Abstract:
Passive and active suppression of casing signals is used to improve the ability to measure formation velocities. Passive suppression includes the use of a magnet slidably coupled to the casing. Active suppression uses measurement of casing signals to control the output of an electromechanical acoustic transducer which cancels the propagating signal. Feedback control may be provided.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related to the field of acoustic wellbore logging. More specifically, the present invention is related to apparatus and methods for reducing ringing in cased wellbores.  
       BACKGROUND OF THE ART  
       [0002]     Acoustic array wellbore logging tools are known in the art for determining acoustic properties of earth formations penetrated by a wellbore. The array acoustic wellbore logging tools known in the art typically include an energy emitting transducer, called a transmitter, and a plurality of receiving transducers, called receivers, axially spaced apart from the transmitter along an elongated tool mandrel. The tool is typically lowered into the wellbore at one end of an armored electrical cable. The transmitter periodically energizes the wellbore with pulses of acoustic energy. The acoustic energy travels through a fluid filling the wellbore and interacts with the interface between the fluid and the wall of the wellbore. Some of the acoustic energy can then travel along the wellbore wall. After traveling along the wall of the wellbore, some of the energy can travel back into the wellbore towards the tool where it can be detected by the receivers. The receivers convert the acoustic energy into electrical signals having amplitudes corresponding to the acoustic energy amplitude.  
         [0003]     A commonly used method for analysis of the acoustic signals is the so-called semblance technique. For example, a method which is known in the an for determining acoustic transmission velocity of the formation, the method being called semblance correlation, comprises determining values of time difference between the signals from each of the receivers at which the degree of correspondence between the signals reaches a maximum. The time difference is then used to calculate a formation acoustic velocity since the distances between the receivers are known. See, for example, U.S. Pat. No. 4,594,691 to Kimball et al., U.S. Pat. No. 5,541,890 to Tang, U.S. Pat. No. 6,023,443 to Dubinsky et al., U.S. Pat. No. 6,930,616 to Tang et al.  
         [0004]     In a cased hole environment, the presence of the casing often prevents the measurement of the formation compressional wave slowness especially when the casing is poorly bonded. This is due to the fact that when an acoustic source is activated, the casing will ring. The ringing signal propagates with substantially no attenuation and will overwhelm m the formation arrival. If, on the other hand, the casing is well bonded to the formation, the ringing of the casing is not an issue. The present invention is a method of determination of formation compressional and shear-wave velocities in a cased borehole.  
       SUMMARY OF THE INVENTION  
       [0005]     One embodiment of the invention is an apparatus for evaluating an earth formation having a cased borehole. An acoustic transmitter on a logging tool conveyed in the borehole generates an acoustic signal that propagates through casing and through the formation. A receiver on the logging tool responds to the casing signal and the formation signal. A device on the logging tool disposed between the transmitter and the receiver suppresses the casing signal at the receiver. The device may include a magnet slidably coupled to the casing so as to increase a casing density between the transmitter and the receiver. The device may include segments having different diameters and/or at least an attached mass. The transmitter may be a quadrupole transmitter operated below a cut-off frequency of the casing. The device may include a sensor that produces an output indicative of the casing signal and a transducer that produces an output that suppresses the casing signal using the output of the sensor. A plurality of receivers may be used with a processor that estimates a compressional-wave velocity and/or a shear-wave velocity of the formation using the output of the plurality of receivers.  
