Patent Publication Number: US-11391130-B2

Title: Gas-lift system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 16/884,763, which was filed on May 27, 2020 and claims priority to U.S. Provisional Patent Application No. 62/950,526, which was filed on Dec. 19, 2019. This application also claims priority to U.S. Provisional Patent Application No. 63/084,608, which was filed on Sep. 29, 2020, and to U.S. Provisional Patent Application 63/050,192 filed on Jul. 10, 2020. Each of these priority applications is incorporated by reference in its entirety here. 
    
    
     BACKGROUND 
     In oil and gas wells, hydrostatic pressure of fluid in the well may be too high to allow for unassisted production of fluids from within the formation. Gas lift, sometimes referred to as “artificial lift,” may thus be employed to alleviate the hydrostatic pressure above the lower area of the well and thereby allow hydrocarbons to be recovered therefrom. 
     To this end, a production tubing with a gas-lift valve placed proximal to the bottom of the production tubing may be deployed into the well. The valve may be open, and fluid may initially fill the annulus between the well and the production tubing, as well as the inside of the production tubing. Gas may then be supplied into the annulus at pressure, which may drive the gas-liquid interface in the annulus downward, below the level of the gas-lift valve. The gas may then flow into the production tubing through the open gas-lift valve and may partially fill the production tubing. This may reduce the hydrostatic pressure at the bottom of the production tubing, thereby allowing the pressure of the fluid in the reservoir to draw the hydrocarbons through the production tubing and to the surface. 
     In some cases, multiple gas-lift valves may be used at different positions along the length of the production tubing. The function may be similar to the single-valve system discussed above. The gas-lift valves may initially all be open, e.g., as the hydrostatic pressure provided by the column of fluid in the annulus may be above a closing pressure of the gas-lift valves. Gas may be injected into the annulus, pushing the column of fluid downward until the shallowest valve is in communication with the gas. The gas may then proceed through the shallowest valve, as explained above. Gas may, however, continue to be injected, further driving the gas-liquid interface downward in the annulus, until the gas reaches the next-shallowest valve. When this occurs, the gas may begin flowing into the production tubing via the second valve. Further, the gas pressure in the annulus at the shallowest valve may drop below the closing pressure of the first valve, resulting in the first valve shutting. This process may repeat for each subjacent valve. 
     However, gas-lift valve systems generally use the injection pressure in the annulus to actuate the valves. This can potentially limit the number of valves that can be used while still staying within practical injection pressure constraints. 
     SUMMARY 
     Embodiments of the disclosure include a gas-lift system including a first valve configured to be coupled to a production tubing. The first valve is configured to provide selective communication of a wellbore fluid between an interior of the production tubing and an annulus defined exterior to the production tubing. The system also includes a second valve configured to be coupled to the production tubing at a position that is subjacent to the first valve. The second valve is configured to provide selective communication of the wellbore fluid between the interior of the production tubing and the annulus. The system also includes a control line coupled to the first valve and the second valve. The control line is configured to apply a control line pressure to the first and second valves, the control line pressure applied by the control line is independent of an annulus pressure in the annulus and a production tubing pressure in the production tubing. The first valve is configured to actuate from an open position to a closed position, or from the closed position to the open position, at least partially in response to the control line pressure, and the second valve is configured to actuate from an open position to a closed position, or from a closed position to an open position, at least partially in response to the control line pressure. 
     Embodiments of the disclosure also include a method for operating a gas-lift system including injecting a gas into an annulus between a production tubing and a well. The gas flows from the annulus into the production tubing through a first valve that is open. The method includes closing the first valve by controlling a pressure in a control line that is coupled to the first valve, without causing or permitting a second valve that is subjacent to the first valve to close. The pressure in the control line is independent of a pressure of the gas in the annulus. The method includes increasing the pressure of the gas in the annulus after closing the first valve, such that the gas flows through the second valve and into the production tubing, and closing the second valve by controlling the pressure in the control line, which is also coupled to the second valve, independently of the pressure of the gas in the annulus and independently of a pressure in the production tubing, while maintaining the first valve in a closed position. The method includes retrieving the first valve from within the well without removing the production tubing from the well. 
     Embodiments of the disclosure further include a gas-lift system including a production tubing extending into a wellbore. An annulus is defined radially between the production tubing and the wellbore. The system also includes a plurality of side-pocket mandrels coupled to the production tubing, each of the side-pocket mandrels defining a primary bore in communication with the production tubing, and a pocket that extends radially outward from an angular interval of the primary bore, a plurality of gas-lift valves configured to selectively communicate the annulus with an interior of the production tubing, each of the plurality of gas-lift valves being received into the pocket of a respective one of the side-pocket mandrels, a surface system comprising a pump configured to pump a hydraulic fluid, and a control line extending from the surface system to the plurality of gas-lift valves, the control line being configured to deliver the hydraulic fluid from the pump to the plurality of gas-lift valves to control opening and closing of the gas-lift valves independently of a pressure in the annulus and independently of a pressure in the production tubing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may best be understood by referring to the following description and accompanying drawings that are used to illustrate some embodiments. In the drawings: 
         FIG. 1  illustrates a side, schematic view of a dual-line gas-lift system in a well, according to an embodiment. 
         FIG. 2  illustrates a side, schematic view of a single-line gas-lift system in the well, according to another embodiment. 
         FIG. 3  illustrates a side, cross-sectional view of a gas lift valve of the gas-lift system in a side-pocket mandrel, according to an embodiment. 
         FIG. 4  illustrates a cross-sectional view of the gas lift valve in the side-pocket mandrel, taken along line  4 - 4  in  FIG. 3 , according to an embodiment. 
         FIG. 5  illustrates a partial, side, cross-sectional view of the valve of the gas lift system, according to an embodiment. 
         FIG. 6  illustrates a partial, side, cross-sectional view of another embodiment of the valve of the gas-lift system. 
         FIG. 7  illustrates a side, cross-sectional view of another embodiment of the valve of the gas-lift system. 
         FIG. 8  illustrates a flowchart of a method for operating a gas-lift system, according to an embodiment. 
         FIG. 9  illustrates a side, cross-sectional view of a valve in a closed position, for use in the single control-line embodiment of the gas-lift system, according to an embodiment. 
         FIG. 10  illustrates a side, cross-sectional view of the valve of  FIG. 9  in an open position, according to an embodiment. 
         FIG. 11  illustrates a side, cross-sectional view of another valve for use in a single control line gas-lift system, according to an embodiment. 
         FIG. 12  illustrates a flowchart of a method for operating a gas lift system, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure describes several embodiments for implementing different features, structures, or functions of the invention. Embodiments of components, arrangements, and configurations are described below to simplify the present disclosure; however, these embodiments are provided merely as examples and are not intended to limit the scope of the invention. Additionally, the present disclosure may repeat reference characters (e.g., numerals) and/or letters in the various embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed in the Figures. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Finally, the embodiments presented below may be combined in any combination of ways, e.g., any element from one exemplary embodiment may be used in any other exemplary embodiment, without departing from the scope of the disclosure. 
     Additionally, certain terms are used throughout the following description and claims to refer to particular components. As one skilled in the art will appreciate, various entities may refer to the same component by different names, and as such, the naming convention for the elements described herein is not intended to limit the scope of the invention, unless otherwise specifically defined herein. Further, the naming convention used herein is not intended to distinguish between components that differ in name but not function. Additionally, in the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” All numerical values in this disclosure may be exact or approximate values unless otherwise specifically stated. Accordingly, various embodiments of the disclosure may deviate from the numbers, values, and ranges disclosed herein without departing from the intended scope. In addition, unless otherwise provided herein, “or” statements are intended to be non-exclusive; for example, the statement “A or B” should be considered to mean “A, B, or both A and B.” 
       FIG. 1  illustrates a side, schematic view of a gas-lift system  100 , according to an embodiment. The gas-lift system  100  may be configured to reduce a hydrostatic pressure in a production tubing  102  that is deployed into a well  103 . Thus, the gas-lift system  100  may be configured to aid in the production of reservoir fluid (e.g., hydrocarbons) from the well  103 , at the lower extent of the production tubing  102 , through the production tubing  102 , and up to the surface above the production tubing  102 . 
     The gas-lift system  100  may include a plurality of valves (by way of example, four are shown:  104 ,  106 ,  108 ,  110 , although any number may be employed), which may be positioned in an annulus  111  between the production tubing  102  and the well  103 . For example, the valve  104  may be the shallowest valve, the valve  106  may be subjacent to the valve  104 , the valve  108  may be subjacent to the valve  106 , and the valve  110  may be the deepest valve and subjacent to the valve  108 . In some embodiments, additional valves may be employed, e.g., between the valve  108  and the valve  110 . 
     First and second control lines  112 ,  116  may extend from a surface system (e.g., located at the ground-level) to the valves  104 ,  106 ,  108 ,  110  and may be connected thereto in parallel as shown. As used herein, the terms “connected” and “coupled” mean directly connected/coupled (i.e., without intervening components) or connected/coupled via one or more intermediate components. That is, both possibilities are contemplated by the use of either term. 
     The surface system may include a pressurized fluid source  114 , such as a pump, and a tank  118 . The tank  118  may include one or more devices configured to modulate a pressure of the fluid therein, e.g., a piston. In other embodiments, the tank  118  may simply hold the fluid, and such that hydrostatic pressure generated by the height of the control line  112 ,  116  coupled thereto acts on the valves  104 - 110 . In some embodiments, the surface system may also include one or more valves  150 ,  151 ,  152  that are configured to control which control line  112 ,  116  is connected to the pressurized fluid source  114  and the tank  118 . For example, in a first configuration, the pressurized fluid source  114  may be connected via the valves  150 ,  151  to the first control line  112 , while the tank  118  may be connected via the valve  152  to the second control line  116 . In a second configuration, the valves  150 - 152  may be modulated such that the tank  118  may be connected to the first control line  112  via the valve  151 , and the pressurized fluid source  114  may be connected to the second control line  116  via the valves  150 ,  152 . 
     Each of the valves  104 ,  106 ,  108 ,  110  may have a different actuation pressure differential. The actuation pressure differential may be the value for the pressure differential between the first and second control lines  112 ,  116  at which the valves  104 ,  106 ,  108 ,  110  actuate, either to close or open, as will be described in greater detail below. For example, the valve  104  may be configured to close in the presence of a first pressure differential value and remain closed at lower pressure differential values. The valve  106  may be configured to close in the presence of a second pressure differential value that is greater than the first pressure differential value and remain closed at lower pressure differential values. This pattern of increasing closing pressure differential values may continue for subjacent valves  106 ,  108 ,  110 . In other embodiments, the pressures may vary as between valves  104 - 110  in any suitable pattern. 
     When the valves  104 - 110  are open, the valves  104 - 110  permit fluid to flow from the annulus  111  into the production tubing  102 . Specifically, gas may be injected into the annulus  111 , which is otherwise full of liquid (or a combination of liquid and gas, i.e., a fluid). As the gas pressure in the annulus  111  (the “annulus” pressure) increases, the interface between the gas and liquid is driven downwards. When the valves  104 - 110  are closed, the valves  104 - 110  block fluid flow therethrough and into production tubing  102 . When the valves  104 - 110  are open, they allow gas flow into the production tubing  102 , generally at the depth where the top-most open valve  104 - 110  is positioned. 
     By using pressure in one or more control lines  112 ,  116  (the “control line” pressure(s)) to control actuation, rather than the annular pressure of the wellbore fluid that resides in the annulus  111  and is received into the production tubing  102  through any of the valves  104 - 110  that are open, the gas-lift system  100  may be able apply a greater range of pressures to actuate the valves  104 - 110 . For example, the range of actuation pressures supplied by the control line pressure of the control lines  112 ,  116  may be above pressures that, if experienced in the wellbore fluid in the annulus  111 , would damage the well  103  and/or are beyond the practical capabilities of wellbore pumping equipment that is commonly used for artificial lift. In some applications, gas injection into the annulus  111  may be generally performed at between about 20 psi to about 80 psi, and thus, if injection pressure is used to actuate the valves  104 - 110 , valve actuation pressures are also in this range. However, since a separate hydraulic pressure differential is employed in the present system  100 , the valve actuation pressures can be outside of this range, e.g., through pumping a generally incompressible, hydraulic fluid that can be raised to much higher pressures, if desired. Further, the hydrostatic pressure acting through one or both of the lines  112 ,  116  can be employed either to “balance” the pressure in the valve  104 - 110  or to assist in opening or closing the valve  104 - 110 , as will be described in greater detail below. It will be appreciated that one or more valves that are not actuated via the control lines  112 ,  116  may also be included in the system  100 . 
       FIG. 2  illustrates a side, schematic view of another gas-lift system  200 , according to an embodiment. The gas-lift system  200  of  FIG. 2  may be similar and used in a similar context as the gas-lift system  100 , and like elements are given like numbers between  FIGS. 1 and 2 . The second control line  116  is omitted from the gas-lift system  200 , and thus the gas-lift system  200  may be referred to as a “single control line” gas-lift system  100 . Further, the gas-lift system  200  includes valves  204 ,  206 ,  208 ,  210 , which may be positioned and configured to permit or block fluid from communicating from the annulus  111  to the production tubing  102 , similarly to valves  104 ,  106 ,  108 , and  110 . 
     More particularly, the first control line  112  is connected to the valves  204 - 210 , e.g., in parallel, is isolated from the annulus  111 , and, in this embodiment, is the only line employed to supply pressure to the valves  204 - 210  from the pressurized fluid source  114 . Accordingly, the tank  118  and valves  151 ,  152  may also be omitted, or could be included in some embodiments for pressure control, etc. in a suitable arrangement. The valves  204 - 210  may be configured to be opened or closed in response to the first control line  112  supplying above or below a certain pressure. For example, the valves  204 - 210  may be biased closed, and, when the pressure supplied by the first control line  112  reaches the actuation pressure, the pressure may force the valve open. Alternatively, the valves  204 - 210  may be biased open, and a pressure supplied by the first control line  112  may force the valves  104 - 110  closed, and may be lowered to allow the valves  104 - 110  to open. 
     The valves  104 - 110  of  FIG. 1  and the valves  204 - 210  of  FIG. 2  are illustrated schematically as fixed to the exterior of the production tubing  102 . This is merely one possibility, however. In another example, any one or more of the valves  104 - 110 ,  204 - 210  may be positioned in a “side-pocket” mandrel, and may thus be retrievable through the production tubing  102 , e.g., without removing the production tubing  102  from the well  103 , using wireline equipment deployed from the surface through the production tubing  102 . In some embodiments, one or more of the valves  104 - 110 ,  204 - 210  may be in such a side-pocket mandrel, and one or more of the other valves  104 - 110 ,  204 - 210  may be attached to the exterior of the production tubing  102 . 
       FIG. 3  illustrates a side, cross-sectional view of an embodiment of a valve  300  positioned in a side-pocket mandrel  302  of a gas-lift system, such as the gas-lift system  100  or  200  discussed above.  FIG. 4  illustrates an axial cross-sectional view of the valve  300  installed in the side-pocket mandrel  302 , according to an embodiment. The valve  300  may be representative of any one or more of the valves  104 - 110  and/or  204 - 210  or others. 
