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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 11/468,631, filed Aug. 30, 2006, which is continuation-in-part of U.S. patent application Ser. No. 11/263,753, filed Oct. 31, 2005, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present invention generally relate to controlling the flow of fluids and gases in a wellbore. More particularly, the present invention relates to a valve for selectively closing a flow path in a single direction. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, a completion string may be positioned in a well to produce fluids from one or more formation zones. Completion devices may include casing, tubing, packers, valves, pumps, sand control equipment, and other equipment to control the production of hydrocarbons. During production, fluid flows from a reservoir through perforations and casing openings into the wellbore and up a production tubing to the surface. The reservoir may be at a sufficiently high pressure such that natural flow may occur despite the presence of opposing pressure from the fluid column present in the production tubing. However, over the life of a reservoir, pressure declines may be experienced as the reservoir becomes depleted. When the pressure of the reservoir is insufficient for natural flow, artificial lift systems may be used to enhance production. Various artificial lift mechanisms may include pumps, gas lift mechanisms, and other mechanisms. One type of pump is the electrical submersible pump (ESP). 
         [0006]    An ESP normally has a centrifugal pump with a large number of stages of impellers and diffusers. The pump is driven by a downhole motor, which is typically a large three-phase AC motor. A seal section separates the motor from the pump for equalizing internal pressure of lubricant within the motor to that of the well bore. Often, additional components may be included, such as a gas separator, a sand separator, and a pressure and temperature measuring module. Large ESP assemblies may exceed 100 feet in length. 
         [0007]    The ESP is typically installed by securing it to a string of production tubing and lowering the ESP assembly into the well. The string of production tubing may be made up of sections of pipe, each being about 30 feet in length. 
         [0008]    If the ESP fails, the ESP may need to be removed from the wellbore for repair at the surface. Such repair may take an extended amount of time, e.g., days or weeks. Typically, a conventional check valve is positioned below the ESP to control the flow of fluid in the wellbore while the ESP is being repaired. The check valve generally includes a seat and a ball, whereby the ball moves off the seat when the valve is open to allow formation fluid to move toward the surface of the wellbore and the ball contacts and creates a seal with the seat when the valve is closed to restrict the flow of formation fluid in the wellbore. 
         [0009]    Gas lift is another process used to artificially lift oil or water from wells where there is insufficient reservoir pressure to produce the well. The process involves injecting gas through the tubing-casing annulus. Injected gas aerates the fluid to make it less dense; the formation pressure is then able to lift the oil column and forces the fluid out of the wellbore. Gas may be injected continuously or intermittently, depending on the producing characteristics of the well and the arrangement of the gas-lift equipment. 
         [0010]    The amount of gas to be injected to maximize oil production varies based on well conditions and geometries. Too much or too little injected gas will result in less than maximum production. Generally, the optimal amount of injected gas is determined by well tests, where the rate of injection is varied and liquid production (oil and perhaps water) is measured. 
         [0011]    Although the gas is recovered from the oil at a later separation stage, the process requires energy to drive a compressor in order to raise the pressure of the gas to a level where it can be re-injected. 
         [0012]    The gas-lift mandrel is a device installed in the tubing string of a gas-lift well onto which or into which a gas-lift valve is fitted. There are two common types of mandrel. In the conventional gas-lift mandrel, the gas-lift valve is installed as the tubing is placed in the well. Thus, to replace or repair the valve, the tubing string must be pulled. In the “sidepocket” mandrel, however, the valve is installed and removed by wireline while the mandrel is still in the well, eliminating the need to pull the tubing to repair or replace the valve. 
         [0013]    Like other valves discussed herein, gas lift valves are typically “one way” valves and rely on a check valve to prevent gas from traveling back into the annulus once it is injected into a tubing string. 
         [0014]    Although the conventional check valve is capable of preventing the flow of fluid in a single direction, there are several problems in using the conventional check valve in this type of arrangement. First, the seat of the check valve has a smaller inner diameter than the bore of the production tubing, thereby restricting the flow of fluid through the production tubing. Second, the ball of the check valve is always in the flow path of the formation fluid exiting the wellbore which results in the erosion of the ball. This erosion may affect the ability of the ball to interact with the seat to close the valve and restrict the flow of fluid in the wellbore. 
         [0015]    Therefore, a need exists in the art for an improved apparatus and method for controlling the flow of fluid and gas in a wellbore. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention generally relates to controlling the flow of fluids and gases in a wellbore. In one aspect, a valve for selectively closing a flow path in a first direction is provided. The valve includes a body and a piston surface formable across the flow path in the first direction. The piston surface is formed at an end of a shiftable member annularly disposed in the body. The valve further includes a flapper member, the flapper member closable to seal the flow path when the shiftable member moves from a first position to a second position due to fluid flow acting on the piston surface. 
