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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to devices and methods for operating a valve in an incremental fashion. 
         [0003]    2. Description of the Related Art 
         [0004]    Sliding sleeve valves are often used in a hydrocarbon production string to selectively control the flow of production fluid into the production string. A sliding sleeve valve typically includes an outer cylindrical housing that defines a flowbore and a sleeve member that is moveably disposed within the housing. Both the housing and the sleeve member include openings. When the openings are aligned, fluid can be communicated through the openings and into the flowbore. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention provides systems for operating one or more sliding sleeve valves in an incremental, step-type fashion between open and closed positions. This permits the valve or valves to be choked to progressively smaller flow areas. The systems of the present invention also permit the valve or valves to be fully closed without having to progress through incremental steps. 
         [0006]    In a preferred embodiment, a sliding sleeve valve is interconnected with hydraulic open and close lines so that fluid flow through the hydraulic lines will actuate the sleeve valve between open and closed positions. In preferred embodiments, the close line incorporates a fluid metering assembly which is operable to flow discrete increments of hydraulic fluid into or out of the sleeve valve. The fluid metering assembly preferably includes a bi-directional check valve assembly and an incremental piston assembly. 
         [0007]    During exemplary operation of the system, the sliding sleeve valve is incrementally choked from a fully open position to a partially open position by flowing hydraulic fluid into the close line at a pressure that is below a predetermined level. The incremental piston assembly will be operated to transmit a predetermined discrete amount of fluid to the sleeve valve, thereby moving the sleeve member incrementally toward a closed position. 
         [0008]    In the event that it is desired to fully close the sliding sleeve valve, fluid is flowed into the close line at a pressure that is above the predetermined level. The pressurized fluid will open a check valve within the check valve assembly and permit fluid to pass in an unrestricted manner through the fluid metering assembly. The sleeve valve will then be shifted to a fully closed position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  is a side, cross-sectional view of an exemplary wellbore containing a production tubing string which incorporates sliding sleeve valves and a control system in accordance with the present invention. 
           [0011]      FIG. 2  is a schematic side view of an exemplary sliding sleeve valve in a fully open position and an operably associated hydraulic metering assembly, in accordance with the present invention. 
           [0012]      FIG. 3  is a schematic side view of the sleeve valve shown in  FIG. 2 , now having been moved to a partially choked open position. 
           [0013]      FIG. 4  is a schematic side view of the sleeve valve shown in  FIGS. 2 and 3 , now having been moved to a further partially choked open position. 
           [0014]      FIG. 5  is a schematic side view of the sleeve valve shown in  FIGS. 2-4  now in a fully closed position. 
           [0015]      FIG. 6  is a side, cross-sectional view of an exemplary bi-directional check valve assembly used in the hydraulic metering assembly of the present invention 
           [0016]      FIG. 7  is a side, cross-sectional view of an exemplary incremental piston assembly used in the hydraulic metering assembly of the present invention. 
           [0017]      FIG. 8  is a side, cross-sectional view of the incremental piston assembly shown in  FIG. 7 , now having been actuated. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]      FIG. 1  illustrates an exemplary wellbore  10  that that has been drilled from the surface  12  through the earth  14 . The wellbore  10  passes through production formations  16 ,  18  and  20 , which are separated from one another by substantially impermeable layers  22 . The wellbore  10  has been lined with metallic casing  24 , in a manner known in the art, and perforations  26  extend through the casing  24  and into the formations  16 ,  18 ,  20 . 
         [0019]    A hydrocarbon production string  28  is disposed within the wellbore  10 . An annulus  30  is defined between the outer radial surface of the production string  28  and the casing  24 . The production string  28  may be made up of sections of standard production tubing or, alternatively, be formed of coiled tubing, in a manner known in the art. The production string defines an interior flowbore  32  by which production fluids may be conveyed to the surface  12 . The production string  28  includes production nipples  34 ,  36 ,  38  which are located proximate each of the production zones  16 ,  18 ,  20 , respectively. 
         [0020]    Although three production zones and three production nipples are depicted in  FIG. 1 , it should be understood that this is for purposes of illustration only, and that there may be more or fewer than three such nipples, as needed or desired. Each of the production nipples  34 ,  36 ,  38  is a sliding sleeve valve which can be opened or closed to selectively permit production fluid entering the wellbore annulus  30  from the perforations  26  to enter the central flowbore  32 . 
         [0021]    A common hydraulic “open” line  40  extends from a surface-based pump, or open side fluid source,  42  and is interconnected with each of the production nipples  34 ,  36 ,  38  in a manner which will be described in further detail shortly. In addition, a separate hydraulic “close” line extends from a surface pump, or close side fluid source,  44  to each of the production nipples  34 ,  36 ,  38 . Close line  46  extends from the pump  44  to production nipple  34 . Hydraulic close line  48  extends from the pump  44  to the second production nipple  36 , and hydraulic close line  50  extends from the pump  44  to the third production nipple  38 . A hydraulic metering assembly  52  is integrated into each of the close lines  46 ,  48 ,  50 . It is noted that, while the open side fluid source  42  and closed side fluid source  44  are depicted schematically as separate fluid sources, they may, in fact be a single pump or other fluid source. 
         [0022]      FIG. 2  illustrates an exemplary sliding sleeve valve  54  of the type used for each of the production nipples  34 ,  36 ,  38 . The sliding sleeve valve  54  includes an outer cylindrical housing  56  that defines a central flowbore  58 . The housing  56  has threaded axial ends  60  to permit the housing  56  to be integrated into the production tubing string  28 . Multiple lateral flow ports  62  are disposed through the housing  56 . A generally cylindrical sleeve member  64  is disposed within the central flowbore  58  of the housing  56  and is axially moveable with respect to the housing  56 . Lateral fluid openings  66  are disposed through the sleeve member  64 . 
         [0023]    A radially-enlarged recess  68  is formed in a portion of the flowbore  58  of the housing  56 . A flange  70  extends radially outwardly from the sleeve member  64  and into the recess  68  to divide the recess  68  into first and second fluid chambers  72 ,  74 . The fluid chambers  72  and  74  are defined between the inner sleeve member  64  and the recess  68  of the housing  56 . Fluid seals  76 , of a type known in the art, ensure fluid tightness for the chambers  72 ,  74 . One of the hydraulic fluid “close” lines  46 ,  48  or  50  is interconnected with the first chamber  72 . The hydraulic fluid “open” line  40  is interconnected with the second chamber  74 . 
         [0024]    The fluid metering assemblies  52  each include a bi-directional check valve assembly  78  and an incremental piston assembly  80  which are incorporated into the close line  46 ,  48  or  50  in a parallel fashion by the use of fluid line splitters  82 . The check valve assembly  78  is shown in greater detail in  FIG. 6  and includes a housing  84  with a fluid inlet  86  and a fluid outlet  88 . Parallel first and second flow paths  90 ,  92  extend from the fluid inlet  86  to the fluid outlet  88 . A first check valve  94  is located within the first flow path  90  and is oriented so as to block fluid flow from the inlet  86  toward the outlet  88  but selectively permit fluid flow along the flow path  90  from the outlet  88  toward the inlet  86 . In the depicted embodiment, the first check valve  94  includes a closure member  96  that is biased by compression spring  98  against a valve seat  100 . The spring  98  provides a bias force upon the closure member  96  that can be overcome by a first, relatively low, level of pressure by fluid flowing along the first flow path  90  toward inlet  86 . As a non-limiting example, a fluid pressure of 100-200 psi would unseat the closure member  96  from the valve seat  100  and allow fluid to pass through the first flow path  90 . 
         [0025]    A second check valve  102  is located within the second flow path  92 . The second check valve  102  blocks fluid flow from the outlet  88  toward the inlet  86 , but it will selectively permit fluid flow from the inlet  86  toward the outlet  88 . In the depicted embodiment, the second check valve  102  includes a closure member  104  that is biased by a compression spring  106  against a valve seat  108 . The spring provides a bias force upon the closure member  104  that can be overcome by a second level of pressure by fluid flowing along the second flow path  92  toward the outlet  88 . The second level of pressure is greater than the first level of pressure. As a non-limiting example, a fluid pressure of about 5000 psi would unseat the closure member  104  from the valve seat  108  and allow fluid to pass through the second flow path  92 . 
         [0026]    Referring now to  FIGS. 7 and 8 , the incremental piston assembly  80  is shown apart from the other components of the fluid metering assembly  52 . The incremental piston assembly  80  includes a tubular piston housing  110  with upper and lower end subs  112 ,  114  secured at opposite axial ends. Fluid passages  116  are disposed axially through each of the end subs  112 ,  114 . An incremental piston chamber  118  is defined within the piston housing  110  between the end subs  112 ,  114 . End sub  112  provides a fluid inlet for the chamber  118  while end sub  114  provides a fluid outlet. The piston chamber  118  contains an incremental piston pump, generally shown at  120 . The incremental piston pump  120  is useful for sequentially displacing a predetermined, known amount of fluid through the piston chamber  118  of the incremental piston assembly  80  and includes a piston sleeve  122  which radially surrounds a piston member  124 . The exemplary piston member  124  features an enlarged pressure-receiving end  126 , a reduced-diameter shaft portion  128 , and an enlarged piston head  130 . It is noted, however, that the piston member  124  could have other geometrical designs. The piston member  124  is moveable with respect to the sleeve  122  between a retracted position ( FIG. 7 ) and an extended position ( FIG. 8 ). When moved to the extended position, the enlarged piston head  130  of the piston member  124  displaces a volume of fluid through the fluid outlet  116  of end sub  114  and substantially the same volume of fluid is drawn into the fluid inlet of end sub  112  from the close line  46 ,  48  or  50 . The enlarged piston head  130  of the piston member  124  contacts an end portion  132  of compression spring member  134 , which is disposed within the chamber  88 . The spring  134  biases the piston member  124  toward the retracted position. Although the spring illustrated in the drawings is a spiral-type compression spring, those of skill in the art will understand that other compressible spring designs could be used, including, for example, Belleville washers or fluid springs, as are known in the art. When fluid pressure is increased within the close line  46 ,  48 , or  50 , it bears upon the pressure-receiving end portion  126  to urge the piston member  124  to move axially with respect to the sleeve  122  toward the extended position, and the spring member  134  is compressed by the piston head  130 . It is noted that, while the pressure-receiving end  126  of the piston member  124  may be disposed within the surrounding sleeve  122  with a relatively close fit, there are preferably no elastomeric or other fluid-tight seals located between the piston member  124  and the sleeve  122 . As a result, it is contemplated that some fluid pressure will seep between the piston member  124  and sleeve  122  during operation. 
         [0027]    In operation, the fluid metering assemblies  52  are used to operate each of the production nipples  34 ,  36 ,  38  by increments between an extreme open position (i.e., the fully open position depicted in  FIG. 2 ) and an extreme, or fully closed position (see  FIG. 5 ). In other words, the fluid metering assemblies  52  will operate the production nipples  34 ,  36 ,  38  between fully open, fully closed and partially open, or “choked” positions. It is noted that, in  FIGS. 2-5 , the valve is shown in a fully open position (i.e., the fluid openings  66  of the sleeve member  64  are fully aligned with the ports  62  of the housing  56 ) when the sleeve member  64  is in a raised or upper position within the housing  56 , and is closed by moving the sleeve member  64  downwardly with respect to the housing  56 , the valve  54  may be constructed so that the valve is fully opened when the sleeve member  64  is in a lower position with respect to the housing  56  and is shifted upwardly or even rotated with respect to the housing for choking and closure to occur. In an exemplary method of operation, the production string  28  is run into the wellbore  10  and typically secured in place with sets of packers (not shown) of a type known in the art. At this point, the production nipples  34 ,  36 ,  38  may all be fully opened by pressurizing the common open line  40  with surface-based pump, or open side fluid source,  42 . This will flow pressurized fluid into the second fluid chamber  74  of each of the production nipples  34 ,  36 ,  38  and urge the flange  70  and sleeve  64  of each upwardly until each of the production nipples  34 ,  36 ,  38  are in the fully open position shown in  FIG. 2 . It is noted that, as fluid enters the second chamber  74  and urges the sleeve  64  upwardly, the first chamber  72  will be reduced in volume, and fluid within the first chamber  72  will exit the first chamber  72  via the respective close lines  46 ,  48  or  50 . The fluid displaced from the first chamber  72  will flow through the bi-directional check valve assembly  78 . In order to pass through the check valve assembly  78 , the displaced fluid will urge closure member  96  off of its valve seat  100 , permitting the fluid to pass through the first flow path  90  of the check valve assembly  78 . 
         [0028]    When it is desired to choke the production flow into the production nipples  34 ,  36 ,  38 , the hydraulic metering assemblies  52  may be actuated to sequentially move their respective production nipples to choked position of smaller flow area, as illustrated in  FIGS. 3 and 4  and, eventually, the fully closed position depicted in  FIG. 5 . It is noted that due to the use of separate and independent close lines  46 ,  48 ,  50  for each individual production nipple  34 ,  36  and  38 , each production nipple may be choked separately and to a different degree than the other production nipples. To choke the production nipple, fluid is flowed by pump  44  into the respective close line  46 ,  48  or  50  at a pressure that is below a predetermined level. The predetermined level is the level of fluid pressure that would lo unseat the closure member  104  from its valve seat  108  in the bi-directional check valve assembly  78 . The fluid flow into the close line will cause the incremental piston assembly  80  to displace a predetermined amount of fluid, as described previously, into the first chamber  72  of the respective production nipple  34 ,  36  or  38 . The predetermined amount of fluid entering the first chamber  72 , will act upon the flange  70  and urge the sleeve member  64  axially downwardly with respect to the surrounding housing  56 . From the fully opened position shown in  FIG. 2 , the sleeve valve  54  will be moved to the partially choked position shown in  FIG. 3 . The fluid openings  66  will be less aligned with the fluid ports  62  of the housing  56 , thereby reducing the amount of available fluid flow area and choking the production nipple  34 ,  36  or  38 . 
         [0029]    Once the sleeve member  64  has been moved axially downwardly in an incremental manner, as described, fluid pressure within the close line  46 ,  48  or  50  is reduced or bled off to permit the compression spring  134  of the incremental piston assembly  80  to return the piston  124  to its retracted position. The compression spring  134  will urge the piston member  124  back to its retracted position. Fluid will pass around the piston portion  126  to refill the piston chamber  118  with fluid. Following the reduction in pressure, the close line  46 ,  48  or  50  can be repressurized as described above to move the sleeve member  64  a further incremental distance axially downwardly with respect to its surrounding housing  56 . From the partially choked position shown in  FIG. 3 , the sleeve valve  54  will be moved incrementally to an even more choked position, depicted in  FIG. 4 . The pressure within one or more of the close line(s)  46 ,  48 ,  50  can then again be bled off, and pressure reapplied to move the sleeve member(s)  64  of the respective production nipple(s)  34 ,  36 ,  38  further downwardly with respect to the surrounding housing  56 , thereby further choking the flow area provided by those production nipple(s)  34 ,  36 ,  38 . 
         [0030]    The hydraulic metering assemblies  52  also may be actuated to move an associated production nipple  34 ,  36  or  38  to the fully closed position shown in  FIG. 5  in a single step from either a fully opened position or a choked position. In order to do this, fluid is flowed by fluid source  44  into the respective close line  46 ,  48  or  50  at a pressure that is above the predetermined level necessary to unseat the closure member  104  from its valve is seat  108 . This permits fluid to pass in an unrestricted manner through the second flow path  92  of the bi-directional check valve assembly  78 . The pressurized fluid will enter the first chamber  72  of the production nipples  34 ,  36 ,  38 , act upon the flange  70  and urge the sleeve member  64  axially downward to the fully closed position depicted in  FIG. 5 . Thus, by applying a fluid pressure to the close line(s)  46 ,  48 ,  50  at a level that is above the predetermined fluid pressure level, the incremental piston assembly  80  can be bypassed. 
         [0031]    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.

Summary:
Systems for operating one or more sliding sleeve valves in an incremental, step-type fashion between open and closed positions, permitting the valve or valves to be choked to progressively smaller flow areas. The systems of the present invention also permit the valve or valves to be fully closed without having to progress through incremental steps.