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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to hydraulic metering devices used to operate downhole devices, such as sliding sleeve valves. 
         [0003]    2. Description of the Related Art 
         [0004]    A number of downhole devices are operated hydraulically. At times, it is desirable to operate these devices in a stepped manner to respond to changes in downhole conditions. For example, a production tubing string might have a sliding sleeve valve associated with a production nipple to control the flow of fluid into the production tubing string. It would be desirable to be able to shift the sliding sleeve by increments between an open and a closed position. This adjustability would allow fluid flow into the tubing string through the production nipple to be balanced with fluid flowing into the tubing string from other production nipples. 
         [0005]    Attempts have been made to use metering devices to adjustably operate a downhole device in a stepped manner. Unfortunately, most of these arrangements have proven to be complex in construction and operation. For example, PCT Application No. PCT/US00/12329 by Schultz et al., entitled “Hydraulic Control System for Downhole Tools” describes a hydraulic control system for multiple well tool assemblies that includes a metering device. The metering device uses two pumps. One of the pumps transfers fluid from a first hydraulic line to an actuator of the well tool assembly in response to fluctuations in pressure on a second hydraulic line, and the other pump transfers fluid from the second hydraulic line to the actuator in response to fluctuations in pressure on the first hydraulic line. The fact that this system requires multiple pumps with associated hydraulic lines makes the system complex in practice and costly. 
         [0006]    U.S. Pat. No. 6,585,051 issued to Purkis describes a number of metering apparatuses for use in a downhole environment to discharge a known volume of fluid into a well tool actuator. These metering devices are relatively complex and, therefore, may be prone to failure during use. Additionally, several of the described metering devices incorporate numerous elastomeric O-rings to create fluid tight seals within the metering devices. The O-rings are prone to wear and failure during operation, making metering of a known volume unreliable. 
         [0007]    Additionally, the prior art metering arrangements all meter fluid into a fluid input on the downhole device. This can be problematic in some instances 
         [0008]    The present invention addresses the problems of the prior art. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention provides devices and methods for operating a sliding sleeve valve or other downhole well tool that is axially shiftable among a finite number of increments between two extreme configurations such as open and closed configurations. A metering device is described having a pair of piston metering assemblies that operate in parallel fluid flow paths. The first piston metering assembly is a “zero position” piston assembly, which when actuated, moves the sleeve of the well tool from a fully closed position to the zero position. The second piston metering assembly is an incremental piston assembly, which can be repeatedly pressurized and depressurized to meter predetermined amounts of fluid from an actuator sequentially to move the sleeve of the sleeve valve in consecutive increments toward a fully open position. The sleeve valve may be moved back to a fully closed position by reverse pressurizing the metering device. 
         [0010]    In other aspects, the invention relates to methods of operating a downhole tool, such as a sliding sleeve valve, using a hydraulic metering device so that the tool is adjusted in increments between two extreme configurations, such as open and closed positions. In practice, the metering device and methods of the present invention are less complex than prior art metering arrangements, and the nature of the components used makes the metering device less prone to wear-induced problems, such as the deterioration of elastomeric O-ring seals. 
         [0011]    A further advantage of the metering assembly of the present invention is that the metering assembly can be operably interconnected to either the “open” line (fluid inlet) or “close” line (fluid outlet) of a well tool actuator in order to operate the well tool. In a currently preferred embodiment, the metering assembly is connected to the fluid output of the well tool actuator to meter fluid out of the actuator in incremental known amounts to cause the well tool to be actuated in a stepped, incremental manner. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    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 drawings and wherein: 
           [0013]      FIG. 1  is a schematic view of a downhole well tool system having a fully closed sliding sleeve valve associated with a hydraulic metering valve in accordance with the present invention. 
           [0014]      FIG. 2  is a schematic view of the arrangement shown in  FIG. 1 , now with the sliding sleeve valve in the zero position. 
           [0015]      FIG. 3  is a schematic view of the arrangement shown in  FIGS. 1 and 2 , now with the sliding sleeve valve in a partially open position. 
           [0016]      FIG. 4  is a schematic view of the arrangement shown in  FIGS. 1-3 , now with the sliding sleeve valve in a fully open position. 
           [0017]      FIGS. 5A-5B  present a side, cross-sectional view of portions of the exemplary hydraulic metering valve, in an unpressurized condition as used in the well tool system shown in  FIGS. 1-3 , constructed in accordance with the present invention. 
