contact with the sample with a piston or other force applying device. 
   Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  provides a cross-sectional side view of an end plug for an ultrasonic cement analyzer having the inventive transducer mounted therein. 
       FIG. 2  provides a cross-sectional side view of an end plug for the ultrasonic cement analyzer having the inventive shear wave transducer mounted therein showing the acoustic element and isolator in greater detail. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the construction illustrated and the steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation. 
   A typical end plug  10  for an ultrasonic cement analyzer (“UCA”) is shown in  FIG. 1 . Preferably, end plug  10  has transducer  12  mounted therein such that a contacting surface  14  of transducer  12  can contact a cement sample contained in a UCA cell (not shown) to be capped with end plug  10 . It should be noted that surface  14  may protrude slightly below bottom surface  60  and end  22  to improve contact with the sample. 
   Preferably, transducer  12  includes: transmitter  13 , body  38 , and acoustic couplant  45 . Delay line  15  includes isolator  16 ; and acoustic element  18 . Isolator  16  is essentially cylindrical in appearance having: flange  20  extending radially outward from end  22 ; an exterior side wall  24  extending upward from flange  20  to distal end  26 ; cylindrical cavity  28  formed by interior wall  30 ; and top wall  32  having aperture  34  opening into cavity  28 . 
   Ultrasonic transmitter  13  is preferably a commercially available transducer. One such ultrasonic transducer is the Harisonic transducer used with prior art UCA devices. Preferably transmitter  13  includes: a body  42  and an electrical connector  44  for receiving electrical pulses to excite transmitter  13 . Preferably connector  44  is a BNC-type connector. 
   Referring now to  FIGS. 1 and 2 , acoustic element  18  is typically a unitary structure formed from metal, or other rigid material, and includes: a cylindrical top end  36 ; a cylindrical body portion  38 ; a flange  40  separating end  36  from body  38 . 
   Transducer  12  is assembled by pressing top end  36  in to aperture  34  until flange  40  contacts top wall  32 . Most preferably, cavity  28  has a diameter somewhat larger than the diameter of body  38  so that a gap  46  is formed between body portion  38  and interior wall  30 . Acoustic energy is coupled from transmitter  13  to acoustic element  18  through acoustic couplant  45 . 
   Preferably, end plug  10  includes a bottom surface  60  having cavity  62  therein. Cavity  62  is generally a series of coaxial cylindrical cavities of varying diameter giving cavity  62  a “stepped” appearance. In a preferred embodiment, cavity  62  has a lower cylindrical cavity  64  which is preferably sized to receive flange  20  of isolator  16 . Flange  66  is defined by a second cylindrical cavity  68  having a smaller diameter than that of cavity  64 . Above flange  66 , a third cavity  69  is formed of a larger diameter than that of body  38  creating a cylindrical gap  70  and, finally, an upper cavity  72  is formed, preferably extending to the top surface (not shown) of end plug  10 . 
   When transducer  12  is installed in end plug  10 , housing  16  is inserted into cavity  62  until flange  20  contacts flange  66 . Transmitter  13  is placed above body  38  and held in place with sleeve  100  and snap ring  102  placed in snap ring groove  104  near the upper end of cavity  72 . Wave spring  110  maintains pressure between transmitter  13  and acoustic couplant  45  and body  38 . As will be apparent to those skilled in the art, body  38  is thus only supported at its lower end. When a pulse is applied to transmitter  13 , acoustic element  18  is driven in its axial direction which will generate a compressional wave in a cement sample in contact with end  14 . In addition, an electrical pulse applied to transmitter  13  will also result in lateral movement of acoustic element  18 . In prior art designs, transmitter  13  was pressed directly into end plug  10  at surface  60 . The mass of end plug  10  and the rigid mounting of transmitter  13  near the sample greatly limited the generation of a shear wave in the sample. In contrast, in the inventive transducer, the mass of the end plug  10  is, to a large degree, isolated from transducer  12  since isolator  16  is secured at its lower end and acoustic element  18  is secured at its upper end. Gaps  46  and  70  reduce the degree to which end plug  10  attenuates lateral movement of acoustic element  18 . Thus, the amplitude of the shear wave produced by the inventive transducer  12  will be many times greater than would be produced if transmitter  13  were simply mounted directly in end plug  10 . 
   To further improve the transmission and reception of acoustic energy, the transducer may be forced into contact with the sample by a piston  112  ( FIG. 1 ). As shown in  FIG. 1 , piston  112  acts on the sleeve  100  to compress the wave spring  110 , and force transmitter  13 , isolator  16 , and transducer  12  to move so that transducer  12  is forced into contact with a sample. Although piston  112  is shown in  FIG. 1  as an example, other force applying devices may be used. Further piston  112  may be located elsewhere to impart force to transducer  12 . For example, piston  112  may be keyed to end plug  10  and act directly on isolator  16  or may be configured in other ways. 
   As will be apparent to those skilled in the art, to determine the velocities of the compressional wave and shear wave independently, there must be some method for distinguishing each wave independently at a receiver. Two techniques are possible with the inventive apparatus. 
   First, in a typical sample, the shear wave will arrive at a receiver later than the compressional wave. Thus, a time based method may be used to distinguish between the two waves. Those familiar with cement analyzers will appreciate that, typically, a second end plug, capping the opposite end of the UCA cell, may be configured to house an ultrasonic receiver. After the generation of the waves in the sample, the first ultrasonic pulse received will indicate the arrival of the compressional wave. The velocity of the wave may simply be calculated by dividing the distance between the receiver and transmitter by the elapsed time between transmittal of the pulse and its arrival at the receiver. The second ultrasonic pulse received will be as a result of the shear wave. The shear wave velocity is likewise calculated by dividing the distance between the transmitter and receiver by the time period between excitation of the transmitter and the and receipt of the second pulse. 
   Alternatively, it is also possible to generate the shear wave at a frequency different from that of the compressional wave which allows the two waves to be measured independently through filtering of the received signal. Since the shear wave velocity in steel is typically half the speed of the compressional wave velocity, the shear wave frequency generated by the inventive transducer  12  will be approximately half the frequency of the compressional wave. As will be apparent to those skilled in the art, after the compressional and shear wave are received, the two waves may be separated with relatively simple electrical filtering. 
   The importance in measuring the velocities of the shear wave and compressional wave is that, when such values are known, it is possible to calculate dynamic mechanical properties of the sample such as, for example, Poisson&#39;s ratio and Young&#39;s modulus. As will be apparent to those skilled in the art, known relationships have been demonstrated between the compressional and shear wave velocities and various mechanical properties of specific materials. 
   For example, it has previously been shown that Poisson&#39;s ratio and Young&#39;s modulus for a rock specimen may be determined if the acoustical compressional and shear wave velocities are known for that specimen. In such a specimen, Young&#39;s modulus (E) is given by:
 
 E=C   S   2 ρ[3( C   P   /C   S ) 2 −4]/( C   P   /C   S ) 2 −1]
         where:
           C S  is the velocity of the shear wave through the specimen;   C P  is the velocity of the compressional wave through the specimen; and   ρ is the density of the material.   
               

   Poisson&#39;s ratio (μ) is given by:
 
μ=½[( C   P   /C   S ) 2 −2]/[( C   P   /C   S ) 2 −1]
 
   As will be apparent to those skilled in the art, the above equations may vary somewhat depending on the particular geometry of the specimen measured, however it is within the skill level of one of ordinary skill in the art to empirically determine such modifications. Likewise, while these equations would provide a good estimate for Young&#39;s modulus and Poisson&#39;s ratio for a cement sample tested in a UCA, modifying the equations to account for the particular geometry of the UCA test cell and differences between a rock specimen and a cement specimen could improve the accuracy of the calculation. Again, it is within the skill level of one of ordinary skill in the art to empirically determine such modifications. 
   It should thus be noted that, when using an UCA employing a shear transducer, such as the inventive shear transducer, a cement sample may be tested at elevated pressures and temperatures which actually simulate those encountered in casing an oil or gas well. In addition, as an individual sample sets, it is possible to determine a number of mechanical properties of the sample as the cement transitions from a slurry, through the gelation stage, and into a solid. As discussed above, the setting time may be determined from the point in time at which a shear wave will propagate through the sample, the compressive strength may be determined from the velocity of the compressional wave as the cement sets, and the dynamic mechanical properties (i.e., Poisson&#39;s ratio and Young&#39;s modulus) may be determined from the velocities of the shear and compressional waves as the cement transitions into the solid state. In such a UCA, it is possible to measure, display, and record changes in the mechanical properties of the sample as setting occurs. 
   In an additional embodiment of the invention, further improvement to the UCA process can be achieved by orienting the transducer, receiver and sample horizontally rather than vertically. 
   Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention.