Abstract:
The present invention provides for an apparatus and method to displace fluids and to prevent the mixing or bypassing of fluids used in gravel pack operations. A moveable plug is used to create a physical partition between fluids.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The present invention pertains to the pumping of fluids into a wellbore, and particularly to the pumping of fluids during a gravel pack operation.  
         [0003]     2. Related Art  
         [0004]     It is often desired to place sand or other filtering medium between a sand screen and the wellbore in wells having poorly or loosely consolidated production formations. Without the presence of such filtering media, screens can become plugged or production equipment can be damaged by fines (small particulates of the formation matrix) or formation sand produced with the production fluids.  
         [0005]     The sand is typically conveyed entrained in a fluid. The entrained sand is generally referred to as “gravel” and the gravel and conveyance fluid combination is generally referred to as a “gravel slurry.” The gravel slurry is normally pumped into the well through tubing until it reaches a crossover, at which point the slurry enters the annulus between the sand screen and the wellbore or casing. As the conveyance fluid is either lost to the formation or returns to the surface, the gravel settles out, packing the annulus.  
         [0006]     A gravel pack operation usually requires the pumping of more than one type of fluid. The volume of gravel slurry pumped is generally calculated based on the anticipated volume necessary to fill the annular space with gravel. Spacer fluids are generally placed ahead of and behind the gravel slurry to provide for fluid isolation and separation during the gravel pack operation. Other fluids, such as brine or drilling fluid “mud”, may be pumped behind the gravel slurry as displacement fluid to force the gravel slurry into the proper location in the well. Thus, different fluids may be present or introduced into the well both ahead of and behind the gravel slurry.  
         [0007]     Because the different fluids have different physical properties, such as density and viscosity, they can sometimes mix, or one fluid may flow past the other. Those undesired effects may be exacerbated in horizontal or highly deviated portions of the wellbore or if fluids are traveling down the wellbore at low velocities. That may lead to a reduction in displacement efficiency, preventing a particular fluid from reaching its intended location in the well. That, in turn, may lead to an incomplete gravel pack operation.  
       SUMMARY OF INVENTION  
       [0008]     The present invention provides for an apparatus and method to displace fluids and to prevent the mixing or bypassing of fluids used in gravel pack operations. A moveable plug is used to create a physical partition between fluids.  
         [0009]     Advantages and other features of the invention will become apparent from the following description, drawings, and claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]      FIGS. 1A, 1B , and  1 C are schematic views of a fluid displacement system constructed in accordance with the present invention showing different stages of operation.  
         [0011]      FIGS. 2A and 2B  are schematic views of a plug used in the fluid displacement system of  FIGS. 1A   1 C.  
         [0012]      FIG. 3  is a perspective view of a plug head used in the fluid displacement system of  FIGS. 1A   1 C. 
     
    
     DETAILED DESCRIPTION  
       [0013]     Referring to  FIGS. 1A, 1B , and  1 C, a fluid displacement system  10  comprises a work string  12 , a bottom plug  14 , a top plug  16 , and a plug catcher  18 . In some embodiments, there may be no bottom plug  14 .  
         [0014]     Work string  12  is a tubular member extending from the surface to some desired depth in a wellbore  20 . Typically work string  12  terminates in a crossover tool or service tool  22  so that fluid pumped through work string  12  exits into an annulus  24  between wellbore  20  (or casing, if present) and a sand screen  26 . Work string  12  may be used to convey various fluids such as brine or drilling mud  28  and gravel slurry  30 .  
         [0015]     Bottom plug  14 , as shown in  FIGS. 2A and 2B , comprises a plug body  32 , having a central passageway  34  therethrough, and a diaphragm  36 , pre-set to rupture at a desired pressure differential, extending across passageway  34 . Diaphragm  36  sealingly engages plug body  32  and prevents flow through passageway  34  until diaphragm  36  is ruptured or otherwise removed. Bottom plug  14  can be made of various materials but is preferably made of slightly compressible material to enhance its ability to seal against the inner diameter of work string  12  (bottom plug  14  is shown compressed in  FIG. 2B ). Plug body  32  may also comprise ribs  38 . Ribs  38  are preferably tapered and slightly compressible to further enhance the sealability of bottom plug  14 .  
         [0016]     Top plug  16  is shown in  FIGS. 1A-1C . Top plug  16  comprises the plug body  32  and is very similar and may be identical to bottom plug  14 , with a central passageway  34  therethrough and a diaphragm  36 . As in bottom plug  14 , diaphragm  36  is pre-set to rupture at a certain pressure differential. The rupture pressure for top plug  16  is preferably greater than that of bottom plug  14 , though they could be equal. Top plug  16  can also be made of various materials, but is preferably made of slightly compressible material to enhance its ability to seal against the inner diameter of work string  12 . Plug body  32  may also comprise ribs  38 . As above with bottom plug  14 , ribs  38  on plug body  32  are preferably tapered and slightly compressible to further enhance the sealability of top plug  16 .  