     The side-pocket mandrel  302  may connected to, fit on, be received around, or otherwise used in conjunction with the production tubing  102  (e.g.,  FIGS. 1 and 2 ). For example, a primary bore  304  may be formed through the side-pocket mandrel  302 , which may serve to permit fluid flow through the production tubing  102 , e.g., between either axial end of the primary bore  304 . The side-pocket mandrel  302  may also include a pocket  306 , e.g., formed by the wall of the side-pocket mandrel  302  extending radially outward along an angular interval of generally less than 180 degrees around the primary bore  304 . Accordingly, the side-pocket mandrel  302  may be non-axisymmetric at the pocket  306 . At least a portion of the pocket  306  may be open to communication with the primary bore  304 , allowing for access to the pocket  306 , e.g., from the surface via suitable installation or removing tools, e.g., wireline or slickline tools. 
     The valve  300  may be positioned at least partially in the pocket  306 . An axially-extending bore  310  may be formed through the side-pocket mandrel  302  on a lower end of the pocket  306 . The bore  310  may be configured to communicate with the annulus  111  via an opening  312 . The bore  310  may also be configured to communicate with the interior of the production tubing  102  via a radial port  314 . A lower portion of the valve  300  may be received into the bore  310  and secured therein. The valve  300  may include an adaptive feature configured to be engaged by a wireline retrieval tool for installing the valve  300  into and/or removing the valve  300  from the pocket  306 . In an embodiment, as shown, the adaptive feature may be an adapter  316  that is connected to (e.g., received at least partially in) the upper end of the valve  300 , but in other embodiments, could be formed integrally with the remainder of the valve  300 . 
     In this embodiment, there are two control lines  112 ,  116 , although, as discussed above, the gas-lift system  100  could be implemented with a single control line  112 . The control lines  112 ,  116  may extend axially through the mandrel  302  where the mandrel  302  defines the pocket  306 , e.g., through bores  320 ,  322  defined therein, as shown. In other embodiments, the control lines  112 ,  116  may be connected to the bores  320 ,  322 , but may not extend therethrough. The bores  320 ,  322  may be parallel and offset from one another, as shown, e.g., at about the same radial distance from the center of the production tubing  102 . In other embodiments, the bores  320 ,  322  may be formed elsewhere in the pocket  306 . Further, the control lines  112 ,  116  may extend radially through at least a portion of the pocket  306  via radial ports  324 ,  326 , respectively, so as to communicate with the bore  310 . For example, the first line  112  may communicate with the bore  310  at an axially lower position than the second line  116 , as shown, although this could be reversed. The control lines  112 ,  116  may extend within the annulus  111  above and below the pocket  306 . 
     As will be explained in greater detail below, the valve  300  positioned in the side-pocket mandrel  302  may be configured to be actuated (i.e., opened or closed) responsive to the pressure (or pressure differential) in the control lines  112 ,  116 , independently of pressure in the production tubing  102  and/or in the annulus  111  (e.g.,  FIG. 1 ). When opened, the valve  300  may permit fluid communication between the opening  312  and the radial port  314 , thereby allowing fluid (generally, gas injected into the annulus  111 ) into the production tubing  102  from the annulus  111 . When closed, the valve  300  may block such fluid flow therethrough. Thus, the valve  300  may provide a retrievable gas-lift valve that is controllable separately from pressure within the annulus  111 . 
       FIG. 5  illustrates a partial, side, cross-sectional view of a valve  500 , according to an embodiment. The valve  500  may be an example of one or more of the valves  104 - 110  and/or  300  used in the gas-lift system  100 . In some embodiments, an adapter, such as the adapter  316  of FIG.  3 , may be added to the valve  500  so that it may be installed into and retrievable from a side-pocket mandrel (e.g., side-pocket mandrel  302 ). The valve  500  may have a default open position, such that a pressure differential between the first control line  112  and the second control line  116  may be generated or otherwise used to close the valve  500 . 
     For example, the valve  500  may include a housing  502 , a seat  504 , a valve closure element  506 , an elongate rod  508 , and a piston  510 . The housing  502  may be a unitary structure, as shown, or may be made from two or more bodies that are connected (e.g., threaded) together. The housing  502  may define an open axial end or “opening”  512 , which may be in communication with the interior of the production tubing  102  (e.g., providing the orifice  202  of  FIG. 2 ). The housing  502  may also be open to the annulus  111  on its opposite axial end  513 . A primary port  514  may be defined through the housing  502 , which may communicate the surrounding environment within the annulus  111  with the interior of the housing  502 . 
     The seat  504  may be interposed between the port  514  and the open axial end  512 . For example, the seat  504  may be defined by or connected to the housing  502 . Further, the valve closure element (e.g., a dome-shaped or otherwise partially spherical member, a conical member, etc.)  506  may be engageable with the seat  504 , to selectively permit or block communication of fluid from the port  514  to the open axial end  512 . 
