Patent Publication Number: US-7896082-B2

Title: Methods and apparatus for negating mineral scale buildup in flapper valves

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to devices and methods for controlling and removing the buildup of mineral scales and the like upon subsurface safety valves. 
     2. Description of the Related Art 
     Surface-controlled, subsurface safety valves (“SCSSV&#39;s”) are typically used in production string arrangements to quickly close off the production flowbore in the event of an emergency, such as a blowout. A usual form for an SCSSV is a flapper-type valve that includes a flapper member that is pivotally movable between open and closed positions within the flowbore. The flapper member is actuated between the open and closed positions by a flow tube that is axially movable within the flowbore. 
     After being placed into a wellbore, mineral scale typically forms and builds up on all portions of the production tubing string that are exposed to wellbore fluids. Portions of the flowbore that have a pressure drop are particularly vulnerable to scale buildup. Scale and other buildup forming on and around the flow tube of the SCSSV can make it difficult to move the flow tube axially and thereby prevent proper operation of the SCSSV. Of particular concern is the interior surface of the flowbore within the valve housing that is located above the flow tube, as scale buildup in that location can prevent the flow tube from moving axially and prevent the valve from closing. 
     Wireline brushes can be used to try to clean the scale buildup from the flow tube and surrounding valve housing. However, this is costly as it necessitates stopping production operations to run the brush in and then conduct the cleaning. 
     SUMMARY OF THE INVENTION 
     In preferred embodiments, the invention provides exemplary subsurface safety valve designs that are operable to clean and remove or to prevent buildups of scale that might prevent operation of the valve. In other aspects, the invention provides methods and devices for cleaning and removing or preventing scale buildups on interior surfaces of a sliding sleeve valve housing above the flow tube. In some exemplary embodiments, the flow tube of the valve includes a wiper member that extends radially outwardly from the flow tube and into contact with the interior surface of the valve housing. The wiper member provides a physical spacer that increases the spacing between the flow tube and housing, which counteracts the effect of scale buildup and permits operation of the valve even after some buildup has occurred. The wiper member is also operable to physically wipe away or otherwise remove the scale buildup. In particular preferred embodiments, the wiper member contains or is formed of a scale dissolving material that helps to dissolve and remove the scale buildup from the interior surface. A wiper member that releases small amounts of the scale dissolving substance on the interior surface above the flow tube helps prevent scale deposition in this area of the valve. 
     In other embodiments, the interior surface of the valve housing is provided with a sleeve that is disposed between the interior surface of the valve housing and the general flowbore passing through the valve housing to protect the interior surface against scale buildup. In addition, the sleeve serves to provide a substantially smooth and continuous interior surface of substantially uniform diameter and, therefore, minimizing a pressure drop across the valve that would tend to permit scale buildup. In varied embodiments, the sleeve is formed of an elastomeric material or a metallic material that is axially compressible. In further embodiments, the sleeve is substantially rigid and retained in a recess formed in either the valve housing or the flow tube. As the valve is actuated to a closed position, the sleeve retracts into the recess. In some embodiments, the sleeve is biased axially outwardly from the recess. In other embodiments, the sleeve is securely affixed to the flow tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein: 
         FIG. 1  is a side, cross-sectional view of an exemplary hydrocarbon production tubing string within a wellbore and containing a SCSSV in accordance with the present invention. 
         FIG. 2  is a side, one-quarter cross-sectional view of a currently preferred embodiment for a SCSSV constructed in accordance with the present invention. 
         FIG. 3  is a side, one quarter cross-sectional view of the SCSSV shown in  FIG. 2 , now with the valve closed. 
         FIG. 4  is a detail cross-section taken along lines  4 - 4  in  FIG. 2 . 
         FIG. 5  is a side, one-quarter cross-sectional view of a second preferred embodiment for a SCSSV constructed in accordance with the present invention. 
         FIG. 6  is a side, one-quarter cross-sectional view of a further preferred embodiment for a SCSSV constructed in accordance with the present invention. 
         FIG. 7  is a side, one-quarter cross-sectional view of a further preferred embodiment for a SCSSV constructed in accordance with the present invention. 
         FIG. 8  is a detail drawing depicting an exemplary j-slot arrangement used with the SCSSV of  FIG. 7 . 
         FIG. 9  is a side, one-quarter cross-sectional view of a further preferred embodiment for a SCSSV constructed in accordance with the present invention. 
         FIG. 10  is a side, one-quarter cross-sectional view of the SCSSV shown in  FIG. 9 , now in a closed position. 
