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BACKGROUND 
       [0001]    This invention relates to a vibrating device for use in sand control and formation stimulation in an oil and gas recovery operation. 
         [0002]    Many oil and gas downhole recovery operations, especially high-rate, high-permeability completions, produce reservoir fluids that contain fines, or formation sand. Therefore, support and screening devices, such as screens, slotted liners, and the like, have been utilized to support a gravel pack, or the like, in the well to stabilize the formation while permitting the recovered fluids to pass from the formation into the wellbore while preventing passage of fines or formation sand with the recovered fluids. 
         [0003]    These support devices are often placed in a pressure-drop zone which subjects the devices to contamination from scaling (salt crystal growth) and other materials that are precipitated during production of the reservoir fluids. Thus, the devices must be cleaned either mechanically, which adds to the labor and cost of the project, or chemically, which may harm the metal parts of the devices. 
         [0004]    Also, during the recovery operation from the wellbore, a “skin” develops around the wall of the wellbore which impedes the flow of fluid from the formation requiring stimulation techniques to remove the skin and stimulate the formation. 
         [0005]    Therefore, what is needed is a device of the above type that simultaneously performs the above screening and stimulation functions yet eliminates the above problems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a diagrammatic view of an embodiment of the sand control system of the present invention shown in a downhole environment. 
           [0007]      FIG. 2  is a flow chart depicting steps of a method according to an alternate embodiment of the invention. 
           [0008]      FIG. 3  is a graph depicting two variables in accordance with the embodiment of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring to  FIG. 1  of the drawings, the reference  10  refers, in general, to a downhole tool installed in a wellbore  12 . The tool  10  is connected to a wireline or coiled tubing  14 , and is lowered to a depth in the wellbore  12  penetrating a producing formation F. It is also understood that a casing (not shown) can be provided in the wellbore  12  and that production tubing (not shown) is installed in the wellbore  12  above the tool  10 . 
         [0010]    Four axially-spaced, cylindrical gravel pack support and screening devices  16   a - 16   d  are mounted, in any conventional manner, to the wall of the wellbore  12  penetrating the formation F. The devices  16   a - 16   d  can be in the form of screens, slotted liners, or any similar type of gravel support device. Although not clear from the drawing due to scale limitations, it is understood that the devices  16   a - 16   d  define an annular space with the wall of the wellbore  12  that receives one or more gravel packs, or the like, (not shown). The purpose of each gravel pack is to improve the integrity of the wall of the wellbore  12 , yet allow recovered fluids to pass to and through the devices  16   a - 16   d  and into the wellbore  12 , while preventing the passage of any fines or sand from the fluids. Since these gravel packs are conventional, they will not be described in any further detail. 
         [0011]    Two electrical drivers  18   a  and  18   b  are mounted on the tool  10  in an axially-spaced relation. The drivers  18   a  and  18   b  are conventional and, as such, are connected to a source of AC or DC power received from ground surface and are adapted to supply electrical power for reasons to be described. 
         [0012]    An acoustic transducer  20   a  is mounted on the wall of the wellbore  12  between the devices  16   a  and  16   b;  an acoustic transducer  20   b  is mounted on the wall of the wellbore  12  between the devices  16   b  and  16   c;  and an acoustic transducer  20   c  is mounted on the wall of the wellbore  12  between the devices  16   c  and  16   d.    
         [0013]    The acoustic transducers  20   a - 20   c  can be in the form of conventional electromechanical transducers, or converters, such as tuning forks, cantilevers, oval-mode tools, magnetostrictive drivers, or piezoelectric transducers that vibrate in response to an input signal. Preferably, the acoustic transducers  20   a - 20   c  are driven, or excited by electrical power output from the drivers  18   a  and  18   b  and operate efficiently at a desired, predetermined frequency when actuated. For example, one of the drivers  18   a  and  18   b,  as well as one or two of the acoustic transducers  20   a - 20   c,  can operate at a relatively high frequency; while the other driver and the other acoustic transducer(s) can operate at a relatively low frequency. 
         [0014]    As a particular example of the type of acoustic transducers  20   a - 20   c  that can be used, if the desired frequency is above 4 kHz, one or more of the acoustic transducers  20   a - 20   c  can be in the form of piezoelectric transducers, such as those marketed under the designation PZT-4 by the Edo Corporation of Salt Lake City, Utah. If it is desired to operate below 4 kHz, one or more of the acoustic transducers  20   a - 20   c  can be in the form of conventional magnetostrictive drivers. In either case, the frequency, or frequencies, of the electrical output of the drivers  18   a  and  18   b  are matched to the frequencies of the acoustic transducers  20   a - 20   c,  so that the acoustic transducers  20   a - 20   c  are driven at their resonant frequencies, and the devices  16   a - 16   d  are designed to resonate at these frequencies. 