         [0006]     Another embodiment of the invention is a method of evaluating an earth formation having a cased borehole. An acoustic signal is generated that propagates through the casing and the formation using a transmitter on a logging tool. A receiver on the logging tool produces an output responsive to the casing signal and the formation signal. The casing signal is suppressed using a device on the logging tool between the transmitter and the receiver. The suppression may be accomplished by using as the device a magnet slidably coupled to the casing thereby increasing a casing density. The signal may be generated by using a quadrupole transmitter operated below a cutoff frequency of the casing. The suppression may be accomplished by using a sensor disposed between the transmitter and the receiver that produces an output indicative of the casing signal and using the output of the sensor to operate a transducer. The suppression may be based on an estimated time of propagation of the casing signal between the transmitter and the receiver. The suppression may be done by attenuating selected frequencies in the casing signal using a segment having different diameters and/or an attached mass. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     For detailed understanding of the present invention, reference should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawing and in which:  
         [0008]      FIG. 1  is a schematic illustration of a wireline logging system;  
         [0009]      FIG. 2  is an illustration of a logging tool according to the present invention within a cased borehole with poor cementing;  
         [0010]      FIG. 3   a  shows simulation results of waveforms with a light casing in an earth formation;  
         [0011]      FIG. 3   b  shows simulation results of the waveforms of  FIG. 3   a  after windowing and filtering;  
         [0012]      FIG. 4   a  shows simulation results of waveforms with a light casing in an earth formation;  
         [0013]      FIG. 4   b  shows simulation results of the waveforms of  FIG. 4   a  after windowing and filtering;  
         [0014]      FIG. 4   c  shows a semblance display of the data in  FIG. 4   a;    
         [0015]      FIG. 4   d  shows a semblance display of the data in  FIG. 4   b;    
         [0016]      FIG. 5   a  shows an alternate embodiment of the present invention in which the magnet is provided with different internal diameters;  
         [0017]      FIG. 5   b  shows an alternate embodiment of the present invention in which the magnet is provided with attached masses;  
         [0018]      FIG. 6  shows an embodiment of the present invention in which active noise cancellation is used; and  
         [0019]      FIG. 7  is a circuit diagram representing  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The present invention is discussed with reference to specific logging instruments that may form part of a string of several logging instruments for conducting wireline logging operations. It is to be understood that the choice of the specific instruments discussed herein is not to be construed as a limitation and that the method of the present invention may also be used with other logging instruments as well.  
         [0021]     A typical configuration of the logging system is shown in  FIG. 1 . This is a modification of an arrangement from U.S. Pat. No. 4,953,399 to Fertl et al., having the same assignee as the present invention, the contents of which are incorporated herein by reference. Shown in  FIG. 1  is a suite of logging instruments  10 , disposed within a borehole  11  penetrating an earth formation  13 , illustrated in vertical section, and coupled to equipment at the earth&#39;s surface, in accordance with various illustrative embodiments of the method and apparatus of the present invention. Logging instrument suite  10  may include a resistivity device  12 , a natural gamma ray device  14 , and/or two porosity-determining devices, such as a neutron device  16  and/or a density device  18 . Collectively, these devices and others used in the borehole for logging operations are referred to as formation evaluation sensors. The resistivity device  12  may be one of a number of different types of instruments known to the art for measuring the electrical resistivity of formations surrounding a borehole so long as such device has a relatively deep depth of investigation. For example, a HDIL (High Definition Induction Logging) device such as that described in U.S. Pat. No. 5,452,761 to Beard et al., having the same assignee as the present invention, the contents of which are fully incorporated herein by reference, may be used. The natural gamma ray device  14  may be of a type including a scintillation detector including a scintillation crystal cooperatively coupled to a photomultiplier tube such that when the crystal is impinged by gamma rays a succession of electrical pulses is generated, such pulses having a magnitude proportional to the energy of the impinging gamma rays. The neutron device  16  may be one of several types known to the art for using the response characteristics of the formation to neutron radiation to determine formation porosity. Such a device is essentially responsive to the neutron-moderating properties of the formation. The density device  18  may be a conventional gamma-gamma density instrument such as that described in U.S. Pat. No. 3,321,625 to Wahl, used to determine the bulk density of the formation. A downhole processor  29  may be provided at a suitable location as part of the instrument suite.  
         [0022]     The logging instrument suite  10  is conveyed within borehole  11  by a cable  20  containing electrical conductors (not illustrated) for communicating electrical signals between the logging instrument suite  10  and the surface electronics, indicated generally at  22 , located at the earth&#39;s surface. The logging devices  12 ,  14 ,  16 , and/or  18  within the logging instrument suite  10  are cooperatively coupled such that electrical signals may be communicated between each of the logging devices  12 ,  14 ,  16 , and/or  18  and the surface electronics  22 . The cable  20  is attached to a drum  24  at the earth&#39;s surface in a manner familiar to the art. The logging instrument suite  10  is caused to traverse the borehole  11  by spooling the cable  20  on to or off of the drum  24 , also in a manner familiar to the art.  