         [0017]    In another aspect, a valve for selectively closing a flow path through a wellbore in a single direction is provided. The valve includes a housing and a variable piston surface area formable across the flow path in the single direction. The valve also includes a flow tube axially movable within the housing between a first and a second position, wherein the variable piston surface is operatively attached to the flow tube. Further, the valve includes a flapper for closing the flow path through the valve upon movement of the flow tube to the second position. 
         [0018]    In yet another aspect, a method for selectively closing a flow path through a wellbore in a first direction is provided. The method includes positioning a valve in the wellbore, wherein the valve has a body, a formable piston surface at an end of a shiftable member, and a flapper member. The method further includes reducing the flow in the first direction, thereby forming the piston surface. Further, the method includes commencing a flow in a second direction against the piston surface to move the shiftable member away from a position adjacent the flapper member. Additionally, the method includes closing the flapper member to seal the flow path through the wellbore. 
         [0019]    In another embodiment, a valve embodying aspects of the invention is used in a gas lift arrangement to prevent the back flow of oil or gas injected into a tubing string from an annular area while reducing any obstruction of flow through the gas lift apparatus. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0020]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0021]      FIG. 1  is a view illustrating a control valve disposed in a wellbore. 
           [0022]      FIG. 2  is a view illustrating the valve in an open position. 
           [0023]      FIG. 3  is a view illustrating the piston surface formed in a bore of the valve. 
           [0024]      FIG. 4  is a view taken along line  4 - 4  of  FIG. 3  to illustrate the piston surface. 
           [0025]      FIG. 5  is a view illustrating the valve in the closed position. 
           [0026]      FIG. 6  is a view illustrating a sidepocket mandrel assembly for use in a gas lift well. 
           [0027]      FIG. 7  is a view taken along line  7 - 7  of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION  
       [0028]      FIG. 1  is a view illustrating a control valve  100  disposed in a wellbore  10 . As shown, the control valve  100  is in a lower completion assembly disposed in a string of tubulars  30  inside a casing  25 . An electrical submersible pump  15  may be disposed above the control valve  100  in an upper completion assembly. As illustrated, a polished bore receptacle and seal assembly  40  may be used to interconnect the electrical submersible pump  15  to the valve  100  and a packer arrangement  45  may be used to seal an annulus formed between the valve  100  and the casing  25 . Generally, the valve  100  is used to isolate the lower completion assembly from the upper completion assembly when a mechanism in the upper completion assembly, such as the pump  15 , requires modification or removal from the wellbore  10 . 
         [0029]    The electrical submersible pump  15  serves as an artificial lift mechanism, driving production fluids from the bottom of the wellbore  10  through production tubing  35  to the surface. Although embodiments of the invention are described with reference to an electrical submersible pump, other embodiments contemplate the use of other types of artificial lift mechanisms commonly known by persons of ordinary skill in the art. Further, the valve  100  may be used in conjunction with other types of downhole tools without departing from principles of the present invention. 
         [0030]      FIG. 2  is a view of the valve  100  in an open position. The valve  100  includes a top sub  170  and a bottom sub  175 . The top  170  and bottom  175  subs are configured to be threadedly connected in series with the other downhole tubing. The valve  100  further includes a housing  105  disposed intermediate the top  170  and bottom  175  subs. The housing  105  defines a tubular body that serves as a housing for the valve  100 . Additionally, the valve  100  includes a bore  110  to allow fluid, such as hydrocarbons, to flow through the valve  100  during a production operation. 
         [0031]    The valve  100  includes a piston surface  125  that is formable in the bore  110  of the valve  100 . The piston surface  125  shown in  FIG. 2  is in an unformed state. The piston surface  125  is maintained in the unformed state by a fluid force acting on the piston surface  125  created by fluid flow through the bore  110  of the valve  100  in the direction indicated by arrow  115 . The piston surface  125  generally includes three individual members  120 . Each member  120  has an end that is rotationally attached to a flow tube  155  by a pin  195  and each member  120  is biased rotationally inward toward the center of the valve  100 . Additionally, each member  120  is made from a material that is capable of withstanding the downhole environment, such as a metallic material or a composite material. Optionally, the members  120  may be coated with an abrasion resistant material. 
         [0032]    As illustrated in  FIG. 2 , the valve  100  also may include a biasing member  130 . In one embodiment, the biasing member  130  defines a spring. The biasing member  130  resides in a chamber  160  defined between the flow tube  155  and the housing  105 . A lower end of the biasing member  130  abuts a spring spacer  165 . An upper end of the biasing member  130  abuts a shoulder  180  formed on the flow tube  155 . The biasing member  130  operates in compression to bias the flow tube  155  in a first position. Movement of the flow tube  155  from the first position to a second position compresses the biasing member  130  against the spring spacer  165 . 
         [0033]    The valve  100  further includes a flapper member  150  configured to seal the bore  110  of the valve  100 . The flapper member  150  is rotationally attached by a pin  190  to a portion of the housing  105 . The flapper member  150  pivots between an open position and a closed position in response to movement of the flow tube  155 . In the open position, a fluid pathway is created through the bore  110 , thereby allowing the flow of fluid through the valve  100 . Conversely, in the closed position, the flapper member  150  blocks the fluid pathway through the bore  110 , thereby preventing the flow of fluid through the valve  100 . 
         [0034]    As shown in  FIG. 2 , a upper portion of the flow tube  155  is disposed adjacent the flapper member  150 . The flow tube  155  is movable longitudinally along the bore  110  of the valve  100  in response to a force on the piston surface  125 . Axial movement of the flow tube  155 , in turn, causes the flapper member  150  to pivot between its open and closed positions. In the open position, the flow tube  155  blocks the movement of the flapper member  150 , thereby causing the flapper member  150  to be maintained in the open position. In the closed position, the flow tube  155  allows the flapper  150  to rotate on the pin  190  and move to the closed position. It should also be noted that the flow tube  155  substantially eliminates the potential of contaminants from interfering with the critical workings of the valve  100 . 
         [0035]      FIG. 3  illustrates the piston surface  125  formed in the bore of the valve  100 . To seal the bore  110 , the flow of fluid through the bore  110  of the valve  100  in the direction indicated by the arrow  115  is reduced. As the flow of fluid is reduced, the fluid force holding the piston surface  125  in the unformed state becomes less than the biasing force on the piston surface  125 . At that point, each member  120  of the piston surface  125  rotates around the pin  195  toward the center of the valve  100  to form the piston surface  125  illustrated in  FIG. 4 . After the piston surface  125  is formed, the flow of fluid in the direction indicated by arrow  145  is commenced, thereby creating a force on the piston surface  125 . As the force on the piston surface  125  increases, the force eventually becomes stronger than the force created by the biasing member  130 . At that point, the force on the piston surface  125  urges the flow tube  155  longitudinally along the bore  110  of the valve  100 . 
         [0036]      FIG. 5  is a view illustrating the valve  100  in the closed position. After the piston surface  125  is formed, the flow tube  155  moves axially in the valve  100 . This moves the upper end of the flow tube  155  out of its position adjacent the flapper member  150 . This, in turn, allows the flapper member  150  to pivot into its closed position. In this position, the bore  110  of the valve  100  is sealed, thereby preventing fluid communication through the valve  100 . More specifically, flow tube  155  in the closed position no longer blocks the movement of the flapper member  150 , thereby allowing the flapper member  150  to pivot from the open position to the closed position and seal the bore  110  of the valve  100 . 
         [0037]    The flapper member  150  in the closed position closes the flow of fluid through the bore  110  of the valve  100 , therefore no fluid force in the bore  110  acts on the members  120 . To move the flapper member  150  back to the open position, the flow of fluid in the direction indicated by arrow  145  is reduced and the fluid on top of the flapper member  150  is pumped or sucked off the top of the flapper member  150 . At a predetermined point, the biasing member biasing the flapper member  150  is overcome and subsequently the biasing member  130  extends axially to urge the flow tube  155  longitudinally along the bore  110  until a portion of the flow tube  155  is adjacent the flapper member  150 . In this manner, the flapper member  150  is back to the open position, thereby opening the bore  110  of the valve  100  to flow of fluid therethrough, as illustrated in  FIG. 2 . 
         [0038]    In one embodiment, the valve  100  may be locked in the open position as shown in  FIG. 2  by disposing a tube (not shown) in the bore  110  of valve  100 . The tube is configured to prevent the axial movement of flow tube  155  from the first position to the second position by preventing the formation of the piston surface  125 . Thus, the flapper member  150  will remain in the open position and the valve  100  will be locked in the open position. To lock the valve  100 , the tube is typically pulled into the bore  110  from a position below the valve  100 . In a similar manner, the valve  100  may be unlocked by removing the tube from the bore  110  of the valve  100 . 
         [0039]    In another embodiment, the valve may be used in a gas lift application to prevent the back flow of gas (or production fluid) as gas is injected into a string or strings of production tubing. In one example, gas lift valves are disposed at various locations along the length of an annulus formed between production tubing and well casing. Gas lift valves are well known in the art and are described in U.S. Pat. No. 6,932,581, which is incorporated by reference in its entirety herein. Pressurized gas is introduced into the annulus from the well surface and when some predetermined pressure differential exists between the annulus and the tubing at a certain location, that valve opens and the gas is injected into the tubing string to lighten the oil and facilitate its rise to the surface of the well. The control valve of the invention is used in conjunction with the gas lift valves to prevent a backflow of gas or fluid from the production tubing to the annulus. Typically, the control valve is located adjacent the gas lift valve in the annulus. The valve permits gas to flow into the gas lift valve when it is open. However, when the gas lift valve closes, the control valve, with its closing members restricts the flow of gas or fluid back toward the annulus. 
         [0040]    In gas lift applications, control valves according to the invention may be fixed in a sidepocket mandrel. A conventional sidepocket mandrel has a pocket bore size of about 1.750 inches and the control valve dimensions are designed accordingly. Employing control valves according to the invention permits fluid path dimensions to be maximized. Thanks to the flapper sealing member, no flow restriction or significant pressure drop occurs across the valve, and a more efficient operation of the pump is possible. Moreover, control valves according to the invention prove more reliable because they do not present any erosion related problems like conventional check valves. 
         [0041]    As illustrated in  FIG. 6 , in order to allow a larger amount of gas flowing into the tubing and optimizing the fluid flow path, a sidepocket mandrel  200  may be provided with two lateral bores  210  flowing into a main bore  220  which is connected in correspondence of its lower portion to the inside of the tubing string through a slot (not shown). The lateral bores  210  communicate with the main bore  220  through a drilled portion  230  which crosses the entire cross section of the main bore  220  and projects with its ends respectively into both the lateral bores  210 . Each of the two lateral bores  210  in the sidepocket mandrel is provided with a seat  211  a control valve  100  (not shown) can be threadably connected thereto, whereas the main bore  220  is provided with a conventional gas lift valve (not shown).  FIG. 7  illustrates a cross section of the sidepocket mandrel assembly in correspondence of the drilled portion  230 . 
         [0042]    A sidepocket mandrel as shown in  FIGS. 6-7  is fixed to a tubing string located inside a wellbore and provided with control valves according to the invention in the respective seats  211 . Pressurizing gas in the annulus between the tubing string and the wellbore and opening the gas lift valve at the same time, initiate gas flowing through the mandrel  200  into the tubing so that the control valves  100  are driven in an open condition, wherein the gas is permitted to flow through the mandrel  200  and exercise the necessary pressure to keep the control valves opened. Two different streams of gas are created respectively inside each lateral bore  210  which finally commingle inside the main bore  220 . The gas then flows downwards inside the main bore  220  and finally enters the tubing string. The total amount of gas flowing through the mandrel  200  is directly dependent on the gas lift valve and, because in the opened condition the control valves do not cause any flow restriction, an optimization of the gas flow is obtained. Once the gas flow is either reduced or stopped the control valves close so as to prevent a backflow of gas or fluid from the production tubing to the annulus. The operation of the control valves according to the invention applied in gas lift applications is the same one as previously described in relation with  FIGS. 2 to 5 . 
         [0043]    Although a sidepocket mandrel with two lateral bores has been described hereinabove, it is apparent that with regard to the object of the invention the same considerations here apply for a sidepocket mandrel including only one lateral bore. 
         [0044]    Although the invention has been described in part by making detailed reference to specific embodiments, such detail is intended to be and will be understood to be instructional rather than restrictive. For instance, the valve may be used in an injection well for controlling the flow of fluid therein. It should be also noted that while embodiments of the invention disclosed herein are described in connection with a valve, the embodiments described herein may be used with any well completion equipment, such as a packer, a sliding sleeve, a landing nipple, and the like. 
         [0045]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
The present invention generally relates to controlling the flow of fluids in a wellbore. In one aspect, a valve for selectively closing a flow path through a wellbore in a first direction is provided. The valve includes a body and a piston surface formable across the flow path in the first direction. The piston surface is formed at an end of a shiftable member annularly disposed in the body. The valve further includes a flapper member, the flapper member closable to seal the flow path when the shiftable member moves from a first position to a second position due to fluid flow acting on the piston surface. In another aspect, a valve for selectively closing a flow path through a wellbore in a single direction is provided. In yet another aspect, a method for selectively closing a flow path through a wellbore in a first direction is provided.