         [0015]    The acoustic transducers  20   a - 20   c  are mechanically coupled to the devices  16   a - 16   d  in a manner so that vibrations of the acoustic transducers  20   a - 20   c  are imparted to the devices  16   a - 16   d.  The acoustic transducers  20   a - 20   c  can be designed to form, with the devices  16   a - 16   d,  one assembly, or package, that is inserted as a unit in the wellbore  12  and mounted to the wellbore  12  in a conventional manner. 
         [0016]    The devices  16   a  and  16   b  provide equal and opposite loads on the acoustic transducer  20   a,  so that the acoustic transducer  20   a  can be used to vibrate the two devices  16   a  and  16   b  simultaneously. Similarly, the devices  16   b  and  16   c  provide equal and opposite loads on the acoustic transducer  20   b  so that the acoustic transducer  20   b  can be used to vibrate the two devices  16   b  and  16   c  simultaneously; and the devices  16   c  and  16   d  provide equal and opposite loads on the acoustic transducer  20   c  so that the acoustic transducer  20   c  can be used to vibrate the two devices  16   c  and  16   d  simultaneously. 
         [0017]    A pressure sensor  22   a  is mounted to the outer surface of the device  16   b;  a pressure sensor  22   b  is mounted between the outer surfaces of the devices  16   c  and  16   d;  and two pressure sensors  22   c  and  22   d  are mounted to the inner surfaces of the devices  16   a  and  16   c,  respectively. The pressure sensors  22   a  and  22   b  are adapted to sense the pressure of production fluid outside the devices  16   a - 16   d,  and the pressure sensors  22   c  and  22   d  are adapted to sense the pressure of the fluid inside the devices  16   a - 16   d.  Thus, the pressure sensors  22   a - 22   d  can be said to sense a condition of the devices  16   a - 16   d.    
         [0018]    A control unit  24 , which can be in the form of a microprocessor, or the like, is mounted to the wall of the wellbore  12  just above the device  16   a  and, although not shown in the drawing in the interest of clarity, is electrically connected to the pressure sensors  22   a - 22   d.  The control unit  24  is adapted to process signals from the pressure sensors  22   a - 22   d  and generate a corresponding output signal, for reasons to be described. 
         [0019]    A telemetry device  26  is mounted on the tool  10  and is adapted to collect data from the control unit  24  and transmit the data to the ground surface for processing. Since the telemetry device  26  is conventional it will not be described in detail. 
         [0020]    A conductor assembly  28 , shown by a dashed line, extends from the drivers  18   a  and  18   b,  the telemetry device  26 , through the wireline or coiled tubing  14  and to ground surface; and a conductor assembly  28   a  extends from the control unit  24  to the conductor assembly  28 . It is understood that the conductor assemblies  28  and  28   a  include sufficient cables to transmit electrical power and telemetry between the ground surface and the drivers  18   a  and  18   b,  the control unit  24 , and the telemetry device  26 . In the latter context, it is understood that the telemetry can include depth, pressure, and temperature data, and any other necessary wellbore data. Although not shown in the drawings in the interest of clarity, it is understood that the pressure sensors  22   a - 22   d  can also be electrically connected to the conductor assembly  28  for the same reasons. 
         [0021]    In operation, the devices  16   a - 16   d  and the acoustic transducers  20   a - 20   c  are inserted in, and mounted to, the wellbore  12 , preferably as a package, adjacent the formation F as shown in  FIG. 1 , and the devices  16   a - 16   d  are packed with sand, or the like, to form a gravel pack. Production is started and, as a result, fluids recovered from the formation F pass through the gravel packs and the devices  16   a - 16   d  and upwardly in the wellbore  12  to the above-mentioned production tubing (not shown) for passing to the ground surface, while the devices  16   a - 16   d  prevent fines or sand from the fluids from passing with the fluids. 
         [0022]    The pressure sensors  22   a  and  22   b  sense the pressure of the recovered fluid outside the devices  16   a - 16   d  and generate corresponding signals which are transmitted to the control unit  24 . Similarly, the pressure sensors  22   c  and  22   d  sense the pressure of the recovered fluid inside the devices  16   a - 16   d  and generate corresponding signals which are also transmitted to the control unit  24 . The control unit  24  processes the above signals and is programmed to respond when the fluid pressure outside the devices  16   a - 16   d  exceeds the fluid pressure inside the devices  16   a - 16   d  by a predetermined amount, indicating that the devices  16   a - 16   d  are at least partially clogged with scale, and/or any other foreign materials. When this happens, the control unit  24  sends a corresponding signal to the telemetry device  26  which, in turn, sends a corresponding signal, via the conductor assembly  28 , to the ground surface. 
