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BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to the field of cementing plugs downhole in wellbores. More particularly, the invention relates to an improved cementing plug system for identifying the location of one or more cementing plugs downhole in a wellbore.  
           [0002]    Cement plugs and wipers are placed in a wellbore to facilitate and control the positioning and cement, fluids and other materials downhole in the wellbore. In cementing operations, a bottom plug is positioned in wellbore casing and liquid cement is pumped into the casing to move the plug downhole. The bottom plug displaces mud used in drilling operations and wipes the interior surface of the casing string. When the bottom plug reaches floating equipment such as a float collar, the fluid pressure differential ruptures the bottom plug to permit movement of the cement through the bottom plug and into the annulus between the casing and the wellbore rock.  
           [0003]    As the cement is pumped into the casing, a top cement plug is often released into the casing to follow the cement and to wipe cement from the casing interior wall. When the top plug reaches the float collar further progress is impeded so that a resulting pressure increase notifies the operator that the top cement plug has reached the selected destination.  
           [0004]    Many variations of cement plugs have been developed. For example, U.S. Pat. No. 6,196,311 to Treece (2001) described an improved cementing plug which can be used as a lower or top plug. U.S. Pat. No. 6,237,686 to Ryll et al. disclosed an elastomeric cement plug. U.S. Pat. No. 6,244,350 to Gudmedstad et al. (2001) disclosed a cement plug having hollow upper and lower sections and ports for selectively placing cement in a wellbore. U.S. Pat. No. 6,263,968 to Freeman et al. disclosed a combination of top and bottom cement plugs for placing cement in a wellbore.  
           [0005]    Nonrotating cement plugs are used when polycrystalline diamond compact drill bits remove the plug and excess cement from the casing interior. U.S. Pat. No. 4,858,687 to Watson et al. disclosed a nonrotating cement plug, and U.S. Pat. No. 5,165,474 to Buisine et al. (1992) disclosed a cementing plug having deformable fins for resisting rotation of the plug.  
           [0006]    To cement a casing in a wellbore, volumetric calculations consider the borehole dimentions and depth of the float valve. If a displacement plug does not reach the seat when the calculated total displacement volume has been pumped, pumping operations are typically stopped so that overdisplacement of the cement does not occur. Overdisplacement of the cement moves all of the cement outside of the casing. Underdisplacement of the cement leaves cement within the casing, requiring expensive and time consuming drilling operations to remove the misplaced cement. U.S. Pat. No. 5,095,988 to Bode (1992) disclosed a ball injection apparatus for launching cement plugs into well casing. Upper and lower plug assemblies sequentially launched balls, bombs or darts into the cement to control cement placement. To verify the location of the balls or other devices in the cement, a magnetic sensor was placed downhole to detect small magnets implanted in the balls and to trip a light indicating passage of the ball or other device.  
           [0007]    Various techniques have been proposed to control the amount of cement pumped into a wellbore. For example, U.S. Pat. No. 6,170,574 to Jones (2001) disclosed a pump bailer for transporting cement downhole into the wellbore and for releasing the cement at a selected position within the wellbore.  
           [0008]    Other techniques use wireline logging tools to locate the position of a cement column within a casing. For example, U.S. Pat. No. 6,189,383 to Tello et al. (2001) described a wireline logging tool for measuring the density of fluids downhole in a casing.  
           [0009]    Various techniques lock a tool into a selected position downhole in a casing. Mechanical locking techniques are well known as a technique to positioning a tool, and one form of mechanical location device is identified in U.S. Pat. No. 6,199,632 to Shy (2001). Another form of mechanical locking device was disclosed in U.S. Pat. No. 4,491,178 to Terrell et al. (1985), which disclosed a wireline controlled setting tool having an explosive mechanism for activating operation of the tool downhole in the wellbore.  
           [0010]    Cementing operations are subject to many variables. Variances in internal pipe diameter can significantly affect the column height of cement within the pipe. Recognized displacement factors do not account for the volume within each tool joint, leading to additional discrepancies in cement placement. In pipe including collars, drift in the internal diameter and gaps between the pipe and couplings create additional variables in the pipe string interior volume. Obstructions in the pipe string interior can cause a cement plug from landing short of the correlative landing collar. Lost circulation, washouts, and obstructions can interfere with cement placement, thereby jeopardizing the accuracy of identifying desired cement placement within the wellbore. SUMMARY OF THE INVENTION  
           [0011]    The invention provides a system for identifying the location of a body in a wellbore extending downwardly from the surface through subsurface geologic formations. The system comprises a body moveable in the wellbore, a transmitter for sending a signal through the wellbore which identifies the location of the body within the wellbore, a receiver for collecting the signal, and a processor engaged with said receiver for processing said signal and for identifying the location of the body within the wellbore.  
           [0012]    In another embodiment, the invention provides a system for identifying the location of a cement plug in a wellbore extending downwardly from the surface through subsurface geologic formations, and comprises a body moveable in the wellbore, a transmitter attached to the body for sending a signal through the wellbore which identifies the location of the body within the wellbore, a receiver for collecting the signal, and a processor engaged with the receiver for processing the signal and for identifying the location of the body within the wellbore.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 illustrates a transmitter attached to a body downhole in a wellbore and a receiver located at the wellbore surface.  
