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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a wellhead isolation tool (“WIT”) and, more specifically, to such a tool which locates the fluid control and connection devices at the lower end of the WIT.  
           [0002]    A WIT is typically used in an oil or gas well to protect the internal surfaces of the wellhead assembly that is installed at the top of the well bore from corrosive or erosive materials during stimulation of the well. The WIT is normally mounted on the top of the wellhead assembly and comprises a tubular mandrel which is inserted through the wellhead assembly and sealed to the production tubing string. The well stimulation fluid is then pumped through the mandrel and into the production tubing string. Means, such as one or more hydraulic cylinders, are usually provided to raise and lower the mandrel through the wellhead assembly. Because of the large stroke required to do this, the WIT is usually quite tall—at least as tall as the wellhead assembly. In previous WIT designs, the mandrel extends beyond the top of the hydraulic cylinders and the stimulation fluid is injected into the top end of the mandrel. To make the necessary connections, workers have to access the top of the WIT, which requires the construction of platforms, ladders and the like. This not only increases costs, but also creates a safety concern.  
         SUMMARY OF THE INVENTION  
         [0003]    In accordance with the present invention, these and other limitations in the prior art are overcome by providing a wellhead isolation tool for use with a wellhead assembly from which a tubing string is suspended, the wellhead isolation tool comprising a tubular mandrel which includes an axial passage that extends therethrough and a lower end that is adapted to engage the tubing string, a pumping head which is connected over the wellhead assembly and which includes an internal chamber that is in fluid communication with the axial passage and a port that extends through the pumping head to the chamber, and an actuator assembly which is connected over the pumping head and which functions to move the mandrel axially through the pumping head and into engagement with the tubing string. In this manner, when the mandrel is engaged with the tubing string, fluid may be communicated through the port, the chamber and the mandrel and into the tubing string.  
           [0004]    In accordance with a preferred embodiment of the invention, the wellhead isolation tool also comprises a sleeve which is connected between the actuator assembly and the mandrel and which is positioned at least partially within the chamber when the mandrel is engaged with the tubing string. The sleeve comprises an axial bore that communicates with the axial passage in the mandrel and at least one generally radial bore that communicates between the chamber and the axial bore.  
           [0005]    In addition, the wellhead isolation tool preferably includes a generally cylindrical diffusion element which is positioned within the chamber. The diffusion element includes an outer diameter surface, an inner diameter surface which surrounds at least a portion of the sleeve when the mandrel is engaged with the tubing string, and a plurality of holes which extend generally radially between the inner and outer diameter surfaces.  
           [0006]    Thus, the present invention allows the well stimulation fluid to be injected from the side of the pumping head, which is located between the wellhead assembly and the actuator assembly. Consequently, all the control, injection and lockdown functions are located in one convenient area at the lower end of the WIT. Therefore, no need exists to access the top of the WIT, which reduces costs and safety concerns. In addition, the diffusion element disperses the flow of the incoming fluid and thus prevents the fluid from impinging on isolated spots within the sleeve. Therefore, the diffusion element prevents the fluid from unduly eroding the sleeve.  
           [0007]    These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a longitudinal cross-sectional view of the WIT of the present invention;  
         [0009]    [0009]FIG. 2 is an enlarged longitudinal cross-sectional view of the WIT of the present invention, but with the cross section taken at a different radial angle than the cross section of FIG. 1;  
         [0010]    [0010]FIG. 3 is a cross-sectional view of the pumping head portion of the WIT shown installed on an exemplary wellhead assembly;  
         [0011]    [0011]FIG. 4 is an enlarged cross-sectional view of the pumping head portion of the WIT depicted in FIG. 3; and  
         [0012]    [0012]FIG. 5 is an isometric view of the pumping head portion of the WIT, with some components shown in partial section. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    The wellhead isolation tool (“WIT”) of the present invention is especially useful in protecting the internal surfaces of a wellhead assembly from erosion or corrosion during stimulation of an oil or gas well over which the wellhead assembly is installed, while at the same time providing convenient access to the fluid injection ports at the lower end of the WIT. For purposes of the present application, the WIT, which is indicated generally in the Figures by reference number  10 , is shown in conjunction with an exemplary wellhead assembly. However, it should be understood that the WIT may be used with a variety of wellhead and christmas tree assemblies, either surface or subsea, and that the present invention should not be considered as limited to the wellhead assembly described herein.  
