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TECHNICAL FIELD OF THE INVENTION 
     This invention relates, in general, to tools and equipment for completing a subterranean well that traverses a hydrocarbon bearing formation and, in particular, to a downhole packer having a slidable cover over a seal assembly to protect the seal assembly from damage during high rate circulation of viscous fluids prior to setting. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the present invention, its background will be described in relation to setting packers, as an example. 
     In the course of treating and preparing a subterranean well for production, well packers are commonly run into the well on a conveyance such as a work string or production tubing. The purpose of the packer is to support production tubing and other completion equipment, such as sand control assemblies adjacent to a producing formation, and to seal the annulus between the outside of the production tubing and the inside of the well casing to block movement of fluids through the annulus past the packer location. 
     Typically, packers may have an upper and a lower set of anchor slips with opposed camming surfaces, which cooperate with complementary opposed wedging surfaces, whereby the anchor slips are outwardly radially extendable into gripping engagement against the well casing bore in response to relative axial movement of the wedging surfaces. Packers may also carry annular seal assembly including one or more seal elements, which are radially expandable into sealing engagement against the bore of the well casing in response to axial compression forces. 
     Prior to actuation and the subsequent radial expansion of the seal elements, many adverse environmental conditions may exist around the outer diameter of the seal elements. For example, certain completion operations require viscous fluids to be circulated in the annulus between the well casing and the tubing string at high rates. 
     It has been found that these high flow rates of viscous wellbore fluids may create a low pressure region around and adjacent to the outer diameter of the packers and the seal elements. It has also been found that this low pressure region may cause the seal elements to radially expand prematurely, thus causing their exposed surfaces to be further damaged by abrasive contact with the high flow rate wellbore fluids. 
     This premature contact with the high flow rate wellbore fluids may further accelerate the destruction of the seal elements prior to setting, which in turn, reduces the capability of the packer to provide the desired seal between the outside of the production tubing and the inside of the well casing. 
     Therefore, a need has arisen for a packer that is capable of being deployed in adverse environments such that its seal elements are not affected by the adverse environments, such as high circulation rate fluids, prior to setting the packer. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a high circulation rate packer. The high circulation rate packer of the present provides protection for the seal elements from the adverse downhole environments, such as high circulation rates of wellbore fluids, prior to radial expansion of the seal elements of the packer. 
     In one aspect, the high circulation rate packer of the present invention includes a slidable cover that covers the seal elements prior to radial expansion of the seal elements downhole. The slidable cover is rigid and impervious to pressure drops and abrasive fluids downhole. The slidable cover is located over the expandable seal elements until a setting sequence of the packer is initiated. During the setting sequence, the slidable cover may be longitudinally or axially shifted thereby exposing the seal elements and allowing radial expansion thereof. 
     In another aspect, the present invention is directed to a high circulation rate packer for establishing a sealing and gripping engagement with a well casing disposed in a wellbore. The high circulation rate packer includes a packer mandrel and a seal assembly slidably disposed about the packer mandrel, the seal assembly having a running position and a radially expanded sealing position. The high circulation rate packer also includes a cover slidably disposed relative to the packer mandrel, the cover having a running position wherein the cover is disposed about the seal assembly and a retracted position wherein the cover is at least partially removed from about the seal assembly. 
     In one embodiment, the cover that is made from a rigid impervious sheet type material. The cover may also be a material that is impervious to downhole fluids. The cover, in its running position, may cover substantially the entire outer surface of the seal assembly. The cover may further include a locking mechanism that locks the cover in the retracted position. 
     In another aspect, the present invention is directed to a high circulation rate packer for establishing a sealing and gripping engagement with a well casing disposed in a wellbore. The high circulation rate packer includes a packer mandrel and a seal assembly disposed about the packer mandrel. The seal assembly has a running position and a radially expanded sealing position. A slip wedge is slidably disposed about the packer mandrel, the slip wedge has a running position and a setting position. A slip assembly is slidably disposed about the packer mandrel and operably associated with the slip wedge. The slip assembly has a running position and a radially expanded gripping position. A cover is slidably disposed relative to the packer mandrel and operably associated with the slip wedge. The cover has a running position wherein the slidable cover is disposed about the seal assembly and a retracted position, when the slip wedge is in the setting position, wherein the cover is at least partially removed from about the seal assembly. 
     The packer mandrel may further include a first outer diameter portion and a second outer diameter portion wherein the second outer diameter portion is greater than the first outer diameter portion, wherein the second outer diameter portion supports the seal assembly in the sealing position and the first outer diameter portion supports the seal assembly in the running position. The high circulation rate packer may include a piston slidably disposed about the packer mandrel and operably associated with the seal assembly for forcing the seal assembly between the running position and the sealing position. The high circulation rate packer may also include an element backup shoe slidably disposed about the packer mandrel and operably associated with the seal assembly, the element backup shoe having a running position wherein the element backup shoe is not in sealing engagement with the well casing and a sealing position wherein the element backup shoe is in sealing engagement with the well casing. 
     In a further aspect, the present invention is directed to a high circulation rate packer for establishing a sealing and gripping engagement with a well casing disposed in a wellbore. The high circulation rate packer includes a packer mandrel, a piston slidably disposed about the packer mandrel, the piston having a running position and a setting position, a slip wedge slidably disposed about the packer mandrel, the slip wedge having a running position and a setting position and a cylinder slidably disposed about the piston and operably associated with the slip wedge. A pick-up ring is disposed radially between the packer mandrel and the cylinder and longitudinally between the slip wedge and the piston. The pick-up ring is in fluid communication with an interior cylindrical bore of the packer mandrel. The pick-up ring, the piston and the cylinder define a first chamber and the pick-up ring, the slip wedge and the cylinder define a second chamber that may be in fluid communication with the first chamber. A slip assembly is slidably disposed about the packer mandrel and operably associated with the slip wedge. The slip assembly has a running position and a radially expanded gripping position, when the slip wedge is in the setting position. A cover is slidably disposed relative to the packer mandrel and operably associated with the slip wedge. The cover has a running position wherein at least a portion of the cover is disposed about the seal assembly and a retracted position, when the slip wedge is in the setting position, wherein the cover is at least partially removed from about the seal assembly. 
     In one embodiment, fluid pressure in the second chamber acts upon the slip wedge in a first direction to operate the slip wedge from the running position to the setting position, thereby operating the slip assembly from the running position and the radially expanded gripping position. In another embodiment, fluid pressure in the first chamber acts upon the piston in a second direction to operate the seal assembly from the running position to the sealing position. In a further embodiment, a shear ring prevents the piston from operating the seal assembly to the sealing position until the slip assembly is in the gripping position. Also, a locking mechanism may be operably associated with the cover that locks the cover in the retracted position when the slip assembly is in the gripping position. 
     In yet another aspect, the present invention is directed to a method for setting a packer to establish a sealing and gripping engagement with a well casing including lowering the packer into the well casing to a selected location, applying fluid pressure to an expandable chamber within the packer, responsive to the fluid pressure, radially setting a slip assembly into gripping engagement with the well casing and sliding a cover disposed about a seal assembly to at least partially expose a seal assembly, and responsive to the setting of the slip assembly and the fluid pressure, radially outwardly extending the seal assembly into sealing engagement with the well casing. 
     The method may also include contacting the slip assembly with a slip wedge, forcing the seal assembly from a first outer diameter portion of the packer mandrel to a second outer diameter portion of the packer mandrel, the second outer diameter portion being greater than the first outer diameter portion, longitudinally sliding a piston slidably disposed about the packer to operate the seal assembly into the sealing engagement with the well casing, and setting at least one element backup shoe positioned substantially adjacent to the seal assembly into sealing engagement with the well casing. 
     In another aspect, the present invention is directed to a method for setting a packer to establish a sealing engagement with a well casing including lowering the packer into the well casing to a selected location, sliding a cover disposed about a seal assembly to expose the seal assembly and radially outwardly extending the seal assembly into sealing engagement with the well casing. 
     The method may also include sliding a piston slidably disposed about the packer to force the seal assembly into the sealing engagement with the well casing, shearing shear pins retaining the piston, forcing the seal assembly from a first outer diameter portion of the packer mandrel to a second outer diameter portion of the packer mandrel, the second outer diameter portion being greater than the first outer diameter portion, and setting at least one element backup shoe positioned substantially adjacent to the seal assembly into sealing engagement with the well casing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
         FIG. 1  is a schematic illustration of an offshore platform operating a high circulation rate packer in accordance with the present invention; 
         FIGS. 2A-2B  are quarter-section views of an exemplary high circulation rate packer of the present invention in a running configuration in accordance with the present invention; 
         FIGS. 3A-3B  are quarter-section views of the tool of  FIGS. 2A-2B  in a partially set configuration in accordance with the present invention; and 
         FIGS. 4A-4B  are half-section views of the tool of  FIGS. 2A-2B  in a fully set configuration in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
     In the following description of the representative embodiments of the invention, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. In general, “above”, “upper”, “upward” and similar terms refer to a direction toward the earth&#39;s surface along a wellbore, and “below”, “lower”, “downward” and similar terms refer to a direction away from the earth&#39;s surface along the wellbore. 
     Referring initially to  FIG. 1 , a pair of packers in a work string deployed in an offshore oil or gas well is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to wellhead installation  22 , including blowout preventers  24 . Platform  12  has a hoisting apparatus  26  and a derrick  28  for raising and lowering pipe strings such as work string  30 . 
     A wellbore  32  extends through the various earth strata including formation  14 . A casing  34  is cemented within wellbore  32  by cement  36 . Work string  30  includes various tools including sand control screens  38 ,  40 ,  42  positioned in an interval of wellbore  32  adjacent to formation  14  between packers  44 ,  46  of the present invention. 
     Importantly, even though  FIG. 1  depicts a vertical well, it should be understood by one skilled in the art that the packers of the present invention are equally well-suited for use in deviated wells, inclined wells or horizontal wells. Also, even though  FIG. 1  depicts an offshore operation, it should be understood by one skilled in the art that the packers of the present invention are equally well-suited for use in onshore operations. 
     Note that, in this specification, the terms “liner” and “casing” are used interchangeably to describe tubular materials, which are used to form protective linings in wellbores. Liners and casings may be made from any material such as metals, plastics, composites and the like, may be expanded or unexpanded as part of an installation procedure and may be segmented or continuous. Additionally, it is not necessary for a liner or casing to be cemented in a wellbore. Any type of liner or casing may be used in keeping with the principles of the present invention. 
     Referring now to  FIGS. 2A-2B , therein is depicted a packer of the present invention that is generally designated  50 . Packer  50  includes a substantially tubular, longitudinally extending mandrel  52  having a substantially cylindrical bore  54  defining a longitudinal production flow passageway. Mandrel  52  may be coupled with a substantially tubular, longitudinally extending tubular string such as work string  30 . 
     Mandrel  52  includes a larger outer diameter portion  58  and a smaller outer diameter portion  60 . Mandrel  52  has a shoulder  56  that is adjacent to the larger outer diameter portion  58 . Mandrel  52  may include a transition portion  68  between larger outer diameter portion  58  and smaller outer diameter portion  60  that is formed similar to a ramp or wedge. Slidably positioned around larger outer diameter portion  58  of mandrel  52  is a shear ring  62  having one or more shear pins associated therewith such as shear pin  64 . Shear ring  62  may slide around larger outer diameter portion  58  after shear pin  64  is broken until shear ring  62  contacts shoulder  56 . Shear pin  64  may be designed to shear at any desirable shear force. 
     Substantially adjacent to shear pin  64  is an upper element backup shoe  66  that is slidably positioned around larger outer diameter portion  58  of mandrel  52 . Additionally, a seal assembly, depicted as expandable seal element  72 , is slidably positioned around smaller outer diameter portion  60  between upper element backup shoe  66  and a lower element backup shoe  70 . Adjacent to lower element backup shoe  70  and slidably positioned around smaller outer diameter portion  60  of mandrel  52  is an element retainer  76 . Also slidably positioned around the smaller outer diameter portion  60  of mandrel  52  is a piston  78  that is substantially adjacent to element retainer  76 . In the illustrated embodiment, one expandable seal element  72  is shown; however, a seal assembly of the packer of the present invention may include any number of expandable seal elements. 
     Upper element backup shoe  66  and lower element backup shoe  70  may be made from a deformable or malleable material, such as mild steel, soft steel, brass, and the like and may be thin cut at their distal ends. The ends of upper element backup shoe  66  and lower element backup shoe  70  will deform and flare outwardly toward the inner surface of the casing or formation during the setting sequence as further described below. In one embodiment, upper element backup shoe  66  and lower element backup shoe  70  form a metal-to-metal barrier between packer  50  and the inner surface of the casing. 
     Packer  50  further includes a slidable cover  74  that has a substantially thinner portion covering expandable seal element  72  and a thicker portion that is slidably positioned around piston  78 . A shear screw  80  is located between slidable cover  74  and piston  78 . Upon shearing, slidable cover  74  and piston  78  may slide relatively independently of each other. In the illustrated running configuration, slidable cover  74  substantially covers expandable seal element  72 . 
     A body lock housing  82  is located substantially adjacent to and partially around slidable cover  74 . Disposed within body lock housing  82  are one or more set screws  84 . A body lock ring  88  is positioned between body lock housing  82  and piston  78 . A spring-loaded O-ring  86  is also positioned between body lock housing  82  and piston  78 . A portion of body lock housing  82  may overlap a portion of the slidable cover  74 . Either or both of body lock housing  82  and slidable cover  74  may be partially notched such that one fits within the other to continue a low profile or thickness over the overlapping region. In one embodiment, slidable cover  74  and body lock housing  82  include threaded portions for threading the two elements together as shown. 
     In one embodiment, body lock ring  88  has internal threads that oppose and mesh with threads located on the external of mandrel  52 . Body lock ring  88  then is capable of locking itself, piston  78 , mandrel  52  and a cylinder  90  as part of the setting sequence of packer  50  to prevent further relative movement once expandable seal element  72  has been set in accordance with the process described below. 
     Slidably positioned substantially around piston  78  is cylinder  90 . Further, a portion of cylinder  90  may overlap a portion of body lock housing  82 . Either or both of cylinder  90  and body lock housing  82  may be partially notched or threaded such that one fits within the other to continue a low profile or thickness over the overlapping region and to couple the two members together. 
     A pick-up ring  92  is located at one end of piston  78  and is positioned between mandrel  52  and cylinder  90 . Pick-up ring  92  is in fluid communication with a port  94  that is in fluid communication with the longitudinal production flow passageway. A chamber  96  is defined by pick-up ring  92 , mandrel  52 , piston  78  and cylinder  90 . A chamber  98  is defined by mandrel  52 , pick-up ring  92 , an upper wedge  100  and cylinder  90 . A pair of seals  102  are located between cylinder  90  and piston  78  to provide a sealing relationship between cylinder  90  and piston  78 . A pair of seals  104  are located between cylinder  90  and upper wedge  100  to provide a sealing relationship between cylinder  90  and upper wedge  100 . In addition, a pair of seals  106  are located between mandrel  52  and piston  78  to provide a sealing relationship between mandrel  52  and piston  78 . Seals  102 ,  104 , and  106  may consist of any suitable sealing element or elements, such as a single O-ring, a plurality of O-rings, as illustrated, and/or a combination of backup rings, O-rings, and the like. 
     In the illustrated embodiment, chamber  96  and chamber  98  are in fluid communication with each other via pick-up ring  92  and could be considered a single chamber instead of multiple chambers. Those skilled in the art, however, will recognize that a pick-up ring could alternatively isolate chamber  96  and chamber  98  from one another. In addition, chamber  96  and chamber  98  may be considered to be expandable chambers because the volume within chamber  96  and chamber  98  increases during the setting sequence of packer  50  as described below. 
     Slidably positioned around mandrel  52  at a preselected distance below pick-up ring  92  and threadably coupled to cylinder  90  is an upper wedge  100 . Upper wedge  100  has a camming outer surface that will engage an inner surface of a slip assembly  112 . As should be apparent to those skilled in the art, upper wedge  100  may have a variety of configurations including configurations having other numbers of wedge sections, such configurations being considered within the scope of the present invention. 
     Slip assembly  112  is located between upper wedge  100  and a lower wedge  108 . In one embodiment, slip assembly  112  may have teeth  110  located along its outer surface for providing a gripping arrangement with the interior of the well casing. As explained in greater detail below, when a compressive force is generated between upper wedge  100 , slip assembly  112 , and lower wedge  108 , slip assembly  112  is radially expanded into contact with the well casing. 
     Referring collectively to  FIGS. 2A-2B ,  3 A- 3 B, and  4 A- 4 B the operation of packer  50  will now be described. Packer  50  is shown before, during and after activation and expansion of the expandable seal element  72 , respectively in  FIGS. 2A-2B ,  3 A- 3 B and  4 A- 4 B. As packer  50  is run into wellbore  32  in work string  30 , it is normally in its running configuration as shown in  FIGS. 2A-2B , with slidable cover  74  positioned such that its upper portion is covering seal element  72  and such that slidable cover  74  is coupled to piston  78  with shear screw  80 . In one embodiment, a plugging device, such as a ball or a flapper, is positioned downhole of packer  50  below port  94  to enable the pressure within the cylindrical bore  54  to be increased by a pump at the surface, for example. 
     This increase in fluid pressure is transmitted through port  94  to pick-up ring  92  where it acts upon chamber  96  and chamber  98  of packer  50 . The pressurized fluid then acts upon piston  78  with an upward force and upper wedge  100  with a downward force. As discussed above, upper wedge  100  is attached or connected to cylinder  90 , thus the downward force is also transmitted to cylinder  90 . 
     Initially, relative movement between piston  78  and cylinder  90  opposed by shear screw  80  attached between slidable cover  74  and piston  78 . Once the shear force between piston  78  and cylinder  90  exceeds a predetermined amount, shear screw  80  breaks allowing the downward force of the pressurized fluid within chamber  98  acting upon upper wedge  100  and cylinder  90  to move them downward towards slip assembly  112 . As upper wedge  100  contacts slip assembly  112 , slip assembly  112  moves downwardly over lower wedge  108 , which sets slip assembly  112  against the inner surface of casing  34 . Once slip assembly  112  is set against casing  34 , the fluid pressure in first chamber  96  begins to increase to a point where it becomes greater than the resistant force of shear screw  64 , which then shears. 
     Once shear screw  64  has been sheared, piston  78 , element retainer  76 , lower element backup shoe  70 , and expandable seal element  72  will begin to move upward relative to mandrel  52 . It is important that a sufficient amount of downward travel of slidable cover  74 , upper wedge  100 , and cylinder  90  has previously occurred to enable the slidable cover  74  to move out of the way of the upwardly moving expandable seal element  72 , as best seen in  FIGS. 3A-3B . In one embodiment, this may be accomplished by providing a sufficient distance of travel between upper wedge  100  and slip assembly  112 . If the distance between upper wedge  100  and slip assembly  112  is sufficient at the start of the actuation process, cylinder  90 , slidable cover  74  and upper wedge  100  will travel a sufficient distance downward prior to the upward movement of expandable seal element  72  and piston  78 . 
     As the upward travel of piston  78 , element retainer  76 , lower element backup shoe  70 , upper element backup shoe  66  and expandable seal element  72  begin, expandable seal element  72  moves from smaller outer diameter portion  60  over transition portion  68  to larger outer diameter portion  58 . 
     As expandable seal element  72  is forced upon the larger outer diameter portion  58 , shear ring  62  and shear pin  64  are forced upward by expandable seal element  72  until they contact shoulder  56  of mandrel  52 . When expandable seal element  72  moves over larger outer diameter portion  58 , it seals against casing  34  and mandrel  52  of packer  50 . In addition, upper element backup shoe  66  and lower element backup shoe  70  flare outward toward casing  34  to provide a metal-to-metal seal in addition to the seal of expandable seal element  72  between casing  34  and mandrel  52 , as best seen in  FIGS. 4A-4B . 
     Upon setting expandable seal element  72  against mandrel  52  and casing  34 , body lock ring  88  prevents further relative movement between piston  78 , body lock ring  88 , and cylinder  90  to lock piston  78 , element retainer  76 , expandable seal element  72 , and upper element backup shoe  66  into place. Thereafter the fluid pressure within cylindrical bore  54  may be decreased and production through the cylindrical bore  54  may proceed. 
     While the setting of the seal element of the present invention has been described as longitudinally shifting the seal element from a radially reduced to a radially increased diameter portion of the packer mandrel, those skilled in the art will recognize that other types of seal elements with other types of setting procedures could also be used and are considered to be within the scope of the present invention, those seal elements including, but not limited to, seal elements that are radially expandable into sealing engagement against the interior of the well casing in response to axial compression forces. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
A high circulation rate packer ( 50 ) for establishing a sealing and gripping engagement with a well casing ( 34 ) disposed in a wellbore ( 32 ). The packer ( 50 ) includes a packer mandrel ( 54 ), a seal assembly ( 72 ) slidably disposed about the packer mandrel ( 54 ) that has a running position and a radially expanded sealing position and a cover ( 74 ) that is slidably disposed relative to the packer mandrel ( 54 ). The cover ( 74 ) has a running position wherein the cover is disposed about the seal assembly ( 72 ) and a retracted position wherein the cover ( 74 ) is at least partially removed from about the seal assembly ( 72 ).