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BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to mechanisms and methods for closing off a subsurface wellbore and, in particular aspects, to surface-controlled subsurface safety valves used within wellbores.  
         [0003]     2. Description of the Related Art  
         [0004]     Surface-controlled subsurface safety valves (SSSVs) are used to selectively close off lower portions of the flowbore of a production tubing string in the event of an emergency. These valves can then be reopened later when the emergency situation has been remedied and it is desired to reestablish flow through the flowbore. Common SSSV&#39;s are flapper-type valves that are biased closed by a spring mechanism and then reopened, under surface control, by an axially moveable flow tube. Although flapper valves have been in use for a long period of time, they continue to suffer from problems that prevent them from being an ideal solution in all situations. Because they are mechanical devices, their components are prone to damage during typical operation. Also, the flow path might become encrusted with scales or hydrates during production, which can cause the valve to become stuck in an open, closed, or partially open position.  
         [0005]     Certain devices are known that utilize plugs that will block fluid flow through the flowbore and can be readily destroyed when needed. U.S. Pat. No. 6,026,903 issued to Shy et al., for example, discloses a frangible disappearing plug that can be ruptured and destroyed to reopen the flowbore of the well. However, this type of structure must be run into the well at the outset in order to be useful as a plug. This makes it unsuitable for use as an SSSV, since it could not be used to rapidly close off a lower portion of the flowbore in an emergency.  
         [0006]     There is a need for an alternative to standard flapper-type surface controlled safety valves. There is a need for an alternative technique that can be used to establish a blockage within the flowbore of a well during operation in a substantially rapid manner under surface control to close a portion of the flowbore in the event of an emergency. This technique should also allow the flowbore to be reopened, if desired.  
         [0007]     The present invention addresses the problems of the prior art.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention provides devices and methods for selectively closing and opening flow of production fluid (oil, gas, or water) through the flowbore of a production string by chemically altering the phase of fluid media flowing through the production tubing to form a substantially solid plug or blockage. In a preferred embodiment, the phase of the fluid media is altered by selectively injecting a predetermined catalyst into a portion of the flowbore wherein it is desired to create a plug or blockage. The composition of the catalyst will vary depending upon the type of media (oil, gas, water) that is flowing through the tubing. The blockage will typically form in a period of time that is short enough to essentially close off flow under emergency conditions.  
         [0009]     When it is desired to remove the blockage, a dissolving agent is selectively added to the flowbore via a reversing sub to reverse the phase of the blockage from solid to liquid/gas. As the blockage is dissolved, flow is reestablished within the flowbore.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a side, cross-sectional view of an exemplary wellbore containing a production tubing string that incorporates a subsurface safety valve constructed in accordance with the present invention.  
         [0011]      FIG. 2  is a side, cross-sectional view of an exemplary SSSV constructed in accordance with the present invention.  
         [0012]      FIG. 3  is a side, cross-sectional view of the SSSV shown in  FIG. 2 , now after having been actuated to create a blockage within the production tubing string.  
         [0013]      FIG. 4  is an enlarged view of portions of the production assembly illustrating a currently preferred location for development of a blockage as well as aspects of the blockage dissolving process. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]      FIG. 1  schematically depicts an exemplary wellbore  10  that has been drilled into the earth  12  from the surface  14  to a subterranean formation  16 . The formation  16  contains a volume of production fluid which may be a mixture of oil, water, and/or gas. The wellbore  10  has been cased with steel casing  18 , in a manner known in the art. Perforations  20  allow fluid communication from the formation  16  into the wellbore  10 .  
         [0015]     An exemplary production assembly  22  is shown that includes a production tubing string  24  that extends downwardly from a wellhead  26  through the wellbore  10 . An annulus  28  is defined between the outer radial surface of the tubing string  24  and the casing  18 . An axial flowbore  29  is defined within the tubing string  24 . A production nipple  30  incorporated into the tubing string  24  is located adjacent the perforations  20 . Packers  32  are set on each axial side of the production nipple  30 . A surface controlled subsurface safety valve (SSSV)  34  is also incorporated into the production assembly  22  above the production nipple  30 . Also incorporated into the production assembly  22  is a reversing sub  36  and a plug formation sub  38 , the structure and function of which will be described in detail shortly.  
         [0016]      FIGS. 2 and 3  depict the SSSV  34  in greater detail.  FIG. 2  depicts the SSSV  34  in an initial, unactuated position, while  FIG. 3  shows the SSSV  34  after having been actuated to form a plug. The SSSV  34  includes a catalyst sub housing  40  having axial ends  42 ,  44  which are threaded so that they can be secured to other components in the production string  24 . The housing  40  has an enlarged central portion  46  and defines an axial flow passage  48  within. Radially outside of the flow passage  48 , an annular chamber  50  is defined within the central section  46 . It is noted that, while the chamber  50  is shown and described herein to be an annular chamber, the chamber  50  may be of other suitable shapes or configurations. Examples of alternative chamber configurations include blind bore and radial. The direction of production fluid flow through the flow passage  48  is illustrated by the arrow  52 . The upper end of the annular chamber  50  includes a fluid transmission port  54  that leads to a multi-positional flow control valve  56 . A lateral flow passage  58  extends from the valve  56  to the flow passage  48 . A valve control line  60  extends from the surface  14  to the flow control valve  56 . The valve control line  60  is typically an electrical line, but might also comprise a hydraulic, optical fiber, or other control lines known in the art. The flow control valve  56  is preferably moveable between three positions. These positions include a first position wherein fluid communication from the chamber  50  through the port  54  is blocked by the valve  56  (i.e., the closed position); a second position wherein fluid from the chamber  50  may flow through the port  54 , valve  56  and into the flow passage  48  via lateral flow passage  58  (i.e., the open position); and a third position wherein the valve  56  allows fluid to flow from a chamber refill conduit  62  through inlet  64  and into the flow passage  54  (i.e., the refill position). Those of skill in the art are familiar with multi-positional valves of this type and, therefore, its design will not be described further herein.  
         [0017]     The chamber refill conduit  62  extends from the surface  14  and is interconnected with the flow control valve  56  via an inlet  64  that is formed into the housing  40 . The refill conduit  62  may incorporate a one-way fluid flow check valve  66  that allows fluid to be flowed toward the valve  56  but not away from it. It is noted that the valve control line  60  is operationally associated with surface-based components, including, for example, a controller (not shown) for operating the valve  56 . The refill conduit  62  is operationally associated with a pump and fluid reservoir (not shown) for providing a flow of fluid along the refill conduit  62 . The structure and operation of devices of this nature are well understood and, therefore, are not described in any further detail herein.  
         [0018]     An annular piston member  68  is disposed within the annular chamber  50 . Fluid seals  70  ensure fluid-tight sealing between the piston member  68  and the surrounding surfaces of the chamber  50 . The piston member  68  provides an upper axial side  72  and a lower axial side  74 . A compression spring  76  is disposed within the chamber  50  below the piston member  68  and contacts the lower side  74  of the piston member  68 , thereby biasing the piston member  68  upwardly within the chamber  50 . A catalyst fluid  78  initially resides within the chamber  50  above the piston member  68 .  
         [0019]     The composition of the catalyst fluid  78  will vary depending upon the makeup of the fluid media that is flowing through the flowbore  29 . In the instance of natural gas production, fresh water is a preferred catalyst fluid  78  as its addition to natural gas will result in the formation of solid hydrates. In the instance of a natural gas/fresh water combination, it is preferred to locate the plug formation sub  38  proximate a preferred hydrate formation depth, which is typically at or near the mud line in a wellbore.  
         [0020]     In the instance of a wellbore wherein crude oil is primarily being produced through the flowbore  29 , a currently preferred catalyst fluid  78  would be naturally-occurring paraffins. Other suitable viscosifiers or precipitants, which are capable of forming a substantially solid plug when mixed with crude oil may also be used.  
         [0021]     In order to create a blockage within the flowbore  26 , the flow control valve  56  is actuated from the closed position ( FIG. 2 ) to the open position ( FIG. 3 ). Catalyst fluid is allowed to escape from the chamber  50  under the impetus of the piston member  68  and spring  76 . The catalyst fluid  78  in the chamber  50  is substantially exhausted into the flowbore  48  as the spring  76  urges the piston member  68  axially upwardly to the position illustrated in  FIG. 3 .  
         [0022]     Referring now to  FIG. 4 , aspects of the formation and subsequent dissolution of a blockage or plug within the flowbore  29  are further depicted. In  FIG. 4 , the catalyst fluid  78  from the chamber  50  of the SSSV  34  has been largely released into the flow passage  48  to mingle with the production fluid. A plug or blockage  80  will be formed within the plug formation sub  38 . It is preferred that there be a flow restriction point formed within the plug formation sub  38  to promote the formation of the blockage  80 . In the embodiment depicted in  FIG. 4 , the flow restriction point is formed by an orifice plate  82  held in position by welds  84 . In alternative embodiments, the flow restriction could comprise a venturi tube apparatus or one or more fins that are located within the plug formation sub  38 .  
         [0023]     When it is desired to remove or dissolve plug  80 , the reversing sub  36  is actuated. The reversing sub  36  is constructed in a manner similar to the SSSV  34 . It includes a housing  86  with a central flow passage  88  and surrounding annular chamber  90 . Preferably, the multi-position fluid control valve  92 , fluid inlet  94 , and lateral fluid outlet  96  are located at the lower end of the reversing sub  36  rather than at the upper end as with the SSSV  34 . The piston member  98  and compression spring  100  are preferably located at the upper end of the chamber  90 . The chamber  90  is filled with a plug dissolving agent  102 . The reversing sub  36  is actuated by control line  104 , by which the control valve  92  is moved to the open position to allow the plug dissolving agent  102  to be exhausted into the flowbore  29  by urging of the spring  100  against the piston member  98 . The plug dissolving agent  102  will tend to settle downwardly onto the plug  80 , as indicated by arrows  106 , and reopen the flowbore  29  to fluid passage.  
         [0024]     The composition of the plug dissolving agent  102  will vary in accordance with the nature of the production media being flowed through the flowbore  29  and the composition of the plug  80 . In the instance of a natural gas producing well, which would be closed with a hydrate plug, preferred plug dissolving agents include glycol, methyl-ethyl-glycol (MEG), diesel, and other suitable solvents. In the instance of a crude oil producing well, a paraffin solvent, such as diesel can be used. Alternatively, a heated fluid may be used as the plug dissolving agent  102  in order to raise the temperature of the paraffin above its melting point, thereby dissolving it. The particular temperature of the heated fluid needed to dissolve the plug  80  will very depending upon the particular composition of the plug  80 .  
         [0025]     It is noted that the SSSV  34 , reversing sub  36 , and plug formation sub  38  collectively form an SSSV system wherein a plug may be formed and removed for selective blockage of the flowbore  29 . It is also noted that mixtures of oil, gas and water may require that the catalyst fluid  78  be made up of a suitable mixture of the components of the fluids that are suitable for mixing with the complimentary components of the fluid media flowing through the flowbore  29 . For example, if the media flowing through the flowbore  36  were a mixture of 30% natural gas and 70% crude oil, the catalyst fluid  78  could be made up of a mixture of about 30% fresh water and about 70% paraffin, for example. Similarly, where there is mixed fluid media, the plug dissolving agent  102  will also be made up of a suitable combination of fluid components for dissolution of the plug  80 . In the example of 30% natural gas and 70% crude oil, the plug dissolving agent  102  might be composed of a combination of 30% glycol, MEG, or diesel and 70% paraffin solvent.  
         [0026]     Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.

Summary:
Devices and methods for selectively closing and opening flow of production fluid (oil, gas, or water) through the flowbore of a production string by chemically altering the phase of fluid media flowing through the production tubing to form a substantially solid plug or blockage. The phase of the fluid media is altered by selectively injecting a predetermined catalyst into a portion of the flowbore wherein it is desired to create a plug or blockage. When it is desired to remove the blockage, a dissolving agent is selectively added to the flowbore via a reversing sub to reverse the phase of the blockage from solid to liquid/gas. As the blockage is dissolved, flow is reestablished within the flowbore.