         [0023]     The surface electronics  22  may include such electronic circuitry as is necessary to operate the logging devices  12 ,  14 ,  16 , and/or  18  within the logging instrument suite  10  and to process the data therefrom. Some of the processing may be done downhole. In particular, the processing needed for making decisions on speeding up (discussed below) or slowing down the logging speed is preferably done downhole. If such processing is done downhole, then telemetry of instructions to speed up or slow down the logging could be carried out substantially in real time. This avoids potential delays that could occur if large quantities of data were to be telemetered uphole for the processing needed to make the decisions to alter the logging speed. It should be noted that with sufficiently fast communication rates, it makes no difference where the decision-making is carried out. However, with present data rates available on wirelines, the decision-making is preferably done downhole.  
         [0024]     Control circuitry  26  contains such power supplies as are required for operation of the chosen embodiments of logging devices  12 ,  14 ,  16 , and/or  18  within the logging instrument suite  10  and further contains such electronic circuitry as is necessary to process and normalize the signals from such logging devices  12 ,  14 ,  16 , and/or  18  in a conventional manner to yield generally continuous records, or logs, of data pertaining to the formations surrounding the borehole  11 . These logs may then be electronically stored in a data storage  32  prior to further processing. A surface processor  28  may process the measurements made by the formation evaluation sensor(s)  12 ,  14 ,  16 , and/or  18 . This processing could also be done by the downhole processor  29 .  
         [0025]     The surface electronics  22  may also include such equipment as will facilitate machine implementation of various illustrative embodiments of the method of the present invention. The surface processor  28  may be of various forms, but preferably is an appropriate digital computer programmed to process data from the logging devices  12 ,  14 ,  16 , and/or  18 . A memory unit  30  and the data storage unit  32  are each of a type to interface cooperatively with the surface processor  28  and/or the control circuitry  26 . A depth controller  34  determines the longitudinal movement of the logging instrument suite  10  within the borehole  11  and communicates a signal representative of such movement to the surface processor  28 . The logging speed is altered in accordance with speedup or slowdown signals that may be communicated from the downhole processor  29 , and/or provided by the surface processor  28 , as discussed below. This is done by altering the rotation speed of the drum  24 . Offsite communication may be provided, for example, by a satellite link, by a telemetry unit  36 .  
         [0026]     Referring now to  FIG. 2 , a logging tool specific to various illustrative embodiments of the present invention is illustrated. The borehole in an earth formation  107  is depicted by a borehole  106  having a wall  104 . Within the borehole  106  is a casing denoted by  103 . In the example shown, the casing is poorly cemented to the borehole wall  104  by pieces of cement  109   a ,  109   b . An acoustic transmitter is indicated by  101  and an acoustic receiver by  103 . Additional receivers and transmitters may be used, but are not shown to simplify the illustration.  
         [0027]     Upon activation of the acoustic transmitter, acoustic waves are propagated into the borehole and the earth formation. A raypath corresponding to a signal that propagates through the formation is depicted by  111 . This is the desired signal whose traveltime gives the formation velocity. In addition, a raypath  113  propagating through the casing is also shown. The raypath  105  defines a casing signal that arrives earlier than the formation signal denoted by raypath  111 . The casing signal  105  is extremely ringy and is only slowly attenuated with time. This ringiness is due to the high Q of the metal casing and may be aggravated by the situation shown in  FIG. 2  where the casing is essentially free to vibrate between the support points indicated by  109   a ,  109   b . As a result of this ringing, it is difficult to identify the signal propagating through the formation. This is illustrated in  FIGS. 3   a - 3   d.    
