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CROSS-REFERENCE TO RELATED APPLICATION 
     This application takes priority from U.S. patent application Ser. No. 60/093,714 filed Jul. 22, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to oil well completion strings and more particularly to a hydrostatically-balanced open hole gravel pack system wherein hydrostatic pressure is maintained on the formation throughout the gravel packing operations. 
     2. Description of the Art 
     To obtain hydrocarbons from earth&#39;s subsurface formations, wellbores or boreholes are drilled into hydrocarbon-bearing formations or producing zones. After drilling a wellbore to the desired depth, a completion string containing various completion and production devices is installed in the wellbore to produce the hydrocarbons from the production zone to the surface. In one method, a fluid flow restriction device, usually containing one or more serially connected screens, is placed adjacent the production zone. Gravel is then packed in the space or annulus between the wellbore and the screen. No casing is installed between the screens and the wellbore. Such completions are called “open hole” completions and the systems used to gravel pack are called open hole gravel pack systems. 
     In commercially used open hole gravel packing system a completion string is frequently utilized for gravel packing. The completion string usually includes a screen near its bottom (or the downhole end), at least one packer or packing element above the screens, and a mechanism above the packer that allows gravel slurry to flow it from the surface to the annulus between the screens and the wellbore, and the clean fluid to return from the completion string to the surface. To gravel pack the annulus between the formation and the completion string, packer is set to form a seal between the completion string and the wellbore, the packer prevents the hydrostatic pressure from being applied to the formation, which prevents, for a period of time, maintaining the hydrostatic pressure above the formation pressure (the “overbalanced condition” or “overburdened condition”) during the gravel pack operation. Thus, the formation pressure can exceed the hydrostatic pressure, which can cause hole damage or well collapse and damage to the filter cake. 
     A substantial number of currently drilled wellbores are highly deviated or horizontal. The horizontal wellbores are extremely susceptible to damage if the overbalanced conditions are not maintained throughout the gravel pack operations or during any other completion operation. Maintaining the wellbore under overbalanced condition throughout the gravel packing, especially in highly deviated and horizontal wells is very desirable. The present invention provides a gravel pack system and method which maintains the pressure on the formation above the formation pressure throughout the gravel packing operation. The present system also is simpler and easier to use, thereby reducing the overall completion or gravel pack operations time and cost. 
     SUMMARY OF THE INVENTION 
     The present invention provides apparatus and method for gravel packing open holes wherein hydrostatic pressure on the formation is maintained above the formation pressure throughout the gravel pack process. In one embodiment, the gravel pack apparatus includes a completion string which contains a fluid flow restriction device, a crossover device uphole of the fluid flow restriction device and a packer above and below the crossover device. The completion string is conveyed in the wellbore to position the flow restriction device adjacent the producing formation while maintaining the wellbore under overburdened conditions. The upper packer and the crossover device are set while maintaining the wellbore under overburdened condition. This allows the gravel fluid to pass to the annulus and return through the completion string. The returning fluid crosses over to the annulus above the upper packer. After gravel packing, the lower packer is set. The portion of the completion string above the lower packer, which includes the crossover device and the upper packer are retrieved from the wellbore, thus leaving the fluid flow restriction device and the lower packer in the wellbore. In this particular embodiment, setting the lower packer after the gravel packing process has been completed enables maintaining the hydrostatic pressure on the formation throughout the gravel packing process. 
     Examples of the more important feature of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 drawings, in which like elements have been given like numerals: 
     FIGS. 1A-1D show a schematic diagram of a gravel pack string for placement in the wellbore and the wellbore fluid flow path to hydrostatically balance the formation. 
     FIGS. 2A-2D show a schematic diagram of the gravel pack string with the upper or service packer set and the fluid flow path which enables maintaining the hydrostatic pressure on the formation. 
     FIGS. 3A-3D show the gravel pack system of FIGS. 1A-1D with the service packer set for a reverse circulation flow path. 
     FIGS. 4A-4D show the gravel pack system of FIGS. 1A-1D after the Run-in tool and the service packers have been removed, leaving the screen and the liner packer in the wellbore. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1A-1D,  2 A- 2 D,  3 A- 3 D, and  4 A- 4 D show a gravel pack system  10  according to one embodiment of the present invention in various stages of gravel pack operations. 