         FIG. 11  is a side, one-quarter cross-sectional view of a further alternative embodiment for an SCSSV constructed in accordance with the present invention. 
         FIG. 12  is a side, one-quarter cross-sectional view of the SCSSV shown in  FIG. 11 , now in a closed position. 
         FIG. 13  is a side, one-quarter cross-sectional view of a further alternative embodiment for an SCSSV constructed in accordance with the present invention. 
         FIG. 14  is a side, one-quarter cross-sectional view of the SCSSV shown in  FIG. 13 , now in a closed position. 
         FIG. 15  is a side, one-quarter cross-sectional view of a further alternative embodiment for an SCSSV constructed in accordance with the present invention. 
         FIG. 16  is a side, one-quarter cross-sectional view of the SCSSV shown in  FIG. 15 , now in a closed position. 
         FIG. 17  is a side, one-quarter cross-sectional view of another alternative embodiment for an SCSSV constructed in accordance with the present invention. 
         FIG. 18  is a side, one-quarter cross-sectional view of the SCSSV shown in  FIG. 17 , now in a closed position. 
         FIG. 19  is an isometric view of a component of the SCSSV shown in  FIGS. 17-18 , shown apart from the other components of the SCSSV. 
         FIG. 20  is a side, one-quarter cross-sectional view of a further alternative embodiment for an SCSSV constructed in accordance with the present invention. 
         FIG. 21  is a side, one-quarter cross-sectional view of the SCSSV shown in  FIG. 20 , now in a closed position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an exemplary wellbore  10  which has been drilled through the earth  12  from a drilling rig  14  located at the surface  16 . The wellbore  10  is drilled down to a hydrocarbon-bearing formation  18 . As is known in the art, perforations  20  extend outwardly into the formation  18 . 
     An exemplary production tubing string  22  extends downwardly within the wellbore  10  from the surface  16 . An annulus  24  is defined between the production tubing string  22  and the wall of the surrounding wellbore  10 . The production tubing string  22  is typically made up of sections of interconnected production tubing, as is know in the art. In alternative embodiments, the production tubing string  22  may be formed of coiled tubing. The production tubing string  22  defines a production flowbore  26  along its length for the transport of production fluids from the formation  18  to the surface  16 . A ported production nipple  28  is incorporated into the production tubing string  22  and is used to flow production fluids from the surrounding annulus  24  to the flowbore  26 . Packers  30 ,  32 , of a type known in the art, secure the production tubing string  22  within the wellbore  10 . 
     The production tubing string  22  also includes a surface-controlled subsurface safety valve (SCSSV)  34 . The SCSSV  34  is used to quickly and easily close off fluid flow through the flowbore  26  in the event of an emergency. The general construction and operation of flapper valves is well known in the art. Flapper valve assemblies are described, for example, in U.S. Pat. No. 7,270,191 by Drummond et al. entitled “Flapper Opening Mechanism” and U.S. Pat. No. 7,204,313 by Williams et al. entitled “Equalizing Flapper for High Slam Rate Applications.” U.S. Pat. Nos. 7,270,191 and 7,204,313 are owned by the assignee of the present application and are hereby incorporated by reference. A hydraulic control line  36  extends from the valve  34  to a control pump  38  at the surface  16 . 
       FIGS. 2 and 3  depict a first exemplary SCSSV  40  constructed in accordance with the present invention and which may be used as the SCSSV  34  in the arrangement depicted in  FIG. 1 . The valve  40  generally includes a housing  42  that is formed of an upper housing sub  44 , a central housing sub  46  and a lower housing sub  48 . A radially inwardly-projecting flange  50  is carried by the central housing sub  46 . The housing  42  defines a central flowbore  52  within which becomes a portion of the flowbore  26  when the housing  42  is integrated into the production tubing string  22 . 
     A pivotable flapper member  54  is retained upon a pivot pin  56  within a flapper member cavity  58  that is defined within the housing  42 . As is known, the flapper member  54  is movable about the pivot pin  56  between an open position, depicted in  FIG. 2 , wherein fluid can pass through the central flowbore  52 , and a closed position, illustrated in  FIG. 3 , wherein flow through the flowbore  52  is blocked by the flapper member  54 . The flapper member  54  is biased toward the closed position, typically by a torsional spring (not shown), in a manner known in the art. 