         [0014]    [0014]FIG. 2 illustrates a transmitter located at the wellbore surface and a receiver attached to the body.  
         [0015]    [0015]FIG. 3 illustrates a transmitter and transmitter located at the wellbore surface.  
         [0016]    [0016]FIG. 4 illustrates a transmitter engaged with casing pipe for transmitting signals through the casing. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The invention provides a unique system for precisely locating a device downhole in a wellbore. The system is particularly suited to the location of plugs and other devices used in downhole cementing operations.  
         [0018]    [0018]FIG. 1 illustrates one embodiment of the invention wherein wellbore  10  is formed through geologic formations  12  and casing pipe  14  is positioned within and becomes part of wellbore  10 . Body  16  is movable within pipe  14  and can be surrounded on either side with well fluids such as cement  18 . Body  16  can comprise an upper or lower cement plug, a valve, a whipstock, a packer, a chemical injection manifold, a pump, or other type of downhole drilling, workover or production equipment capable of use downhole in wellbore  10 . Although, body  16  is moveable within wellbore  10  to permit access and installation therein, body  16  can be permanently or temporarily positioned at a selected location within wellbore  10  suitable for accomplishing a selected function.  
         [0019]    Transmitter  20  is engaged with body  16  for sending a signal through wellbore  10 . Receiver  22  collects the signal from transmitter  20 , and processor  24  is engaged with receiver  22  for processing the signal and for identifying the location of body  16  within wellbore  10 .  
         [0020]    Transmitter  20  can be engaged with body  16  in different ways including proximity with, attachment to, or integration within body  16 . As shown in FIG. 1, transmitter  20  is attached to body  16  in a preferred embodiment of the invention. In other embodiments of the invention, transmitter  20  can be integrated within body  16  to protect transmitter  20  from downhole temperatures, impacts, and corrosion. The location of transmitter  20  is preferably known relative to body  16  so that the signal generated by transmitter  20  indicates the location of body  16 .  
         [0021]    Alternatively, transmitter  20  can be located at the wellbore surface as shown in FIG. 2, with receiver  22  engaged with body  16 . Receiver  22  can communicate with processor  24  through different transmission techniques such as acoustic pulses detectable with pulse echo sensors, electrical signals transmitted through casing pipe  14 , electrical signals transmitted through a wire (not shown) attached to casing pipe  14 , through signals transmitted through geologic formations  12  (particularly useful in horizontal wells) and other transmission techniques known in the art.  
         [0022]    In yet another embodiment of the invention, transmitter  20  and receiver  22  can both be located at the surface of wellbore  10  as shown in FIG. 3, wherein the signal transmitted by transmitter  20  travels downwardly through wellbore  10 , is reflected by body  16  and returns to the surface of wellbore  10  for detection by receiver  22 . In this embodiment of the invention body  16  provides the function of a signal reflector, and the distance between transmitter  20  and body  16  can be calculated by the transmission time required for delivery and receipt of the transmitted signal. Signal reflector  26  can be attached to or integrated within body  16  to facilitate such function.  
         [0023]    The location of transmitter  20  and receiver  22  will impact the physical character and requirements of such components. If transmitter  20  is to be moved within wellbore  10  as indicated in FIG. 1, battery pack  26  can be attached to transmitter  20  to provide sufficient power to generate the signals. Although electric wires can be attached between transmitter  20  and the receiver  22  to provide electrical power and signal transmission, one of the unique benefits provided by the invention is the capability to identify downhole location of body  16  without the encumbrances and disadvantages inherent with hard wires. This feature of the invention is particularly useful in deep wellbores  10  up to twenty thousand feet deep and in deviated and horizontal and multilateral wellbore branches extending thousands of feet in different directions from the surface entry of wellbore  10 .  
         [0024]    As previously indicated, signal transmission from a transmitter  20  located downhole in wellbore  10  can be made through fluids within wellbore  10 , through geologic formations  12  overlaying wellbore  10 , and through casing pipe  14 . As shown in FIG. 4, transmitter  20  includes contact  28  for contacting the inner wall of casing pipe  14 . Receiver  22  has contact  30  attached to casing pipe  14  Contact  28  permits signals to be transmitted from transmitter  20  to casing pipe  14  so that receiver  22  can detect such signals through contact  30 .  
         [0025]    Processor  24  receives the signals detected by receiver  22  and converts such signals into selected information. As representative examples, such information can indicate the distance between body  16  and the surface of wellbore  10 , regardless of the offset of wellbore  10  from vertical, the elevation of body  16 , the orientation of body  16 , the heading of body  16 , the proximity of body  16  to downhole features of wellbore  10  or of other devices within wellbore  10 , the temperature or viscosity or pressure of fluids downhole in wellbore  10 , and other data useful to the formation of or production from wellbore  10 .  
         [0026]    Although the invention has been described in terms of certain preferred embodiments, it will become apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.

Summary:
An apparatus for locating cement plugs and other devices downhole in a wellbore. A cement plug is moved downhole into a wellbore through a casing, and a transmitter sends a signal to a receiver. A processor engaged with the receiver identifies the elevation or location of the cement plug. Accurate placement of the cement plug eliminates problems associated with underplacement and overplacement of cement in a wellbore.