     Referring to FIGS. 1A-1D, the system  10  includes a fluid flow restriction device  100  having a number of serially disposed screen assemblies  110   a - 110   c . The fluid flow restriction device  100  terminates at the bottom end of the string  10  with a plug  112  and a casing joint  114 . Each screen assembly, such as assembly  110   a , includes an outer shroud  120  and an inner sand screen  122 . The shroud  120  protects the internal parts of the screen assembly  110   a  from direct impact of the production fluid  202 , while the screen  122  prevents gravel, sand and other small solid particles from penetrating into the flow restriction device inside  116 . The screen  122 , however, maintains the string inside  116  in fluid communication with the formation  200 . Any fluid  40  supplied from the surface into the opening  116  at a pressure greater than the pressure of the formation  200  travels downhole to the plug  112 . This fluid then returns uphole (return fluid  42 ) via an opening  124  at the casing joint  114 . The returning fluid  42  passes through the screen assemblies  110   a - 110   c  (as shown by arrows  43 ) to the annulus  204  between the flow restriction device  100  and the wellbore  201  and travels uphole via the annulus  204 , as shown by arrows  44 . The purpose of the flow restriction device  100  is to prevent solids present in the production fluid  202  to pass into the opening  116  of the string  10 . It also prevents passage of any gravel though the screens  122  into the completion string inside  116  that is supplied to the annulus  204  from the surface. 
     A liner packer  150  is disposed uphill of (above) the flow restriction device  100 . A casing nipple  160  and a knock-out isolation valve  165  are serially coupled between the liner packer  150  and the flow restriction device  100 . A running tool  140  in the liner packer  150  is used to convey the liner packer  150  and the flow restriction device  100  into the wellbore  201 . An end  140   a  of the running tool couples a swivel sub  162  in the casing nipple  160 . The swivel sub  162  allows the tool portion above or uphole of the swivel sub  162  to rotate while maintaining stationary the tool portion  163  below the swivel sub. 
     The liner packer  150  includes setting slips  151  and one or more packing elements  152 . A liner packer setting dog (not shown) when moved downhole, causes the packer elements  152  to set, i.e., extend outward to the wellbore inside walls. Seals  144  in a junk bonnet  145  at the top of the liner packer  150  allow a polished stinger  143  to maintain seal. In the above-described configuration, the running tool  140  is attached to the section of the completion string that includes the liner packer assembly  150  and the flow restriction device  100  (referred to herein as the “bottom hole assembly” or the “BHA”). This allows an operator to rotate and release the running tool  140  from the bottom hole assembly to pull out the upper section of the completion string  100  out of the wellbore  201 , leaving behind the BHA in the wellbore  201 . 
     A crossover port assembly or device  170  is coupled uphole of the liner packer assembly  150  through the stringer  143 . The crossover port assembly  170  includes a port  172  which is initially closed off by a sleeve  174 . When the port  172  is closed, as shown in FIG. 1C, fluid supplied under pressure from the surface flows down to an opening  176  in the crossover port assembly  170  and continues to flow through the liner packer assembly  150  and the flow restriction device  100  as show by arrows  40 . When the sleeve  174  is moved downward, i.e., downhole, the port  172  opens. If the flow path below the port  172  is blocked, then any fluid supplied to the completion string  10  above the port  172  will flow through the port  172  and into the annulus  204  and eventually return uphole through the central bore  116  along the completion string  10  length. In the particular embodiment of FIGS. 1A-1D, a gravel pack kit  185  and a service packer  180  are disposed uphole of the crossover device  170 . 
     The service packer  180  can be hydraulically set to block or restrict fluid flow through the annulus  204  uphole of the crossover device  170 . The gravel pack kit  185  includes a port  186  that allows the fluid to flow from a reverse fluid flow path  179  in the service packer  180  to the annulus  204  above the service packer  180  as more fully explained below. The service packer  180  includes slips  181  and a plurality of packing elements  183 . Thus, the gravel pack system or completion string  10  shown in FIGS. 1A-1D includes in a substantially serial relation a flow restriction device  100 , a liner packer  150  above the flow restriction device  100 , a crossover port assembly tool  170 , and a service packer  180  uphole of the crossover device  170 . The gravel packing around the flow restriction device  100  while maintaining the hydrostatic pressure above the formation pressure will now be described while referring to FIGS. 1-4. 
     The completion string  10  shown in FIGS. 1A-1D is conveyed into the wellbore  201  to a desired depth to position the flow restriction device  100  adjacent the producing formation  200 . A wellbore fluid  40  is pumped from a source thereof at the surface (not shown) into the completion string  10 . The fluid flows through the string  10  as shown by the arrows  40  and returns to the surface via the annulus  204  as shown by the arrows  43 . The fluid in the wellbore maintains the hydrostatic pressure over the formation  200 , i.e., maintains the wellbore under overburdened condition. 
     Once the string  10  is correctly positioned in the wellbore  201 , the running tool  140  is released (or disengaged) from the liner packer  150  by rotating the pipe or the work string (attached above the string  10 ), which rotates the string  10  above the swivel sub  162 . The work string is then moved up or uphole, which causes the slips  181  of the service packer  180  to move over members  182 , which sets the packer elements  183  of the service packer  180  (See FIGS.  2 A- 2 D). Setting of the service packer  180  blocks any fluid flow through the annulus  204  around the packer elements  183 . Since the fluid in the string  10  remains in fluid communication with the formation  200 , it maintains the hydrostatic pressure on the formation  200 . 
     After setting the service packer  180 , a ball  190  is dropped into the completion string  10 , which moves the sleeve  174 , thus opening the port  172 . The ball  190  seats in position in the crossover assembly  170  and prevents fluid flow through the crossover assembly  170  past the ball  190 . The movement of sleeve  174  also opens a reverse fluid flow path  177  in the crossover port assembly which is further in fluid communication with fluid path  179  in the service packer assembly  180 . Thus, activating or setting the crossover assembly  170  causes any fluid supplied from the surface to flow through the string  10  to the port  172  and then over to the annulus  204  via the port  172 . The fluid then flows downhole through the annulus  204  and passes through the screens  110   a - 110   c  and then into the string opening  116  as shown by arrows  50  (FIGS.  2 A- 2 D). The fluid then flows uphole through the opening  116  in the flow restriction assembly  100  and then through openings  117  and  118  respectively in the liner packer  150  and the crossover tool  170 . The fluid then crosses over to the line or opening  179  through the service packer via crossover opening  177 . The fluid from line  179  passes into the annulus  204  above the packer  180  via port  186  in the crossover kit  195 . The downhole fluid flow path after the setting of the crossover assembly  170  is depicted by arrows  50 , while the uphole fluid flow path of the returning fluid is shown by arrows  52 . Thus, during the setting of the crossover assembly  170  to establish fluid flow below the service packer via the annulus  204 , the fluid in the wellbore  201  remains in fluid communication with the formation  200 , thereby maintaining the hydrostatic pressure on the formation  200 . 
     Still referring to FIGS. 2A-2D, once the service packer  180  has been set, fluid  188  with gravel or sand  189  (also known in the art as “propant”) is pumped into the string  10  from a source at the surface (not shown). The gravel fluid  188  flows to the annulus  204  around the flow restriction device  100 . The flow restriction device  100  prevents the gravel  189  from entering into the tool inside  116 . The gravel  189  deposits or settles in the annulus  204  while the filtered fluid enters the opening  116  and travels uphole as shown by arrows  52 . The supply of the gravel fluid is continued until the annulus  204  around the flow restriction device  100  is packed with the gravel  189 . 
     Referring to FIGS. 3A-3D, after the desired amount of gravel  189  has been packed around the flow restriction device  100 , the work string is picked-up, which opens bypass  220  in the service packer  180 . Clean fluid  222  is pumped downhole, along the annulus fluid flow path shown by arrows  55  and returns uphole though the flow opening  224  via the port  172 . This reverse circulation removes any excess sand or gravel from the work string. 
     The junk bonnet  144  is then sheared off. The packer setting dog sub  154  is then removed. The liner packer  150  is then set and the string above the bottom hole assembly is pulled out of the wellbore  201 . The work string, the gravel pack kit  185 , the service packer  180  and the crossover device  170  are replaced by production tubing  230  (FIGS.  4 B- 4 D). 
     It should be noted that in the particular method of this invention described herein, the liner packer  150  is set after the gravel pack operation has been completed, which allows maintaining the hydrostatic pressure on the formation throughout the gravel pack operations, thus, maintaining overbalanced or over burdened condition during all stages of the gravel packing operations. This system  10  also requires no gravel pack ports in the hook-up. Full inner dimensions or diameter is available throughout the operations. This method causes no swabbing or disturbance of the open hole filter cake. 
     The gravel pack system described herein above may utilize an combination of devices or any configuration that allows maintaining the hydrostatic pressure on the formation throughout the completion operations, such as gravel pack operations described above. The devices, such as packers, run-in tools, flow restriction devices described herein above are known in the oil field and thus are not described in great detail. 
     While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.

Summary:
The present invention provides apparatus and method for gravel packing open holes wherein hydrostatic pressure is maintained above the formation pressure (“overburdened condition”) throughout the gravel pack process. The apparatus includes a completion string which contains a flow restriction device, a crossover device and a packer each above and below the crossover device. The string is set in the wellbore with the flow restriction device adjacent the producing formation. The upper packer and the crossover device are set, which allows the gravel fluid to pass to the annulus, and return through the string. After gravel packing, the lower packer is set. The crossover device and the upper packer are retrieved from the wellbore leaving the flow restriction device and the lower packer in the wellbore. The system maintains the wellbore under overburdened condition throughout the gravel packing process.