         [0017]     Central passageway  34  through both top and bottom plugs  16 ,  14  provides an inner diameter, after rupture of central diaphragm  36 , large enough to allow the passage of balls, bars, and other tools necessary to operate a downhole tool. Central passageway  34  is also designed to pose minimum or preferably no interference with down-hole tools. Rupture of diaphragm  36  is designed to leave no debris or cause any restriction.  
         [0018]      FIG. 3  shows a plug head  42 . Plug head  42  is placed at or near the surface of the well and attaches to the upper end of work string  12 . Plug head  42  comprises a housing  44 , valves  46 ,  48 , and  50 , and an injection pipe  52 . Housing  44  has spacer pipes  54  and  56  to spatially separate valves  46 ,  48 , and  50 . In one embodiment bottom plug  14  resides in spacer pipe  54  and top plug  16  resides in spacer pipe  56 . Plugs  14  and  16  may be held in place in their respective spacer pipes by pins (not shown) or other fastener means. Alternatively, plug head  42  may be similar to the plug launching tool described in U.S. Pat. No. 5,890,537, the description and illustrations of which are incorporated herein for all purposes. Other variations of plug heads  42  may also be suitable to deploy plugs  14 ,  16 .  
         [0019]     In operation, work string  12 , with crossover  22  and sand screen  26 , is run into the well until screen  26  is properly positioned. Brine  28  is commonly circulated in wellbore  20  after positioning of screen  26 . Brine  28  can be pumped through injection pipe  52  through open valve  46  (with valves  48  and  50  closed). Brine  28  passes below bottom plug  14  into the lower portion of plug head  42 .  
         [0020]     When the operator wishes to pump gravel slurry  30 , he or she closes valve  46 , opens valve  48  (keeping valve  50  closed), and pumps slurry  30  into injection pipe  52 . Slurry  30  enters plug head  42  just above bottom plug  14 . Bottom plug  14  is released to flow into work string  12  when the pins or other fastening means shear or are removed from housing  44 . Pressure applied to slurry  30  forces bottom plug  14  downward, displacing brine  28  as bottom plug  14  descends.  
         [0021]     After pumping the desired volume of slurry  30 , the operator can resume pumping brine  28 , but before doing so, he or she will close valve  48  and open valve  50  (keeping valve  46  closed). That directs brine  28 , being pumped into injection pipe  52 , into plug head  42  just above top plug  16 . Once the pins or fastening means holding top plug  16  in spacer  56  are removed or sheared, top plug  16  will move downward into work string  12 , displacing slurry  30  and bottom plug  14  as it moves. Both bottom plug  14  and top plug  16  effectively seal against the inner diameter of work string  12  to isolate the fluids above, between, and below plugs  14 ,  16 .  
         [0022]     As brine  28  is pumped, top plug  16 , slurry  30 , and bottom plug  14  continue to move downward into wellbore  20  until bottom plug  14  comes into abutting contact with plug catcher  18 . Plug catcher  18  is an internal profile in work string  12  that prevents further downward motion of bottom plug  14 . As brine  28  continues to be pumped, pressure builds rapidly above diaphragm  36  due to the generally incompressible nature of the fluids involved. Upon sufficient pressure, diaphragm  36  ruptures, allowing slurry  30  to exit crossover  22  and enter its desired position in annulus  24 . Top plug  16 , being pushed from above by brine  28 , pushes slurry  30  from work string  12  until top plug  16  comes into abutting contact with bottom plug  14 . Similarly, upon reaching the necessary pressure, diaphragm  36  in top plug  16  ruptures, allowing the fluids to continue traveling down work string  12  and out to the wellbore. A pressure indication can be observed at the surface each time diaphragm  36  in either one of the plugs  14 ,  16  ruptures.  
         [0023]     Work string  12  can then be removed from the well, along with plugs  14 ,  16 , plug catcher  18 , and crossover  22 . Sand screen  26 , left in the hole packed in gravel, can then be connected to production tubing (not shown) to produce the desired well fluids.  
         [0024]     The above-described operation can be performed using only plug  14  or plug  16 , if desired.  
         [0025]     In the preceding description, directional terms, such as “upper,” “lower,” “vertical,” “horizontal,” etc., may have been used for reasons of convenience to describe an apparatus and its associated components. However, such orientations are not needed to practice the invention, and thus, other orientations are possible in other embodiments of the invention.  
         [0026]     Although only a few example embodiments of the present invention are described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.