           [0018]      FIGS. 6A-6B  present a side, cross-sectional view of the device depicted in  FIGS. 5A-5B , now in a pressurized position. 
           [0019]      FIG. 7  is an enlarged cross-sectional view of a free piston used within the device shown in  FIGS. 5A-5B  and  6 A- 6 B and surrounding components. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]      FIGS. 1-4  depict a well tool system  10  that includes a well tool actuator  12  and associated well tool  14 . The well tool  14  is of a variety that is operable in a stepped manner between two extreme positions or configurations. It is noted that the components of the system  10  are shown schematically and, in practice, would be integrated into one or more housings or subs (not shown) in a wellbore production tubing string or similar well tool. One example of a suitable well tool actuator  12  is the “HCM-A” sliding sleeve valve hydraulic actuator that is available commercially from Baker Oil Tools of Houston, Tex. The actuator  12  is provided with a hydraulic “open” line  16  and a hydraulic “close” line  18 . As will be described in detail shortly, fluid pressure is increased within the hydraulic “open” line  16  in order to move the well tool  14  toward an open configuration, and fluid pressure is increased within the “close” line  18  in order to move the well tool  14  toward a closed configuration. 
         [0021]    In currently preferred embodiments, and as depicted in  FIGS. 1-4 , the well tool  14  comprises a sliding sleeve valve, of a type known in the art. In this embodiment, the sliding sleeve valve includes a generally cylindrical housing  20  and a tubular sleeve  22  that is shiftable with respect to the housing  20 . Alignable fluid flow ports control fluid flow between the radial exterior of the housing  20  of the sleeve valve  14  and the interior flowbore  24  of the housing  20 . The housing  20  contains fluid flow ports  26  with interior fluid seals  28  located on each axial side thereof. The sleeve  22  has lateral ports  30  disposed therethrough. In a fully closed position, shown in  FIG. 1 , the ports  30  of the sleeve  22  are not aligned with the ports  26  of the housing  20 , and fluid flow between the radial exterior of the housing  20  and the flowbore  24  is blocked by fluid seals  28 . In a fully opened position ( FIG. 4 ), the ports  30  of the sleeve  22  are fully aligned with the ports  26  of the housing  20 , allowing maximum fluid flow through the sleeve valve  14 . In instances wherein the sleeve valve  14  functions as a fluid flow choke within a production tubing string, it would be desirable to be able to move the sleeve  22  in a stepped manner between intermediate positions that lie between the fully opened and fully closed positions. This would allow the amount of fluid flow to be adjusted in response to changing well conditions, such as an increase in the amount of water content within the production fluid obtained from the surrounding formation and the need to balance the production obtained from one formation with that obtained from other formations. 
         [0022]    A hydraulic metering device, generally indicated at  32 , is associated with the close line, or fluid output,  18  of the sleeve valve actuator  12 . Still referring to  FIGS. 1-4 , the metering device  32  generally includes an upstream filter  34 , a pair of piston metering assemblies  36 ,  38 , and a downstream filter  40 . The downstream filter  40  is operably interconnected with a further hydraulic control line  42  that extends to the surface of the wellbore (not shown). Hydraulic fluid conduit  44  interconnects the upstream filter  34  with the first piston metering assembly  36 , and hydraulic fluid conduit  46  interconnects the upstream filter  34  with the second piston metering assembly  38 . Additionally, a hydraulic fluid conduit  48  interconnects the first piston metering assembly  36  with the downstream filter  40 , while fluid conduit  50  interconnects the second piston metering assembly  36  with the downstream filter  40 . It is noted that the upstream and downstream filters  34 ,  40  serve as fluid filters to help remove debris from the hydraulic fluid within the system and also serve to split the flow of fluid into parallel flow paths. Fluid exiting the actuator  12  via the fluid outlet  18  will be split by the upstream filter  34  so that the fluid will pass into both the first piston metering assembly  36  and the second piston metering assembly  38 . Conversely, fluid flowed in the reverse direction, through the control line  42 , the downstream filter  40  will split the flow of fluid into parallel flow paths that will pass through both the first piston metering assembly  36  and the second fluid metering assembly  38 . Thus, there are parallel flow paths through the metering device  32 . 
         [0023]    Portions of the hydraulic metering device  32  are more clearly depicted in  FIGS. 5A-5B  and  6 A- 6 B. The first piston metering assembly  36  is referred to as a “zero position” piston assembly and includes a tubular piston housing  52  with upper and lower end subs  54 ,  56 , respectively, at opposite axial ends thereof. A piston chamber  58  is defined within the housing  52  and end subs  54 ,  56 . Each of the end subs  54 ,  56  contains an axial fluid flow passage  60  defined therein to allow fluid to enter or exit the piston chamber  58 . Thus, end sub  54  serves as a fluid outlet to the piston chamber  58  while end sub  56  provides a fluid inlet. The piston chamber  58  retains a “zero position” free piston  64  that is slidably moveable within the chamber  58 . The free piston  64  contains a spring-biased check valve  66  that permits one-way flow of fluid across the free piston  64 . Details of the construction of the free piston  64  and check valve  66  are more readily apparent with reference to  FIG. 7 . As depicted there, the check valve  66  is housed within a fluid passage  67  in the body  68  of the free piston  64 , and includes a valve ball member  70  that is biased against valve seat  72  by compressible spring  74 . It is noted that annular fluid seals  76  surround the body  68  of the free piston  64  to create a fluid seal against the housing  52 . 
         [0024]    The second piston metering assembly  38  is referred to as an incremental piston assembly and includes a tubular piston housing  80  with upper and lower end subs  82 ,  84  secured at opposite axial ends. Fluid passages  86  are disposed axially through each of the end subs  82 ,  84 . An incremental piston chamber  88  is defined within the piston housing  80  between the end subs  82 , 84 . End sub  84  provides a fluid inlet for the chamber  88  while end sub  82  provides a fluid outlet. The piston chamber  88  contains an incremental piston pump, generally shown at  90 . The incremental piston pump  90  is useful for sequentially displacing a predetermined, known amount of fluid through the piston chamber  88  of the incremental piston assembly  38  and includes a piston sleeve  92  which radially surrounds a piston member  94 . The piston member  94  features an enlarged pressure-receiving end  96 , a reduced diameter shaft portion  98  and an enlarged piston head  100 . The piston member  94  is moveable with respect to the sleeve  92  between a retracted position ( FIG. 5A ) and an extended position ( FIG. 5B ). When moved to the extended position, the enlarged piston head  100  displaces a volume of fluid through the fluid outlet of end sub  82  and substantially the same volume of fluid is drawn into the fluid inlet of end sub  84  from the actuator  12 . The enlarged piston head  100  of the piston member  94  contacts an end portion  102  of compression spring member  104 , which is disposed within the chamber  88 . The spring  104  biases the piston member  94  toward the retracted position. Although the spring  104  illustrated in the drawings is a spiral-type spring, those of skill in the art will recognize that other compressible spring designs could just as easily be used, including, for example, stacks of Belleville washers or fluid springs, as are known in the art. When fluid pressure is increased within the hydraulic fluid conduit  46 , it bears upon pressure-receiving end portion  96  to urge the piston member  94  to move axially with respect to the sleeve  92  toward the extended position, and the spring member  104  is compressed by the piston head  100  (see  FIG. 6A ). It is noted that, while the pressure-receiving end  96  of the piston member  94  may be disposed within the surrounding sleeve  92  with a relatively close fit, there are no elastomeric or other fluid-tight seals located between the piston member  94  and sleeve  92 . As a result, it is contemplated that some fluid pressure will seep between the piston member  94  and sleeve  92  during operation. 
         [0025]    Returning to  FIGS. 1-4 , the general operation of the overall tool system  10  using the metering device  32  will now be described. The tool system  10  is run into a wellbore (not shown) with the sliding sleeve valve  14  in the closed position depicted in  FIG. 1 . During run in, the metering device  32  is in the initial, unpressurized condition depicted in  FIGS. 5A-5B . When it is desired to move the sleeve valve  14  to a partially open position, fluid pressure is decreased in the hydraulic control line  42  relative to the pressure present in the hydraulic line  18 . This pressure differential will cause the zero position free piston  64  to move from its initial position in contact with the lower end cap  56  to the pressurized position shown in  FIG. 6A . In the pressurized position, the free piston  64  is in contact with or proximate to the upper end cap  54 . This movement of the free piston  64  will cause the actuator  12  to move the sleeve  22  axially downwardly within its housing  20  so that the ports  30  of the sleeve  22  are moved to a point (as shown in  FIG. 2 ) wherein they are close to overlapping the ports  26  of the housing  20 . This position is referred to as the “zero position.” In a currently preferred embodiment, the movement of the free piston  64  will cause the sleeve  22  to displace 10.604″ with respect to the housing  20 . 
         [0026]    The first and second piston metering assemblies  36 ,  38  are interconnected in hydraulic parallel. Therefore, the pressure differential across the metering device  32  will also cause the incremental piston pump  90  to move from the initial position shown in  FIG. 5A  to the pressurized position depicted in  FIG. 6A , thereby displacing an additional volume of fluid from the actuator  12 . The sleeve  22  will then be displaced an additional amount with respect to the housing  20  such that the ports  30  of the sleeve  22  now slightly overlap the ports  26  of the housing  20  and permit a small amount of fluid to pass through the sleeve valve  14 . Thus, the sleeve valve  14  will be partially open. It is noted that when the incremental piston pump  90  is in the pressurized position, the enlarged pressure-receiving end  96  of the piston member  94  will engage a restriction  106  in the shaft  108  passing through the body of the sleeve  92 , thereby limiting the movement of the piston member  94  with respect to the surrounding sleeve  92 . When the piston member  94  has been displaced in this manner, the spring  104  is compressed, as shown in  FIG. 6A . 
         [0027]    If it is desired to open the sleeve valve  14  further to allow greater fluid flow, this is accomplished by first reducing the fluid pressure differential across the metering device  32  and then increasing it. As the pressure differential is reduced, the spring  104  of the incremental piston assembly  38  will urge the piston member  94  back to its initial, unpressurized position, as depicted in  FIG. 5A . Because there is no elastomeric seal or other fluid tight sealing between the enlarged end  96  of the piston member  94  and the surrounding sleeve  92 , fluid can seep between the piston member  94  and the sleeve  92  and equalize the pressure, thereby allowing the spring  104  to return the piston member  94  to its original position. The free piston  64  of the zero position piston metering assembly  36  will remain in its pressurized position, as shown in  FIGS. 6A-6B . 
         [0028]    At this point, the pressure differential across the metering device  32  is increased to cause the incremental piston pump  90  to be actuated again so that the piston member  94  is moved to the extended position shown in  FIG. 6A . This actuation meters an additional amount of fluid from the actuator  12  moves the sleeve  22  of the sleeve valve  14  an additional incremental amount toward the fully open position shown in  FIG. 4 . Those of skill in the art will recognize that the pressure differential across the metering device  32  may be repeatedly increased and decreased in order to move the sleeve  22  in a stepped manner to the fully opened position shown in  FIG. 4 . 
         [0029]    To return the sliding sleeve valve  14  to its fully closed position, hydraulic fluid is pumped into the fluid conduit  42  to create a reverse pressure differential across the metering device  32 . The zero position free piston  64  will be moved by the increased fluid pressure to the position shown in  FIG. 5A . Hydraulic fluid entering the zero position piston metering assembly  36  will also urge the valve ball member  70  of the check valve  66  off the valve seat  72  and allow fluid to pass through the free piston and enter the fluid passage  60  of the end sub  56  and to the actuator  12 . This fluid will cause the actuator to return the sleeve valve  14  to the fully closed position depicted in  FIG. 1 . The sliding sleeve valve  14  may be moved to the fully closed position in this manner at any time and regardless of the configuration that the sleeve valve  14  is in (i.e., zero position, partially open, fully open). 
         [0030]    In the embodiment described, the metering device  32  is operably associated with the fluid outlet, or “close” line  18  of the actuator  12 . However, it would also be possible to operate the well tool actuator by installing the metering device at the fluid inlet, or “open” line  16  of the actuator  12 , thereby metering fluid into the actuator  12  from the metering device  32 . It should be understood that, whether interconnected on the inlet or outlet side of the actuator  12 , the metering device  32  operates the well tool  14  in a stepped manner by metering known amounts of fluid through the metering device  32 . 
         [0031]    A metering device constructed in accordance with the present invention is simple in construction and reliable in operation. Additionally, there are few elastomeric elements, such as O-ring seals needed for operation of the metering device, thereby making the device more resistant to wear-related problems or problems associated with high-temperature downhole environments. 
         [0032]    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:
Methods and systems for operating a sliding sleeve valve or other downhole well tool that is axially shiftable among a finite number of increments between two extreme configurations such as open and closed configurations. A metering device is described having a pair of piston metering assemblies that operate in parallel fluid flow paths. The first piston metering assembly moves the sleeve of the well tool from a fully closed position to the zero position. The second piston metering assembly can be repeatedly pressurized and depressurized to meter predetermined amounts of fluid from an actuator sequentially to move the sleeve of the sleeve valve in consecutive increments toward a fully open position.