     The valve closure element  506  may be coupled to the rod  508 , which may be in turn coupled to the piston  510 . In some embodiments, these structures  506 - 510  may be formed as a single piece, but in other embodiments, may be made separately and connected together. Accordingly, the valve closure element  506  may be moved by movement of the piston  510 , with such movement being transmitted therebetween by the rod  508 . The piston  510  may be positioned within a chamber  520  defined in the housing  502 . For example, a first control port  524  and a second control port  526  may be defined through the sub  522 . The first control port  524  may be in fluid communication with the first control line  112 , and the second control port  526  may be in fluid communication with the second control line  116 . Accordingly, fluidic pressure within the control lines  112 ,  116  may be communicated into the chamber  520  via the first and second control ports  524 ,  526 , respectively. The chamber  520  may be sealed from the rest of the interior of the housing  502  via one or more seals  528 ,  529  between the piston  510  and the housing  502 , such that fluid in the control lines  112 ,  116  is maintained separate from the fluid in the annulus  111  that is received into the housing  502  via the ports  514 . 
     The piston  510  may include a radially-enlarged section  530 , which may include a seal  531  for sealing with an inner surface of the chamber  520 , while allowing movement of the piston  510  relative to the housing  502 , e.g., responsive to pressure differentials. The radially-enlarged section  530  may be proximal to a middle of the piston  510 , such that the piston  510  separates the chamber  520  between the first and second control ports  524 ,  526 . Accordingly, a higher pressure in the first control line  112 , communicated into the chamber  520  via the first control port  524 , in comparison to a lower pressure in the second control line  116 , communicated into the chamber  520  via the second control port  526 , may force the piston  510  downward, e.g., to the right, as shown. This may force the valve closure element  506  into engagement with the seat  504 , thereby closing the valve  500  (preventing fluid communication from the port  514  through the open axial end  512 ). Similarly, a higher pressure in the second control line  116  relative to the first control line  112  may force the piston  510  upward, e.g., to the left, as shown, raising the valve closure element  506  away from the seat  504 , opening the valve  500  and permitting fluid communication between the port  514  and the open axial end  512 . 
     In an embodiment, the valve  500  may be a spring-force valve. For example, the valve  500  may also include a biasing member  540 , such as a spring. In an embodiment, the biasing member  540  may be coiled around the rod  508 , as shown. The valve  500  may further include a nut  542 , which may be positioned in the housing  502 , e.g., threaded into position therein. As such, the nut  542  may be configured to retain its position in the housing  502 , despite axial loads from the biasing member  540 . The rod  508  may extend through the nut  542  and may be configured to slide relative thereto. The biasing member  540  may also bear against the piston  510 , e.g., via a connecting member  544  between the rod and the piston  510 . 
     The biasing member  540  may be configured to apply a biasing force that tends to hold the valve  500  open, e.g., with the valve closure element  506  held away from the seat  504 . The nut  542  may be positioned to vary the level of biasing force applied, and it will be appreciated that the biasing force may vary depending on the position of the piston  510 . Accordingly, when the pressure differential across the piston  510  generates a sufficient downward force, the biasing force of the biasing member  540  may be overcome, permitting the piston  510  to move downward, and thus forcing the valve closure element  506  into engagement with the seat  504 , so as to close the valve  500 . 
       FIG. 6  illustrates a partial, side, cross-sectional view of another embodiment of the valve  500 . In this embodiment, the valve  500  may default to being closed. In the illustrated example, the valve  500  may not include the rod  508 . Rather, the piston  510  may be directly connected to the valve closure element  506 . 
     Further, a force-transmission member  600  may engage an opposite side of the piston  510 . The biasing member  540  may bear upon the force-transmission member  600  and the nut  542 , such that the biasing member  540  is configured to press the force-transmission member  600  downward, to the right, as shown, thereby biasing the valve closure element  506  into engagement with the seat  504 . The force-transmission member  600  may also bear against an end of the piston  510 . The force-transmission member  600  may be slidable relative to the housing  502 . Accordingly, to open the valve  500 , the piston  510  is forced upwards by a pressure differential in the chamber  520 , e.g., by pressure in the first control line  112  exceeding pressure in the second control line  116  by a predetermined value, such that the pressure differential overcomes the biasing force generated by the biasing member  540 . 
       FIG. 7  illustrates a side, cross-sectional view of the valve  500 , according to another embodiment. In this embodiment, the valve  500  has a valve closure element  700  and includes a secondary port  702  that communicates with the production tubing  102 . The open axial end  512  may open to the annulus  111  instead of the production tubing  102 . 
     For example, the valve closure element  700  may be or include a piston, which may extend from and move with the piston  510  that is positioned in the chamber  520 . As with other embodiments, the piston  510  is moved by a pressure differential between the control lines  112 ,  116  as communicated into the chamber  520  via the control ports  524 ,  526 . The valve closure element  700  may slide within the housing  502 , such that, in a closed position (as illustrated) the valve closure element  700  blocks fluid flow into the secondary port  702 , e.g., from either/both of the open axial end  512  and/or the port  514 . Furthermore, the valve closure element  700  may include a pair of seals  704 ,  706 , which, in the closed position, are located on both axial sides of the secondary port  702 . In other embodiments, the seals  704 ,  706  may be positioned on both axial sides of the port  514 , or on both axial sides of both ports  514 ,  702 . The seals  704 ,  706  (or others) may form a seal between the housing  502  and the valve closure element  700 , so as to prevent fluid flow into the secondary port  702  when the valve closure element  700  is in the closed position. 