     A flow tube  60  is disposed within the housing  42  and is axially movable with respect to the housing  42  between an upper position ( FIG. 3 ) and a lower position ( FIG. 2 ). The flow tube  60  is biased toward the upper position by compressive spring  62 . In an exemplary embodiment, the spring  62  is compressed between the flange  50  and a radially-projecting arm  64  on the flow tube  60 . An axially-extending piston member  66  is affixed to the arm  64  and is movably disposed within a piston chamber  68 . The piston chamber  68  is operably interconnected with the hydraulic control line  36  such that surface changes by the pump  38  will create fluid pressure fluctuations within the chamber  68  and thereby move the piston member  66  within the piston chamber  68 . 
     The flow tube  60  also preferably includes a screening bleed port  70  which is best depicted in  FIG. 4 . The bleed port  70  preferably features a pair of cross-slots  72  which allow fluid pressure to be equalized between the radial interior and radial exterior of the flow tube  60 . The bleed port  70  is desirable in situations wherein a wiper member is incorporated into the flow tube  60 , as it is desired to equalize pressure around the flow tube  60  and prevent the creation of a pressure differential. The bleed port  70  preferably contains zero-gap until slight fluid pressure flexes the port  70  open to begin allowing fluid flow flow therethrough to begin allowing flow until pressure across the bleed port  70  equalizes. 
     An annular wiper member  74  is secured to the outer radial surface  76  of the flow tube  60 . An exemplary buildup of mineral scale is depicted at  78 . In a preferred embodiment, the wiper member  74  is formed of a relatively soft material that is abraded as the wiper member  74  contacts and moves against the interior surface  79  of the flowbore  52  and scale buildup  78 . The wiper member  74  is preferably formed largely of a soft elastomeric or thermoplastic material. In further embodiments, the wiper member  74  incorporates scale-dissolving material, such as hydrochloric acid, within the wiper material using known processes, such as chemical encapsulation or micro-encapsulation according techniques known in the art. The scale dissolver will act to dissolve or remove scale  78  as the wiper member  74  is moved upon the scale  78 . A number of alternative commercially available scale dissolvers are known in the art which are suitable for this application. 
     In operation, the SCSSV  40  is run into the wellbore  10  in the position depicted in  FIG. 2 , wherein the flapper member  54  is in the open position, and production through the production tubing string  22  can occur as is typical. In this position, the piston chamber  68  is pressurized by the surface pump  38  so that the piston  66  and flow tube  60  are retained in the axially downward position shown in  FIG. 2  and the spring bias of the compression spring  62  is overcome to do so. In the event of an emergency, an operator at the surface  16  can close the SCSSV, or valve,  40  by actuating the pump  38  to evacuate the piston chamber  68 . The spring bias of the compression spring  62  will urge the flow tube  60  axially upwardly to the position depicted in  FIG. 3 . The flapper member  54  will then rotate to the closed position shown in  FIG. 3 , thereby blocking fluid flow upwardly through the flowbore  52  of the valve  40 . 
     As the flow tube  60  is moved axially upwardly within the housing  42 , the wiper member  74  is moved axially along the interior surface  79  of the flowbore  52 . As this axial movement occurs, the wiper member  74  abrades and releases the incorporated scale dissolver to act upon the scale buildup  78 , thereby completely or partially dissolving and removing the scale buildup  78 . Every time the valve moves from the open to the closed position and back again, the wiper member  74  will release an amount of scale dissolver upon the interior surface  79 . The scale dissolver will leave a slick surface  79  that helps to prevent scale particles from sticking and accumulating upon the surface  79 . 
       FIG. 5  illustrates an alternative exemplary SCSSV  80  in accordance with the present invention. The SCSSV  80  may also be used as the SCSSV  34  in the arrangement depicted in  FIG. 1 . In the SCSSV  80 , the flow tube  60  includes a flexible skirt  82  which extends axially upwardly from the body of the flow tube  60 . In currently preferred embodiments, the flexible skirt  82  is formed of a flexible polymer of a type known in the art. In an alternative embodiment, the skirt  82  is formed of overlapping metal sheets. The skirt  82  is flexible in that it can deflect radially inwardly, as illustrated by the dashed lines  82   a . However, it is preferred that the skirt  82  be formed with shape memory so that it will provide a radial outward bias against the interior surface  79 . This bias will help prevent sand and debris from becoming disposed between the flow tube  60  and the housing  42 . The distal end of the skirt  82  includes a radially outwardly extending wiper or scraper member  84 . The scraper member  84  is shaped and sized to contact the interior surface  79  of the flowbore  52 . During movement of the flow tube  60  with respect to the surrounding housing  42 , the scraper member  84  is operable to physically scrape some of the scale buildup  78  from the interior surface  79 . The scraper member  84  is shaped so that, for scale buildup  78  that is  78  that is not removed, the scraper member  84  will flex over the scale buildup  78  like a sled rides over snow. In addition, the scraper member  84  serves as a spacer member disposed between the flow tube  60  and the surrounding housing  42 . The increased clearance afforded by this spacing helps to mitigate the effects of scale accumulation upon the interior surface  79  and will permit the flow tube  60  to move within the housing  42  despite some buildup. 