         [0028]     Shown in  FIG. 3   a  are simulated waveforms for a model in which a casing having a casing density of 7700 kg/m 3  is within a borehole. It should be noted that the term casing density is defined in terms of the weight of the casing divided by the volume occupied by the casing with its ends sealed off, so that a thinner casing has a higher casing density than a thicker casing. The abscissa is time of arrival (0-4 s) and the ordinate is the receiver number of the array. Signals are shown for an array of eight receivers. The formation arrival  133  and the casing arrival  131  are much weaker than the Stoneley wave arrival  135 . Upon selection of a window that excludes the Stoneley wave arrival and applying a bandpass filter, the data of  FIG. 3   b  results. The casing arrival  131 ′ and the formation arrival  133 ′ can now be seen, but it is also clear that the formation signal arrival is hard to identify.  
         [0029]     Returning to  FIG. 2 , the present invention includes a magnet depicted by  121  that attached to the casing  103 . The magnet  121  may be a permanent magnet or an electromagnet. The attaching of the magnet to the casing is not permanent and may be regarded as a slidable coupling, so that the logging tool can be moved along the casing using the wireline  20 . Wheels (not shown) may be provided on the outside of the logging tool to facilitate movement of the tool for logging. The selection of the magnet strength is a straightforward design issue to make sure that the tension in the cable required to move the logging tool with the magnet slidably coupled to the casing does not exceed the cable strength. The effect of having the magnet coupled to the casing is to increase the mass of the casing between the transmitter  101  and the receiver  103 . The effect of this increased mass of the casing can be seen in  FIGS. 4   a - 4   d.    
         [0030]     Shown in  FIG. 4   a  is an array display similar to  FIG. 3   a  except that the casing density is now 77000 kg/m 3 , i.e., ten times the value for  FIG. 3   a . The formation arrival  153  is now identifiable on the raw time display while the casing arrival  151  cannot be seen. After windowing and bandpass filtering, the formation arrival  153 ′ can be clearly identified and separate from the casing arrival  151 ′.  
         [0031]      FIG. 4   c  is a semblance display of the data of  FIG. 4   a  in the p-τ (slowness-arrival time) domain. The abscissa is the slowness from 0-250 μs/ft (0-825 μs/m) while the ordinate is the arrival time (scale of 0-3200 ms). The peak of the semblance  161  corresponding to the formation arrival can be clearly identified with a slowness of about 105 μs/ft (velocity of 9520 ft/s or 2900 ms/s).  
         [0032]     In another embodiment of the invention, the magnet  21  is part of a grooved assembly  221  with different diameters. The assembly is shown in  FIG. 5   a  within the casing  203 . The different diameter portions are selected to attenuate certain frequencies of signals propagating along the casing. Such an arrangement with different diameters has been discussed in U.S. Pat. No. 6,615,949 to Egerev et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference.  
         [0033]     Shown in  FIG. 5   b  is another embodiment of the invention in which the magnet is part of an assembly  321  having attached masses. The assembly is shown slidably coupled to casing  303 . The plurality of heavy mass irregularities are spaced and sized for the maximum attenuation of acoustic pulses in a predetermined frequency range. This too is discussed in Egerev.  
         [0034]     In another embodiment of the invention, instead of a monopole transmitter  101  and a monopole receiver  103  as shown in  FIG. 2 , multipole transmitters and receivers are used. Such a device has been taught in U.S. Pat. No. 6,850,168 to Tang et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference. As disclosed in Tang, the logging tool includes a transmitter conveyed on a drilling collar for exciting a quadrupole signal in a borehole being drilled by a drill bit and a receiver for receiving the signal. The transmitter is operated at a frequency below the cut-off frequency of the quadrupole collar mode. The received signal consists primarily of the formation quadrupole mode which, at low frequencies, has a velocity that approaches the formation shear velocity. The value of the cut-off frequency is primarily dependent on the thickness of the drilling collar. The present invention recognizes that the casing and the slidably coupled magnet have a cut-off frequency for quadrupole waves that depends upon the casing density. By proper selection of the casing density, the arrangement described above with reference to  FIG. 2 ,  FIG. 5   a  or  FIG. 5   b  can be used in conjunction with a quadrupole transmitter to determine the formation shear velocity.  
         [0035]     The embodiment of the invention described above relies on passive suppression of the signal propagating through the casing that reaches the receiver. In an alternate embodiment of the invention, active noise cancellation of the casing signal is done. The apparatus of this embodiment of the invention is illustrated in  FIG. 6 .  
         [0036]     Shown in  FIG. 6  is the formation  407  and the casing  403 . The transmitter  101  and the receiver  103  are as indicated. Again, it should be noted that only a single transmitter-receiver combination is shown—in reality an array of receivers (or an array of transmitters) may be used. The apparatus includes a first sensor  405  and a second sensor  409  positioned between the transmitter  101  and the receiver  103 . The first and second sensors are part of the logging tool that is slidably coupled to the casing  403 . Between the first sensor  405  and a second sensor  409  is an electromechanical acoustic transducer (EMAT)  411  that is also slidably coupled to the casing  404 .  
         [0037]     During operation of the invention, when the transmitter  101  is activated, as discussed above, a signal propagates through the casing (see raypath  105  in  FIG. 2 ). The first sensor  405  detects the casing signal. The measured signal at the first sensor is used to activate, after a specified time delay discussed below, the EMAT  411 . The EMAT  411  is activated to produce a signal in the casing  403  that is equal and opposite to the signal detected at the first sensor  405 , thus substantially canceling any casing signal below the EMAT.  
         [0038]     The time delay r mentioned above is determined from the distance D between the first sensor  405  and the EMAT  411  and the velocity of sound c in the casing. The velocity of sound in the casing is a known quantity, so that the time delay is a known quantity. The relationship is given by τ=D/c.  
         [0039]     To correct for any discrepancies in the time delay due to incorrect knowledge of either c or D, the invention includes the second sensor  409  which measures the casing signal that is still propagating along the casing towards the receiver  103 . The output of the second sensor is an indication of incomplete cancellation of the casing signal by the EMAT and is used through suitable circuitry (not shown) to control the time delay and/or the amplitude of the signal generated by the EMAT.  
         [0040]      FIG. 7  shows a schematic diagram of the operation of the tool. Signals propagating along the casing are denoted by the dashed heavy line. The first sensor  501  measures the casing signal. After a time delay τ  503  and with a suitable gain K  505 , the EMAT  507  is activated. The output of the EMAT is coupled into the casing and the remaining casing signal is detected by the second sensor  509 . The output of the second sensor is then used to control the time delay  503  and or the gain K.  
         [0041]     EMATs are devices that can be used to excited or receive ultrasonic waves in an electrically conductive material such as casing. When a wire, placed near to the surface of casing, is driven by an alternating current at a desired frequency, eddy currents are induced in the casing by electromagnetic induction. In the presence of a static magnetic field (B 0 ), these induced eddy currents (J) will experience a Lorenz forces (f) given by
 
 f=J×B   0 
 
 These Lorentz forces are transmitted into the casing and serve as a source of ultrasonic waves. EMATs are reciprocal devices i.e. they can be used as transmitters or as receivers of ultrasound. There is no requirement for contact, so that problems associated with acoustic coupling to casing do not arise. Those versed in the art would recognize that the invention described above with reference to  FIGS. 6 and 7  could also be practiced with piezoelectric transducers (instead of EMATs). 
 
         [0042]     The processing of the measurements may be done by the surface processor  28 , by the downhole processor  29 , or at a remote location. The data acquisition may be controlled at least in part by the downhole electronics. The processing that may be done may include a semblance analysis of array data such as that shown in  FIGS. 41-4   d  for determination of compressional- and shear-wave velocities of formations. Determination of compressional- and shear-wave velocities of formations is part of the evaluation of the earth formation that may be carried out by the present invention. Implicit in the control and processing of the data is the use of a computer program on a suitable machine readable-medium that enables the processors to perform the control and processing. The machine-readable medium may include ROMs, EPROMs, EEPROMs, flash memories and optical disks. The term processor is intended to include devices such as a field programmable gate array (FPGA).  
         [0043]     While the foregoing disclosure is directed to specific embodiments of the present invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope of the appended claims be embraced by the foregoing disclosure.