     In the illustrated embodiment, the valve  500  of  FIG. 7  may default to closed, similar to the valve  500  of  FIG. 6 , as the biasing force is applied by the biasing member  540  on the force-transmission member  600  presses the valve closure element  700  to the closed position (to the right in this view). For example, the biasing member  540  may press against the force-transmission member  600 , pressing the force-transmission member  600  onto a shoulder of the housing  502 , as shown, which prevents further movement of the force-transmission_member  600 . In other embodiments, the valve closure element  700  may instead be configured to default to the open position, similar to the valve  500  of  FIG. 5 , by configuring the biasing member  540  to bias the valve closure element  700  toward the open position (to the left in this view) rather than the closed position. 
     When the valve closure element  700  is moved to the open position, e.g., by the piston  510  being driven (to the left, as shown) by the pressure differential in the chamber  520 , the valve closure element  700  may permit fluid flow from either/both of the open axial end  512  and/or the port  514  to the secondary port  702 . For example, the valve closure element  700  may be moved uphole (to the left, in this illustration), such that the seals  704 ,  706  no longer block fluid flow into the port  702 . In some embodiments, fluid flow from the port  514  into the port  702  may be blocked even when the valve closure element  700  is moved to the open position, while fluid flow may be permitted to reach the port  702  via the open axial end  512 . 
     The valve  500  may be configured to balance the pressures applied thereto from sources other than the control lines  112 ,  116 . For example, pressure within the annulus  111  may be balanced across the internal components (including the valve closure element  700 ) between the two open axial ends  512 ,  513  and/or the port  514 . Further, because the valve closure element  700  slides across the port  514 , rather than moving directly against a valve seat in the flow path, the valve closure element  700  may not be forced to push against the pressure of the fluid in the production tubing  102 . Such pressure balancing (and/or avoidance) may enable operators to control actuation without consideration for (or at least mitigating the effects of) pressure in the annulus  111  and/or production tubing  102 . 
     In at least one embodiment, an upper end of the valve  500  of  FIGS. 5-7  may include a wireline adapter, such as the wireline adapter  316  of  FIGS. 3 and 4 . The adapter  316  may be connected to the housing  502  or any other suitable structure provided by the valve  500 , such that the adapter  316  is engageable by the wireline tool. Accordingly, the adapter  316  may permit retrieval of the valve  500  using a wireline tool deployed through the production tubing  102 , with the valve  500  being positioned in a pocket  306  of a side-pocket mandrel  302 , as discussed above with reference to  FIGS. 3 and 4 . 
       FIG. 8  illustrates a flowchart of a method  800  for operating a gas-lift system, e.g., the gas-lift system  100  and/or  200 , according to an embodiment. Although the method  800  is described with reference to the gas-lift system  100 , it will be appreciated that the method  800  may, in some embodiments, be executed using other structures. Moreover, the steps of the method  800  discussed herein may be performed in a different order than described, two or more steps may be combined into one, some of the steps may be separated into two or more steps each, steps may be done simultaneously, without departing from the scope of the present disclosure. 
     The method  800  may include injecting a gas into an annulus  111  between a production tubing  102  and a well  103 , as at  802 . The injected gas flows from the annulus  111  into the production tubing  102  through a first valve (e.g., valve  104 ) that is open. 
     The method  800  may also include closing the first valve  104  by controlling a pressure in a control line  112  (and/or  116 ) that is coupled to the first valve  104 , without causing or permitting a second valve  106  that is subjacent to the first valve  104  to close, as at  804 . In some embodiments, the pressure in the control line  112  is independent of a pressure of the gas in the annulus  111  and/or a pressure of the gas or liquid in the production tubing  102  (e.g., the first control line  112  does not rely on pressure in the annulus  111  or in the production tubing  102  to assist in actuating the first valve  104 ). In an embodiment, closing the first valve  104  includes increasing the pressure in the control line  112  such that a pressure differential generated at least partially by pressure in the control line  112  overcomes a biasing force configured to bias the first valve to an open position. In another embodiment, closing the first valve  104  includes reducing the pressure in the control line  112  such that a pressure differential generated at least partially by the pressure in the control line  112  does not overcome a biasing force configured to bias the first valve  104  to a closed position. 
     The method  800  may also include increasing the pressure of the gas in the annulus  111  after closing the first valve  104 , as at  806 . This may drive the gas-liquid interface in the annulus  111  in a downhole direction, such that the gas flows through the second valve  106  and into the production tubing. 
     The method  800  may further include closing the second valve  106  by controlling the pressure in the control line  112 , as at  808 . The control line  112  may be the same control line that is coupled to the second valve  106 , and the pressure in the control line  112  may be controlled independently of the pressure of the gas in the annulus  111 , while maintaining the first valve  104  in a closed position. Further, the control line  112  may control the pressure in the second valve  106  independently of the pressure within the production tubing  102  (e.g., the control line  112  does not rely on the pressure within the production tubing  102  or the annulus  111  to assist in actuating the second valve  106 ). It will be appreciated that controlling the pressure in the control line  112  “independently” of the pressure in the annulus  111  means that the pressure in the control line  112  could, for example, change while the pressure in the annulus  111  remains constant, or remain constant while the pressure in the annulus  111  remains the same, or change by a different amount than the pressure in the annulus changes  111 , etc. 