       FIG. 6  illustrates an alternative exemplary embodiment for a SCSSV  86  in accordance with the present invention. The valve  86  includes a substantially soft wiper member  88  that extends axially upwardly from the flow tube  60 . The wiper member  88  is angled radially outwardly at its upper end  90  and tapered. The outward angle and the taper permit streamlined flow of wellbore fluid, as indicated by the flow arrow  92 . The wiper member  88  is preferably formed of a softer non-elastomeric material, such as a thermoplastic, but other suitable materials may also be used. In a preferred embodiment, the wiper member  88  contains a scale dissolver that is released upon surface  79  to remove and prevent scale buildup  78 . In operation, during movement of the flow tube  60  allows the wiper member  88  to move and flex over scale  78 . Scale dissolver is released to help remove the buildup  78  and prevent buildup from occurring. 
       FIGS. 7 and 8  illustrate a further exemplary embodiment for a SCSSV  94 , in accordance with the present invention. The flow tube  60 ′ of the SCSSV  94  includes an annular wiper member  74 . In addition, a flapper member opening  96  is disposed through the body of the flow tube  60 ′ and is sufficiently large to permit the flapper member  54  to pass through without restriction. A lug pin  98  projects radially inwardly from the flange  50 . The flow tube  60 ′ has a “J-slot” lug path  100  inscribed on its outer radial surface.  FIG. 8  illustrates an exemplary lug path  100 . As  FIG. 8  shows, the lug path  100  is made up of a single inscribed leg  102  which is disposed at an angle with respect to the axial axis of the flow tube  60 ′. The lug pin  98  extends into the lug path  100 . It is noted that the flow tube  60 ′ is movable rotationally with respect to the surrounding housing  42  but need not be movable axially with respect to the housing  42 . Movement of the flow tube  60 ′ is governed by the interface between the lug pin  98  and the lug path  100 . 
     During run-in and typical operation, the SCSSV  94  is in the position depicted in  FIG. 7  with the flapper member  54  retained in the open position by the flow tube  60 ′. The lug pin  98  is located generally in the position indicated at  98   a  in  FIG. 8 . In order to close the SCSSV  94 , the piston chamber  68  is evacuated by the pump  38 , and the piston member  66  moves axially upwardly within the piston chamber  68 . As the flow tube  60 ′ is moved axially upwardly, the lug pin  98  is moved along the vertical leg  104  of the lug path  100  to the general position depicted at  98   c  in  FIG. 8 . Movement of the lug pin  98  along the lug path  100  will cause the flow tube  60 ′ to rotate approximately 90 degrees with respect to the housing  42 . Rotation of the flow tube  60 ′ will align the flapper member opening  96  with the flapper member  54 , thereby allowing the flapper member  54  to move to its closed position under impetus of its torsional spring (not shown). 
     During radial movement of the flow tube  60 ′, the wiper member  74  will physically wipe away some of the scale buildup  78 . 
       FIGS. 9 and 10  depict a further alternative SCSSV  106  wherein the interior surface  79  of the flowbore  52  is covered by a sleeve  108 . Preferably, the sleeve  108  is formed of a flexible material, such as elastomer. The sleeve  108  includes a sheath portion  110  that conforms closely to the interior surface  79 . Scale buildup  78  accumulates on the sleeve  108  rather than the interior surface  79 . The sleeve  108  also includes an axially compressible portion  112 . In a currently preferred embodiment, the compressible portion  112  is made up of a series of folds which may be compressed in the manner of an accordion bellows. A contact arm  114  preferably extends radially outwardly from the flow tube  60  and into engagement with the sheath portion  110  of the sleeve  108 . In operation, when the flow tube  60  is moved axially upwardly within the housing  42 , the sleeve  108  is urged axially upwardly upon the interior surface  79  by the contact arm  114 . The compressible portion  112  of the sleeve  108  is compressed, as depicted in  FIG. 10 , and the sheath portion  110  slides upwardly upon the surface  79  to expose a clean surface  79  which is substantially free of scale buildup  78 . 