         [0014]    Referring to FIG. 3, the WIT  10  is shown connected to the top of an exemplary wellhead assembly  12  that is installed at the upper end of a well bore (not shown). The wellhead assembly  12  comprises a wellhead or tubing spool  14  having a central bore  16  in which a tubing hanger  18  is supported. The tubing hanger  18  in turn is connected to the upper end of a string of production tubing  20  that extends into the well bore. A first valve assembly  22  is connected to the top of the wellhead  14 , for example using a conventional clamp-type connector  24 , and a second valve assembly  26  may be connected to the top of the first valve assembly such as by bolts  28 . The first and second valve assemblies  22 ,  26  are provided to control the flow of fluid through the production tubing  20 , and in the embodiment of the wellhead assembly  12  shown in FIG. 3, the valve assemblies comprise conventional gate valves having respective gates  30  and  32 . In addition, the wellhead assembly  12  may include a connector  34  to facilitate attaching the WIT  10  to the second valve assembly  26 . As shown in FIG. 3, the connector  34  may be secured to the top of the second valve assembly  26  by bolts  36 .  
         [0015]    Referring to FIGS. 1 and 2, the WIT  10  is shown to comprise a tubular mandrel  38 , an actuator assembly  40 , a pumping head  42  and a sleeve  44 . The actuator assembly  40  is selectively operable to lower the mandrel  38  into the wellhead assembly  12  until the lower end of the mandrel engages the top of the production tubing string  20 . The sleeve  44  serves to connect the mandrel  38  to the actuator assembly  40  and to communicate fluid from the pumping head  42  to the mandrel.  
         [0016]    The actuator assembly  40  comprises a lift rod  46  that is threaded into the top of the sleeve  44  generally at  48 . The mandrel  38  in turn is threaded into the bottom of the sleeve  44  generally at  50 . The lift rod  48  extends through an elongated guide tube  52  which is attached to the top of the pumping head  42 . In the embodiment of the invention illustrated in the Figures, the guide tube  52  is clamped to an adapter  54  which in turn is secured to the top of the pumping head  42 , for example using bolts  56 . The upper end of the lift rod  46  protrudes through an axial hole that extends through the top of the guide tube  52 . A stem packing  58  is preferably provided to seal between the lift rod  46  and the guide tube  52 . The stem packing  58  ideally is of the type shown in U.S. Pat. Nos. 4,527,806 or 4,576,385, both of which are hereby incorporated herein by reference, although any suitable type of stem packing could be used. The stem packing  58  is secured in place by a packing nut  60  which in turn is secured in position by a retainer cap  62  that is threaded to the top of the guide tube  52 .  
         [0017]    The top of the lift rod  46  is connected to a pivot connector  64  such as by threads  66 . The pivot connector  64  is connected to a pivot arm  68  via a pin  70 . Each end of the pivot arm  64  is connected to the upper end of a corresponding hydraulic cylinder  72  with suitable means, such as a pin  74 . The lower end of each cylinder  72  is connected to a corresponding riser  76  such as by a pin  78 , and each risers  76  is rigidly attached to a plate  80  that is secured to the bottom of pumping head  42 , for example using bolts  82 .  
         [0018]    Referring again to FIG. 3, when the WIT  10  is used to stimulate the well, the mandrel  38  is lowered downward through the gates  30 ,  32  of the valve assemblies  22 ,  26 , through the tubing hanger  18  and into the top of the production tubing string  22 . An annular cup seal  84  is provided at the end of the mandrel  38  to seal between the outer diameter of the mandrel and the inner diameter of the tubing string  20 . The seal  84  functions to isolate the fluid flow within the mandrel  38  and the tubing string  20 , which is represented by the arrow  86 , from an annulus  88  that surrounds the mandrel above the seal. The seal  84  is energized into sealing engagement with the tubing string  20  when the pressure below the seal is greater than the pressure in the annulus  88 . While the cup seal  84  provides certain operational advantages in the present invention, it should be understood that any other suitable seal could be substituted for the cup seal.  
         [0019]    Referring to FIGS. 4 and 5, the pumping head  42  is shown to comprise an internal diffusion chamber  90 , a number of fluid injection ports  92  which extend radially through the pumping head from the diffusion chamber to the outer diameter of the pumping head, and a corresponding number of valves  94  for controlling the flow of fluid through the injection ports. In a preferred embodiment of the invention, the interior surfaces of the diffusion chamber  90  and the injection ports  92  are coated or clad with a highly wear resistant material to minimize erosion. In addition, the valves  94  are ideally separate components which are bolted or otherwise secured to the outer diameter of the pumping head  42  via suitable connector members  96 .  
         [0020]    The pumping head  42  also comprises a generally cylindrical diffusion element  98  which is supported on a shoulder  100  that is formed in the bottom of the diffusion chamber  90 . The diffusion element  98  optimally comprises an inner diameter which is slightly larger that the outer diameter of the sleeve  44 , an outer diameter which is smaller than the inner diameter of the diffusion chamber  90 , and a plurality of relatively small holes  102  which extend generally radially through the diffusion element between its inner diameter and its outer diameter. The diffusion element  98  is preferably made of a highly wear resistant material, such as tungsten carbide or silicon carbide. In addition, the diffusion element  98  is ideally held in position within the diffusion chamber  90  between the shoulder  100  and an axial extension  104  which depends from the bottom of the adapter  54 .  
         [0021]    The diffusion element  98  is preferably sealed to the diffusion chamber  90  to ensure that the fluid from the injection ports  92  passes through the holes  102 . Accordingly, a first annular seal  106  is positioned between the bottom end of the diffusion element  98  and the shoulder  100 , and a second annular seal  108  is positioned between the top end of the diffusion element and the axial extension  104 . In addition, if the diffusion element  98  is made of a wear resistant material which is brittle in nature, it may be desirable to design the diffusion element such that its axial dimension is slightly smaller than the axial distance between the shoulder  100  and the axial extension  104  so that excessive clamping forces are not exerted on the diffusion element when the adapter  54  is fully connected to the pumping head  42 . Accordingly, the first and second seals  106 ,  108  are adapted to seal across any resulting axial clearances between the bottom of the diffusion element  98  and the shoulder  100  and between the top of the diffusion element and the axial extension  104  to prevent the diffusion element from vibrating or “rattling” within the diffusion chamber  90 . Seals  106 ,  108  are preferably elastomer O-rings, although any suitable seal could be used.  
         [0022]    Referring still to FIGS. 4 and 5, when the mandrel  38  is lowered into the wellhead assembly  12 , the sleeve  44  will land in the pumping head  42  and a number of annular seals  110  which are supported on the sleeve will seal against the pumping head to thereby isolate the diffusion chamber  90  from the annulus  88  that surrounds the mandrel above the seal  84 . The sleeve  44  includes a blind bore  112  and a plurality of apertures  114  that extend radially downwardly from the outer diameter of the sleeve to the blind bore. The exposed surfaces of the sleeve  44  are preferably coated or clad with a highly wear resistant material to minimize erosion. When the sleeve  44  is seated in the pumping head  42 , the apertures  114  are in general axial alignment with the diffusion chamber  90 . The sleeve  44  is locked in this seated position by a number of lockdown screws  116 , which are screwed inwardly until they engage an external groove  118  that is formed on the outer diameter of the sleeve.  
         [0023]    In order to isolate the diffusion chamber  90  from the environment, a seal  120  is ideally provided between the outer diameter of the sleeve  44  and the central bore of adapter  54 , and one or more seals  122 , 124  are optimally positioned between the outer diameter of the axial extension  104  and the central bore of pumping head  42 . The seals  110 , 120  and  122  are preferably of the type disclosed in U.S. Pat. Nos. 5,791,657 or 5,180,008, both of which are hereby incorporated herein by reference, although any suitable seal could be used.  
         [0024]    Referring specifically to FIG. 5, the adapter  54  preferably comprises a first passageway  126  which extends radially outward from the central bore of the adapter, a second passageway  128  which extends generally downwardly through the adapter from adjacent the first passageway, a radial groove  130  which is formed in the outer diameter surface of the axial extension  104  below the seal  120 , and a third passageway  132  which extends between the radial groove and the bottom of the second passageway. Thus, the central bore of the adapter  54  is connected with the diffusion chamber  90  through the first, second and third passageways  126 , 128 , 132  and the radial groove  130 . Furthermore, the first and second passageways  126 , 128  are connected through a conventional needle valve  134  which is mounted in the body of the adapter  54 . Therefore, when the needle valve  134  is opened, the first and second passageways  126 ,  128  are connected and pressure can be equalized between the diffusion chamber  90  and the central bore of the adapter  54 .  
         [0025]    Similarly, the pumping head  42  comprises a first passageway  136  which extends radially outwardly from the central bore of the pumping head below the seals  110 , a second passageway  138  which extends upwardly from the first passageway  136  to the shoulder  100 , and a needle valve  140  which is disposed between the first and second passageways. Thus, the diffusion chamber  90  is connected with the annulus  88  around the mandrel  38  by the first and second passageways  136 , 138 . Therefore, when the needle valve  140  is opened, the first and second passageways  136 , 138  are connected and pressure can be equalized between the diffusion chamber  90  and the annulus  88 . Consequently, when the mandrel  38  is raised and lowered, the needle valves  134 , 140  can be used to overcome hydraulic lock conditions which could impede the movement of sleeve  44 .  
         [0026]    Referring again to FIG. 3, the pumping head  42  ideally also comprises a passage  142  which extends radially from the central bore of the pumping head below the seals  110  to the outer diameter of the pumping head. Flow through passage  142  is controlled by a valve  144 , which is preferably a separate component that is bolted to the outer diameter of the pumping head  42 . When the mandrel  38  is raised or lowered, fluid is injected through valve  144  and the passage  142  to pressurize the annulus  88  around the mandrel  38 . This pressure collapses the cup seal  84  so that the seal does not drag against the tubing string  20  or the bore of the wellhead  14  as the mandrel  38  moves up or down.  
         [0027]    In operation, when the WIT  10  is installed on the wellhead assembly  12 , the hydraulic cylinders  72  are actuated to draw the lift rod  46 , and thus the sleeve  44  and the mandrel  38 , upward. Once the WIT  10  has been secured to the wellhead assembly  12 , the valves  22 ,  26  are opened and the cylinders  72  are actuated to move the mandrel  38  downward. The mandrel  38  passes through the gates  30 ,  32 , the wellhead  14  and the tubing hanger  18  until the bottom end of the mandrel enters and seals to the production tubing string  20 . At this point, the sleeve  44  is landed and sealed in the pumping head  42 , and the lockdown screws  116  are engaged to secure the sleeve, and thus the mandrel  38 , in place.  
         [0028]    Stimulation fluid is now pumped through the inlet valves  94  and the injection ports  92  and into the diffusion chamber  90 . From the diffusion chamber  90 , the fluid is forced through the small holes  102  in the diffusion element  98 , through the angled apertures  114  in the sleeve  44  and down into the mandrel  38 . The stimulation fluid is typically a highly erosive slurry and may also contain corrosive chemicals. However, the diffusion element  98  disperses the flow of the incoming fluid and thus prevents the fluid from impinging on isolated spots within the sleeve  44 . The diffusion element  98  is intended to be a replaceable, sacrificial barrier for protecting the more expensive sleeve  44  from erosion. Moreover, the number and size of the holes  102  in the diffusion element  116  may be optimized for various fluids and flow velocities in order to minimize erosion of the diffusion element  98 .  
         [0029]    It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.

Summary:
A wellhead insolation tool for use with a wellhead assembly from which a tubing string is suspended comprises a tubular mandrel which includes an axial passage that extends therethrough and lower end that is adapted to engage the tubing string, a pumping head which is connected over the wellhead assembly and which includes an internal chamber that is in fluid communication with the axial passage and a port that extends through the pumping head to the chamber, and an actuator which is connected over the pumping head for moving the mandrel axially through the pumping head and into engagement with the tubing string. When the mandrel is engaged with the tubing string, fluid may be communicated through the port, the chamber and the mandrel and into the tubing string.