     In an embodiment, the method  800  may also include opening the first and second valves  104 ,  106  by controlling the pressure in the control line  112  such that a pressure differential generated at least partially by the pressure in the control line  112  causes or permits the first and second valves  104 ,  106  to open, as at  810 . 
     The method  800  may additionally include retrieving the first valve  104 , the second valve  106 , any other gas-lift valves, or a combination thereof, from within the well  103  without removing the production tubing  102  from the well, as at  812 . In an embodiment, the first valve  104  (embodied as the valve  300 ) is positioned in a pocket  306  of a side-pocket mandrel  302 , and retrieving the first valve  104  includes removing the first valve  104  from the pocket  306 . In an embodiment, retrieving the first valve  104  includes removing the first valve  104  from within a bore  310  defined in the side-pocket mandrel  302 . The bore  310  has an opening  312  that communicates with the annulus  111  and a radial port  314  that communicates with the production tubing  102  via a primary bore  304  of the side-pocket mandrel  302 . The control line  112  extends axially through the side-pocket mandrel  302 , where the mandrel  302  defines the pocket  306 . In an embodiment, retrieving the first valve  104  may include engaging an adapter  316  of the first valve  104  using a wireline tool. 
       FIG. 9  illustrates a side, schematic view of one of the valves  204 - 210 , e.g., of the gas-lift system  200  of  FIG. 2 , according to an embodiment. With continuing reference to  FIGS. 2 and 9 , for purposes of discussion, the valve is labeled as valve  204 , but it will be appreciated that the valve  204  may be representative of any or all of the other valves  204 - 210 . The valve  204  is illustrated in a closed position, in which the valve  204  blocks or otherwise prevents fluid flow from the annulus  111  to the interior of the production tubing  102  via the valve  204 . This may be the “normal” or “default” position of the valve  204 . In other embodiments, the valve  204  may default to an open position in which fluid communication between the annulus  111  and the interior of the production tubing  102  is permitted. Further, some gas-lift systems may employ both default-open and default-closed valves. 
     In the illustrated embodiment, the valve  204  may include a housing  900 , which may extend longitudinally, generally parallel to the production tubing  102 . The housing  900  may define an orifice  202 , which may communicate with the interior of the production tubing  102 . The orifice  902  may be formed or defined by a partially or entirely open axial end of the housing  900 . The housing  900  may also include an inlet opening  904 , which may be oriented laterally through part of the housing  900 , so as to allow fluid from the annulus  111  to enter the housing  900 . 
     Within the housing  900 , the valve  204  may include a valve element  910  and a valve seat  912 . The valve element  910  may engage the valve seat  912  and form a seal therewith, as shown, with the valve  204  is in a closed position. The valve element  910  and the valve seat  912  may thus serve to block or otherwise prevent fluid communication between the inlet opening  904  and the orifice  902 . This may prevent fluid flow from the annulus  111  to within the production tubing  102  via the valve  204 . In some embodiments, the valve element  910  may be a generally cylindrical stem with a spherical end that engages the valve seat  912 . 
     The valve  204  may also include a biasing member  920 . The biasing member  920  may be coupled to the valve element  910 . In the illustrated, default-closed embodiment, the biasing member  920  may press the valve element  910  toward the valve seat  912 , such that, in the absence of a sufficient opposing force, the valve element  910  may engage the valve seat  912  and close the valve  204 . In a default-open embodiment, the biasing member  920  may serve to apply a force that drives the valve element  910  away from the valve seat  912 . 
     In some embodiments, the biasing member  920  may be a bellows, such as a gas-charged (e.g., nitrogen-charged) bellows. Operators may charge the biasing member  920  to a certain pressure at the surface. The biasing member  920  may thus be pressured to allow the valve  204  to actuate (from closed to open, or open to closed, depending on the embodiment) in the presence of a pressure that exceeds a hydrostatic pressure at the position in the well at which the valve  204  may be positioned. Accordingly, different valves in a single gas-lift system  200  may have different pressures in the biasing member  920  in such a bellows embodiment. In other embodiments, other types of bellows may be used (e.g., any other suitable gas). In other embodiments, springs, Bellville washers, etc. may be used as the biasing member  920 , and configured to oppose actuation until a certain pressure, above the hydrostatic pressure, is applied. 
     The valve  204  may also include a seal  930 , which may seal with the valve element  910  and the housing  900 . The valve element  910  may be movable with respect to the seal  930 , e.g., able to slide therepast. The seal  930  may the positioned to form a sealed chamber  932  in the valve  104 . In an embodiment, the biasing member  920  may be positioned in the sealed chamber  932 , such that a pressure in the sealed chamber  932 , outside of the biasing member  920 , may act upon the biasing member  920 . The pressure may serve to compress the biasing member  220 . In a spring or other mechanical embodiment of the biasing member  220 , a piston, block, shoulder, etc. may be used to compress (or extend) the biasing member  220 . 