       FIGS. 11 and 12  depict a further alternative SCSSV  116 . The SCSSV  116  includes a modified flow tube  60 ″ which includes an annular recess  118  at its upper end. The recess  118  contains an axially compressible spring  120 . A substantially rigid sleeve  122  is also disposed within the recess  118  and is biased axially upwardly by the spring  120  until it is in contact with a radially-inwardly projecting ledge  121 .  FIG. 11  depicts the SCSSV  116  in an initial position with the SCSSV  116  is open and flow therethrough is occurring naturally. The sleeve  122  substantially prevents fluid flowing through the flowbore  52  from contacting and depositing scale upon the interior surface  79 . Additionally, the sleeve  122  presents an interior radial surface  124 . Scale buildup  78  would occur on the interior surface  124  of the sleeve  122 . 
     When the SCSSV  116  is moved to its closed position, as depicted in  FIG. 12 , the spring  120  is compressed and the sleeve  122  is moved downwardly into the recess  118 . The spring  120  continues to urge the sleeve  122  against the ledge  121 . As the sleeve  122  sleeve  122  is moved into the recess  118 , the scale buildup  78  is scraped from the interior surface  124 . It is preferred that relatively close tolerances be used to aid the effectiveness of the scraping removal of the scale buildup  78 . Additionally, the sleeve  122  minimizes changes in the interior diameter of the flowbore  52  of the valve housing  42 , which helps to prevent scale buildup from occurring within the flowbore  52 . 
       FIGS. 13 and 14  depict a further alternative embodiment for an SCSSV  130  constructed in accordance with the present invention. The upper housing sub  44  of the housing  42  includes an annular axial recess  132  which retains an axially compressible spring  134  and a substantially rigid annular sleeve  136 . The spring  134  biases the sleeve  136  axially downwardly and into contact with the upper end of the flow tube  60 .  FIG. 13  depicts the SCSSV  116  in an initial position with the SCSSV  130  is open and flow therethrough is occurring. The sleeve  136  substantially prevents fluid flowing through the flowbore  52  from contacting and depositing scale upon the interior surface  79 . Additionally, the sleeve  136  presents an interior radial surface  138 . Scale buildup  78  would occur on the interior surface  138  of the sleeve  136 . 
     When the SCSSV  130  is moved to its closed position, as depicted in  FIG. 14 , the spring  134  is axially compressed as the sleeve  136  retracts into the recess  132 . The spring  134  continues to urge the sleeve  132  against the flow tube  60 . As the sleeve  132  is moved into the recess  132 , the scale buildup  78  is scraped from the sleeve  132 . It is preferred that relatively close tolerances be used to aid the effectiveness of the scraping removal of the scale buildup  78 . Additionally, the sleeve  132  minimizes changes in the interior diameter of the flowbore  52  of the valve housing  42 , which helps to prevent scale buildup from occurring within the flowbore  52 . 
       FIGS. 15 and 16  illustrate a further alternative embodiment for an SCSSV  140  constructed in accordance with the present invention. The upper housing sub  44  defines an annular recess  142 . The flow tube  60  includes an axially extending annular shield portion  144  which extends into the recess  142 . The shield portion  144  prevents scale buildup  78  from occurring on the interior surface  79 . Instead, scale buildup  78  will form on the shield portion  144 . As the SCSSV  140  is moved to the closed position, as depicted in  FIG. 16 , a lower shoulder  146  on the upper sub  44  will scrape the scale buildup  78  from the shield portion  144 . Additionally, the shield portion  144  minimizes changes in the interior diameter of the flowbore  52  of the valve housing  42 , which helps to prevent scale buildup from occurring within the flowbore  52 . 
       FIGS. 17 ,  18  and  19  illustrate another alternative embodiment for an SCSSV  150  constructed in accordance with the present invention. The flow tube  60  of the SCSSV  150  presents an upwardly axially-extending shield portion  152 . The axially-extending shield portion  152  has a reduced outer radial diameter surface  154 . The upper axial end of the shield portion  152  is preferably provided with an outwardly and upwardly facing angled edge  156 . 
     A split sleeve element  158  is located within the flowbore  52  above the shield portion  152 .  FIG. 19  shows the split sleeve element  158  apart from the other components of the SCSSV  150 . In the depicted embodiment, the split sleeve element  158  includes multiple radially separated arcuate sections  160 ,  162 ,  164 . Although there are four sections  160 ,  162 ,  164  depicted in  FIGS. 17 ,  18  and  19 , there may be more or fewer than 3, in desired. In a preferred embodiment, each of the sections  160 ,  162 ,  164  include a lower, radially-enlarged diameter portion  166  and an upper, radially-reduced diameter portion  168 . An inwardly and downwardly-facing angled interior surface  170  is defined between the upper and lower portions  166 ,  168  of each section  160 ,  162 ,  164 . The split sleeve element  158  is disposed axially above the shield portion  152  and the flow tube  60  within the flowbore  52 . The angled interior surface  170  of each segment  160 ,  162 ,  164  is located in adjacent, abutting contact with the angled edge  156  of the shield portion  152 . As a result, each of the arcuate sections  160 ,  162 ,  164  are located in a close, generally abutting relation to each other. While the SCSSV  150  is in the open position, as shown in  FIG. 17 , the shield portion  152  and the split sleeve element  158  protect the interior surface  79  against a buildup of scale. 
     When the SCSSV  150  is moved to the closed position, as depicted in  FIG. 18 , the flow tube  60  and affixed shield portion  152  are moved axially upwardly within the flowbore  52 . The angled edge  156  of the shield portion  152  slides against the angled interior surface  170  of each of the arcuate sections  160 ,  162 ,  164  of the split sleeve element  158 , thereby causing the arcuate sections  160 ,  162 ,  194  to separate from one another radially (see  FIG. 18 ). The shield portion  152  slides inside of the upper portions  168  of the segments  160 ,  162 ,  164 , as depicted in  FIG. 18 . 
     Scale buildup  78  on the shield portion  152  or the split sleeve element  158  will be broken up and removed as the shield portion  152  slides axially upwardly and within the upper portions  168  of the segments  160 ,  162 ,  164 . As the segments  160 ,  162 ,  164  separate from one another radially, scale buildup  78  will be broken up and carried away by the flow of production fluids within the flowbore  52 . Also, scale buildup  78  on the interior of the upper portions  168  will be scraped away by the shield portion  152 . 
       FIGS. 20 and 21  depict still a further alternative embodiment for another SCSSV  176  constructed in accordance with the present invention. The SCSSV  176  includes an axially collapsible sleeve  178  which extends from, and is preferably affixed to, the upper end  180  of the flow tube  60 . The sleeve  178  is preferably also affixed at its upper end to a ledge portion  182  of the housing  42  so that the sleeve  178  functions as a shield for the interior surface  79  of the housing  42 . The presence of sleeve  178  also minimizes changes in the interior diameter of the flowbore  52  through the valve  176 , thereby reducing the possibility that scale will accumulate at points within the flowbore  52 . The sleeve  178  is preferably formed of a section of sheet metal. In a currently preferred embodiment, the sleeve  178  is a corrugated sheet  188  that is axially expandable and compressible in the manner of an accordion bellows. The presence of the sleeve  178  minimizes changes in the diameter of the flowbore  52  of the SCSSV  176 , thereby reducing pressure changes within the flowbore that might promote the deposition of scale within the flowbore  52 . 
     When the SCSSV  176  is actuated to a closed position, as illustrated in  FIG. 21 , the flow tube  60  moves axially upwardly to cause the sleeve  178  to be axially compressed. As depicted, the sleeve  178  preferably collapses in the manner of an accordion bellows. As this axial compression occurs, the scale buildup  78  will be broken up and thereafter carried away by the flow of production fluid through the flowbore  52 . 
     It will be appreciated that the invention provides devices and methods for negating buildup of scale and other debris within the flowbore of a sliding sleeve valve. In some aspects, a wiper member is affixed to the flow tube and acts as a spacer between the flow tube and the surrounding valve housing. The increased clearance between the flow tube  60  and the surrounding valve housing  42  as a result of the spacer will permit the valve to operate longer without becoming inoperable due to the expanded clearance area becoming fouled with scale buildup. In addition, the wiper member is operable to physically wipe away scale buildup from the interior radial surface of the valve housing. In some embodiments, the wiper member incorporates a scale dissolver material that can be disposed onto the interior radial surface to assist the breakup and removal of scale buildup. In other aspects of the invention, scale buildup is negated by disposing a shield or sleeve within the flowbore to provide a substantially smooth flowbore without significant changes in diameter. This would eliminate points within the flowbore wherein there are pressure changes that could encourage the growth of scale buildup. In addition, the shield of sleeve would physically protect the interior radial surface from scale buildup. Production fluid flowed through the flowbore of the valve would cause buildup on the shield rather than on the interior radial surface. 
     The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art, that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention.