Abstract:
A pressure-boosting apparatus particularly amenable for use in downhole applications is disclosed. The pressure-boosting apparatus employs an unbalanced piston which is initially fixated in a run-in position. The piston has a flowpath therethrough in which is mounted a check valve. Initially, pressure is applied to above and below the piston which results in an unbalanced force on the piston due to its configuration. Flow to the tool initiates its actuation at this time. When the unbalanced force reaches a predetermined level, the piston is no longer fixated to the housing and begins to accelerate. Acceleration of the piston closes the check valve due to the sudden decrease in pressure behind the check valve and an increase in pressure in front of the check valve as the fluid volume in front of the piston is compressed. Due to the proportional relationship between pressure and area, a magnification of force originally delivered by the pump is achieved for completion of the setting of a downhole tool such as a packer or bridge plug or the like.

Description:
This application is a continuation of application Ser. No. 08/514,876, filed Aug. 14, 1995, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The field of this invention relates to pressure-boosting devices, particularly those that are configurable for use with downhole tools. 
     BACKGROUND OF THE INVENTION 
     In the past, many downhole tools, such as bridge plugs or packers, have been used that are settable hydraulically. In some applications, the downhole tool is positioned in the wellbore with a wireline. Attached to the wireline assembly is a downhole pump which takes suction within the wellbore and builds the pressure up into the downhole tool for its actuation. Typically, these downhole pumps are driven by downhole motors are supplied with electrical power from the wireline and are limited in their pressure output to output pressures in the order of up to about 3,000 psig. Lately, the technology in downhole tools, particularly bridge plugs and packers, has evolved where higher setting pressures are required to assure the sealing integrity of the packer or plug. This is particularly true in environments where larger differential pressures are expected and the sealing force must be enhanced to a sufficient level to withstand the expected differentials across the plug or packer. 
     In the past, the physical configuration of the downhole pumps, as well as the logistics of supplying sufficient power to operate downhole motors, has been a limiting factor in the ability to apply setting pressure to bridge plugs or packers and similar hydraulically settable downhole tools. One solution to the space problem in the wellbore has been to stack a plurality of pistons in parallel so that the available setting pressure acts simultaneously on all the pistons. However, these devices did not magnify the applied pressure and, hence, the applied pressure available for setting the downhole tool. 
     Accordingly, it is an objective of the present invention to provide a simple device which can be readily used in conjunction with the pressure developing pump or a similar device used to create the motive force to set the downhole tool. It can also be used when the tool is run on tubing and a boost force is needed. The boosting device operates automatically and is simple to construct and effective to get a predetermined ratio of increase in applied force to set a downhole tool. 
     SUMMARY OF THE INVENTION 
     A pressure-boosting apparatus particularly amenable for use in downhole applications is disclosed. The pressure-boosting apparatus employs an unbalanced piston which is initially fixated in a run-in position. The piston has a flowpath therethrough in which is mounted a check valve. Initially, pressure is applied to above and below the piston which results in an unbalanced force on the piston due to its configuration. Flow to the tool initiates its actuation at this time. When the unbalanced force reaches a predetermined level, the piston is no longer fixated to the housing and begins to accelerate. Acceleration of the piston closes the check valve due to the sudden decrease in pressure behind the check valve and an increase in pressure in front of the check valve as the fluid volume in front of the piston is compressed. Due to the proportional relationship between pressure and area, a magnification of force originally delivered by the pump is achieved for completion of the setting of a downhole tool such as a packer or bridge plug or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1a-c  are a sectional elevational view of the pressure-boosting device of the present invention in the run-in position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The apparatus A of the present invention is illustrated in detail in  FIGS. 1a-c . At the top of the assembly is a bottom sub extender  10 , which is a conventional design used commonly in wireline applications to communicate the pressure delivered by a downhole pump or other pressure-building device (not shown) into a central fluid passageway  12 , which passes through the body  14  of the apparatus A. Body  14  has four segments: a top sub  16 , an upper housing  18 , a lower housing  20 , and a bottom sub  22 . Bottom sub  22  has a thread  24 , which is used to secure the bottom sub  22  to the downhole tool string (not shown) such as a packer or bridge plug in the preferred embodiment. Top sub  16  is connected to bottom sub extender  10  at thread  26 . Seal  28  secures the connection at thread  26  against fluid leaks. Similarly, thread  30  connects top sub  16  to upper housing  18 , with seal  32  securing the seal between those two components. Thread  34  connects the upper housing  18  to the lower housing  20 . There is no seal backing up the threaded connection at thread  34  for reasons which will be explained below. Finally, thread  36  connects lower housing  20  to bottom sub  22  with seal  38  sealing off the connection between those two components. 
     As seen in  FIGS. 1a-c , the central fluid passageway  12  extends the length of the apparatus A. Disposed in passageway  12  is a ball seat  40 . The ball seat assembly  40  encloses a spring  42  which acts on ball  44 . In the position shown in  FIG. 1a , there is no pressure being applied and the biasing force of spring  42  keeps ball  44  against ball seat  40 . Taken as an assembly, the components, including ball seat  40 , spring  42 , and ball  44 , comprise a check valve assembly. When in the closed position, as shown in  FIG. 1a , the passageway  12  is split into an upper segment, which includes surface  46  on piston  48 , and a lower segment, which includes surface  50  on piston  48 . Other valve or restriction devices can be used without departing from the spirit of the invention, such as a swing check valve, an orifice, or any other valve sensitive to pressure differential for its actuation, or even, less ideally, an orifice. 
     Piston  48  is illustrated in multi-component form. Surface  46  is part of the piston housing  52 . Piston housing  52  is mounted adjacent upper housing  18  with seals  54  and  56  in between. Top sub  16  has a recess  58 . A shear pin or shear screw  60  extends through a portion of piston housing  52  and into recess  58 . As a result, until the shear pin  60  breaks, the position of the piston  48  is fixed with respect to the apparatus A. The remainder of piston  48  comprises of a lower segment  62  which terminates in bottom surface  50 . Lower segment  62  has an annular shape which is sealed against an inner surface  64  of lower housing  20  by virtue of seals  66  and  68 . Piston housing  52  is connected to lower segment  62  at thread  78 , with the connection between those two components sealed by seal  80 . Finally, the piston housing  52  also has a top surface which, along with surface  46  and portions of ball seat  40  at its upper end, comprise the upper surface of the piston  48  which is exposed to applied hydraulic pressure in passageway  12 . It is clear that hydraulic pressure applied from the direction of bottom sub extender  10  cannot go between the piston housing  52  and the upper housing  18  due to the presence of seals  54  and  56 . 
     However, applied pressure from extender  10  acts to initially displace ball  44  away from ball seat  40  by virtue of compression of spring  42 . Accordingly, the axial force due to applied pressure on top surface of piston housing  52  and surface  46 , plus the shear strength of pin  60  in the axial direction, equalizes with the applied pressure in a reverse direction on bottom surface  50 . The pressure at surface  50  occurs because, upon application of pressure into passageway  12 , the, check valve assembly is open, meaning that the pressure can evenly distribute itself throughout passageway  12  down to the bottom surface  50 . Flow to the downhole tool can now occur and initiate the setting. Since by design the bottom surface  50  has a smaller cross-sectional area than the combination of top surface of piston housing  52  and surface  46 , and the upper end of the ball seat  40 , at a given predetermined pressure level, applied in passageway  12 , the net unbalanced force on piston  48  exceeds the ability of the shear pin  60  to retain the piston  48  in its initial -position shown in FIG.  1 a. Ultimately, when a predetermined pressure is exceeded, the shear pin  60  breaks and the piston  48  begins to accelerate toward surface  70  on bottom sub  22 . Those skilled in the art will appreciate that during subsequent movement of the piston  48  downward, the ratio of fluid volume change above to below the closed check valve (at  40  and  44 ) will be inversely proportional to the pressure change above to below the same point when measured over the same interval of time. Movement of the piston in this manner is facilitated by a reduction of the volume of chamber  72 . However, chamber  72  is equalized with the environment around the apparatus A through a port  74 . Arrow  76  illustrates the direction of fluid flow as the volume of chamber  72  decreases by the downward movement of piston  48 . Seals  54 ,  56 ,  66 ,  68  and  80  effectively seal portions of chamber  72  as the piston  48  moves. However, since it is desirable to displace fluid out of chamber  72  upon stroking of piston  48 , port  74  is sized sufficiently large so as not to create any backpressure which would impede the acceleration of the piston  48 . 
     As the piston  48  begins to accelerate toward surface . 70 , the volume in the apparatus A at passageway  12  decreases from the check valve assembly down to bottom sub  22 . This occurs due to the movement of piston  62  into the cavity above surface  70 . Conversely, with the downward movement of the piston  48 , the volume of passageway  12  above the check valve assembly rises. The rise in volume of passageway  12  above the check valve assembly reduces the pressure above the check valve assembly. Conversely, the decrease in volume of the passageway  12  below the check valve assembly increases the pressure in that portion of the passageway until piston  48  has moved sufficiently so that the reduction in pressure in passageway  12  adjacent surface  46  is sufficient to allow spring  42  to move ball  44  against seat  40 . Those skilled in the art will appreciate that these movements occur almost instantaneously upon the breaking of shear pin  60 . Accordingly, for a major portion of its stroke, piston  48  will move downwardly, bringing surface  50  closer to surface  70 , with the check valve assembly in the closed position. 
     Assuming, for the sake of description, that the fluid in passageway  12  is essentially incompressible, the moving piston  48  will try to seek equilibrium as it accelerates towards surface  70 . In so doing, the area ratio as between surface  50  compared to surfaces  70  and  46  and the top end of the check valve seat assembly  40  will dictate the degree of pressure amplification experienced at the lower end of passageway  12  and, hence, to the downhole tool. For example, if the area ratio of surfaces  70 ,  46 , and the top end of ball seat  40  to the bottom surface  50  is 3:1, then stroking of the piston toward surface  70  will ultimately, upon setting the tool, result, in a three-fold increase in the applied pressure to the downhole tool (not shown) which is connectable at thread  24 . There may be some slight variation in the ratio of the resultant pressure build-up depending on the presence of fluid, which may be slightly compressible, and seal friction. Clearly, those skilled in the art will appreciate that the greater the compressibility of the fluid in passageway  12  at the time the piston  48  strokes, the lower the resultant magnification of pressure will be from the ideal direct relationship described above. Those skilled in the art will also appreciate the general relationship between pressure and area which indicates that the combination of the pressure times the area at the top of the piston  48  will be equal to the pressure and the area at the bottom of the piston  48  in an ideal case involving a fully incompressible fluid. This movement of the piston  48  applies the required pressure which the downhole pump itself (not shown) could not deliver to complete the setting of the downhole tool. 
     Those skilled in the art will now understand that what has been illustrated is a very simple pressure-boosting device which works fully automatically. The resultant boost forces can be predetermined by the configuration of the piston  48 , and its adjacent sealing surfaces. Similarly, depending on the boost force designed into the configuration of piston  48 , those skilled in the art can readily select the value of the force required to shear the pin  60  to begin the movement of piston  48 . The apparatus A can be resettable for multiple use without removal from the wellbore, as will be described below. The apparatus A has particular application to use of downhole pumps that are run on wireline whose output capability may only be in the range of 2,000-3,000 psig. With the use of the apparatus A, the output pressure from such a pump can be increased to 5,000 psig or more. The only limitations on the ratio of pressure-boosting available are the physical space requirements of the particular well in question and any length requirements or limitations on the apparatus A. 
     After the apparatus A has been used to set the bridge plug or packer, it can be retrieved to the surface and redressed for subsequent use. 
     It should be noted that minor modifications from the preferred embodiment illustrated are also considered to be part of the scope of the invention. For example, the piston assembly  48 , rather than being initially fixated by a shear pin  60 , can be assembled in the apparatus A so that it is resettable upon withdrawal of pressure from passageway  12  without the need to remove it from the wellbore to redress the shear pin  60 . For example, a spring or other equivalent biasing member  82  is schematically illustrated in cavity  72 . Spring  82  can be a stack of Belleville washers or helical compression spring which will retain the position of piston  48  until a sufficient compressive force is applied to the stack. At that point, the spring can compress, allowing a piston  48  to move toward surface  70 . Other types of biasing mechanisms can be used to return the piston  48  to its run-in position upon the removal of the net unbalanced force created by the application of hydraulic fluid pressure in passageway  12 , all of which are considered to be within the spirit of the invention. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.