     The control line  112  may communicate with the sealed chamber  932 . Accordingly, the pressure in the control line  112  may be fed directly to the sealed chamber  932  to control the pressure within the sealed chamber  932 . The pressure in the control line  112  may thus be used to actuate the valve  204  from the closed positioned illustrated in  FIG. 9  to the open position illustrated in  FIG. 10 . 
     Referring to  FIG. 10  in greater detail, and still referring to  FIG. 2 , as shown, the valve element  910  has lifted away from engagement with the valve seat  912 , opening the valve  204 . As such, there is a fluid communication path opened from the inlet opening  904  to the orifice  902 , and thus from the annulus  111  to within the production tubing  102 . 
     As can be seen in  FIG. 10 , the biasing member  920  has compressed. This is caused by the pressure in the control line  112  increasing, which in turn increases the pressure in the sealed chamber  932 . As the pressure in the sealed chamber  932  increases, eventually it overcomes the biasing force applied by the biasing member  920  and compresses the biasing member  920  by an amount sufficient to disengage the valve element  910  from the valve seat  912 . 
       FIG. 11  illustrates a side, schematic view of the valve  204 , according to another embodiment. Again, the valve  204  is shown in the closed position, with the valve element  910  engaging the valve seat  912 . In this embodiment, the valve element  910  extends through the biasing member  920 . Further, the housing  900  may include a block  1100  below the biasing member  920  and a second seal  1102  above the biasing member  920  and within the housing  900 . Thus, the sealed chamber  932  may be formed above the biasing member  920 . The control line  112  may inject pressure into the sealed chamber  932 . The biasing member  920  may press against the block  1100  and a shoulder or other engaging feature of the valve element  910 , thereby biasing the valve element  210  upwards, away from the valve seat  912 . As such, this embodiment of the valve  204  is biased open. When pressure sufficient to compress the biasing member  920  is received via the control line  112 , the biasing member  920  compresses, allowing the valve element  910  to press into engagement with the valve seat  912 , closing the valve  204 . 
       FIG. 12  illustrates a flowchart of a method  1200  for operating a gas-lift system, according to an embodiment. In at least some embodiments, the method  1200  may be executed using an embodiment of the gas-lift system  100  and/or  200  discussed above, but in other embodiments, may use other structures, and thus should not be considered limited to any particular structure unless otherwise indicated herein. 
     In some embodiments, the method  1200  may include selecting first and second pressure values at which first and second valves (e.g., valves  204  and  206 ) actuate, based on a depth at which the first and second valves  204 ,  206  are to be deployed, respectively, as at  1201 . For example, the pressures may be selected to exceed the hydrostatic pressure at the depth, such that the valves  204 ,  206  are operable by a control line  112  that is independent of the annulus  111 . For example, the pressure may be selected to permit the valves  204 ,  206  to open upon reaching the first pressure value. Further, the pressure may be selected such that, for example, successively deeper-positioned valves  208 ,  210  actuate and successively high pressure. 
     In an embodiment, the method  1200  may also include tuning a biasing member  920  of the first valve  204 , the second valve  206 , or both, as at  1202 . The biasing member  920  of the first valve  204  resists actuation of the first valve  204  until the first pressure value is reached, and the biasing member  920  of the second valve  206  resists actuation of the second valve  206  until the second pressure value that is different from the first pressure value is reached. In some embodiments, the biasing members  920  may be bellows, and tuning the biasing members  920  may include charging the bellows with a gas (e.g., nitrogen) to a predetermined pressure based on the selected first and second pressure values, respectively. 
     The method  1200  may also include positioning the first and second valves  204 ,  206  and the production tubing  102  at the selected depths (from  501 ) in a well  103 , as at  1203 . The method  1200  may include injecting a gas into an annulus  111  between a production tubing  102  and the well  103 , as at  1204 . The gas flows from the annulus into the production tubing through a first valve that is open. 
     The method  1200  also includes closing the first valve  204  by controlling a pressure in a single control line  112  that is coupled to the first valve  204 , without causing or permitting a second valve  206  that is subjacent to the first valve  204  to close, as at  1205 . The pressure in the control line  112  is independent of a pressure of the gas in the annulus. 
     The method  1200  may also include increasing the pressure of the gas in the annulus  111  after closing the first valve  204 , such that the gas flows through the second valve  206  and into the production tubing  102 , as at  1206 . 
     The method  1200  may further include closing the second valve  206  by controlling the pressure in the control line  112 , as at  1208 . The control line  112  is also coupled to the second valve  206 , independently of the pressure of the gas in the annulus  111  (e.g., such that the pressure of the gas in the annulus  111  does not determine or control the pressure of the gas in the control line  112 ), while maintaining the first valve  204  in a closed position. 
     In some embodiments, the method  1200  may, for example, before closing the first and/or second valves  204 ,  206  at  1205  and  1208 , or after closing the first and/or second valves  204 ,  206  at  1205  and  1208 , include opening the first and second valves  204 ,  206 , as at  1210 , by controlling the pressure in the control line  112  such that a pressure generated at least partially by the pressure in the control line  112  causes or permits the first and second valves  204 ,  206  to open. Further, in at least some embodiments, the method  1200  may include retrieving the first and/or second valves  204 ,  206  without removing the production tubing  102  from the well  103 , as discussed above. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.