         [0023]    The operator at the ground surface responds to the above signal from the telemetry device  26 , and lowers the tool  10 , via the wireline or coiled tubing  14 , to a position in which the drivers  18   a  and  18   b  are approximately aligned with the acoustic transducer  20   a  as shown in  FIG. 1 . The drivers  18   a  and  18   b  are activated by the electrical power from the ground surface, which is transmitted to the drivers by the conductor assembly  28 . The drivers  18   a  and  18   b  preferably convert the frequency of the electrical power to drive the acoustic transducer  20   a  to cause corresponding vibration of the devices  16   a  and  16   b  at their resonant frequency in the manner discussed above. These vibrations dislodge, fracture, or break up, the scale, and/or any other foreign materials, accumulating on the devices  16   a  and  16   b.  These vibrations can also mobilize fines inside the gravel packs and fines located in the wellbore  12  and in the formation F near the wellbore  12 . The scale, fines, and/or materials recovered from the devices  16   a  and  16   b  are allowed to fall to the bottom of the wellbore  12 , or could be circulated in any conventional manner to the ground surface for recovery. The circulation can occur with non-production fluids in a well intervention mode. However, this method is especially useful in that the scale, fines, and/or materials can be produced to the ground surface together with production fluids. Consequently, there is no lost production using this intervention mode. 
         [0024]    The audio output from the acoustic transducer  20   a,  which can be in the range of 4,000-30,000 Hz, also stimulates the formation F adjacent the devices  16   a  and  16   b  and reduces the “skin” around the wellbore  12  that can slow the flow of production fluid from the formation to the wellbore  12 . 
         [0025]    The tool  10  is then lowered further to a position in which the drivers  18   a  and  18   b  are approximately aligned with the acoustic transducer  20   b  and the above method is repeated in connection with the devices  16   b  and  16   c,  after which the method is repeated again with the acoustic transducer  20   c  to vibrate the devices  16   c  and  16   d.    
         [0026]    As a result of all of the foregoing, scale, and/or any other foreign materials accumulating on the devices  16   a - 16   d,  are broken up without causing any physical or chemical damage to the devices  16   a - 16   d,  while the formation F is stimulated and the skin around the wellbore  12  is reduced. 
         [0027]    According to another embodiment of the invention as shown in  FIG. 2 , the pressure sensors  22   a - 22   d  are eliminated and a reservoir model can be utilized to provide information relating to the need to vibrate the devices  16   a - 16   d  in the above manner. Otherwise the embodiment of  FIG. 2  contains the same components as the embodiment of  FIG. 1 . According to the embodiment of  FIG. 2 , data is initially collected to generate an initial reservoir model which is inputted to the control unit  24 . After production of fluid from the formation F is initiated, the production information is generated and inputted to the control unit  24  which matches the information to the initial model and adjusts the model as necessary to set a working model. As production continues, the additional production data is collected and inputted to the control unit  24  which compares the data to the working model. If there is a match, the data is fed back to the control unit  24  for further processing; and, if there is no match, the drivers  18   a  and  18   b  are actuated to drive the acoustic transducers  20   a - 20   c  and thus initiate the device vibration/production stimulation cycle as described above. 
         [0028]      FIG. 3  is a graph of the simulated production from the wellbore  12  vs. time and shows the reservoir model of  FIG. 2  by the rectangular data points, and a deviation from the model by the triangular data points, both before and after the scale is removed from the devices  16   a - 16   d  and the formation F is stimulated, including removal of the skin, in accordance with the foregoing “treatment”. It is noted that the treatment brings the production back to the model values. 
         [0029]    Thus, the system and method of the present invention performs the screening and stimulation functions yet eliminates the problems discussed above. 
         [0030]    Several variations may be made in both of the above embodiments without departing from the scope of the invention. These variations are as follows: 
         [0031]    (1) Rather than having an operator at the ground surface activate the drivers  18   a  and  18   b  in response to a corresponding signal received from the control unit  24 , as discussed above, a control unit, such as another microprocessor, the control unit  24 , or other similar device, could be provided to perform this function. 
         [0032]    (2) Rather than use the reservoir model discussed in connection with  FIG. 2  instead of the pressure sensors  22   a - 22   d,  the reservoir model could be used in addition to the pressure sensors  22   a - 22   d.    
         [0033]    (3) The tool  10  could be lowered into the wellbore  12  prior to the initiation of the fluid recovery process or at least prior to the sensing of the presence of scale, and any other foreign materials, as discussed above. 
         [0034]    (4) The control unit  24  can be programmed to adjust the pressure differential required to actuate the drivers  18   a  and  18   b.    
         [0035]    (5) The number and type of screening devices  16   a - 16   d,  drivers  18   a  and  18   b,  acoustic transducers  20   a - 20   c,  and/or pressure sensors  22   a - 22   d  can be varied. Although two drivers  18   a  and  18   b  have been shown driving the acoustic transducers  20   a - 20   c,  a single driver can be used. Placing multiple drivers on the tool  10  can allow multiple acoustic transducers to be driven simultaneously.

Summary:
A system and method for stimulating a formation penetrated by a wellbore and vibrating a device for supporting a gravel pack in the wellbore, according to which the build up of scale on the device is sensed and a corresponding signal is output. A tool is lowered into the wellbore, and includes a driver for driving an acoustic transducer coupled to the device for vibrating the device and stimulating the formation.