Patent Publication Number: US-10329878-B2

Title: Maintaining a downhole valve in an open position

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is the National Stage of, and therefore claims the benefit of, International Application No. PCT/US2014/042759 filed on Jun. 17, 2014, entitled “MAINTAINING A DOWNHOLE VALVE IN AN OPEN POSITION,” which was published in English under International Publication Number WO 2015/195098 on Dec. 23, 2015. The above application is commonly assigned with this National Stage application and is incorporated herein by reference in its entirety. 
     TECHNICAL BACKGROUND 
     This disclosure relates to a downhole fluid valve actuator for downhole tools. 
     BACKGROUND 
     Valves in some downhole tools can be controlled via increasing or decreasing the pressure of the fluid in the annulus surrounding the tool. For example, a ball valve can be opened by increasing the pressure in the annulus above a certain reference pressure. In some cases, decreasing the annulus pressure below the reference pressure closes the ball valve. When the tool is removed from the wellbore, the annulus pressure in the vicinity of the tool decreases, and the valve closes. Any fluid remaining in the tool when the valve closes adds to the weight of the tool string. The additional weight of the remaining fluid can cause damaging strain on the tool string. Furthermore, the additional weight means that more work is required to remove the tool string from the wellbore. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a cross-section view of an example well system that includes a downhole valve system. 
         FIG. 2  illustrates a cross-section view of an example implementation of a valve actuator for a downhole valve system. 
         FIG. 3  illustrates a cross-section view of another example implementation of a valve actuator for a downhole valve system. 
         FIG. 4  illustrates a cross-section view of another example implementation of a valve actuator for a downhole valve system. 
         FIG. 5  illustrates a cross-section view of another example implementation of a valve actuator for a downhole valve system. 
         FIG. 6  illustrates a cross-section view of another example implementation of a valve actuator for a downhole valve system. 
         FIG. 7  illustrates a cross-section view of another example implementation of a valve actuator for a downhole valve system. 
         FIG. 8  illustrates a cross-section view of another example implementation of a valve actuator for a downhole valve system. 
         FIG. 9  illustrates a cross-section view of another example implementation of a valve actuator for a downhole valve system. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to a downhole fluid valve actuator in a wellbore. The actuator is able to open a valve (e.g., a ball valve) in a tool string in response to an increase in pressure in the annulus surrounding the tool string. The actuator is also configured to keep the valve in the open position even as the tool string is removed from the well. In some implementations, the valve is configured to remain in the open position by increasing the annulus pressure above a reference pressure. For example, a rupture disk can be implemented such that rupturing the rupture disk with high applied annulus pressure disables the valve closing mechanism within the actuator. In another example, the valve closing mechanism can be disabled by shearing a shear pin via a high applied annulus pressure. In some implementations, the movement of a piston within the actuator is limited such that once a valve is opened it cannot be closed. In some implementations, the actuator is strained during tool removal such that the actuator mechanism is held in the open valve configuration. 
     Various implementations of a downhole valve system that includes a valve and a valve actuator according to the present disclosure may include none, one or some of the following features. The downhole valve system can reduce strain on the tool string by maintaining the valve in the open position during tool string removal. Some implementations allow the valve to be opened and closed repeatedly until a high annulus pressure is applied that disables the valve closing mechanism. As another example, the downhole valve system can reduce or eliminate pressure trapped between a valve and other downhole tools coupled to the valve system in a downhole string by maintaining the valve in the open position during tool string removal. Further, a downhole tool string that includes the downhole valve system can drain by maintaining the valve in the open position during tool string removal, so that fluid is not brought to a surface and exposed to working personnel. 
       FIG. 1  illustrates a cross-section view of an example well system  100  that includes a downhole valve system  120 . An annulus  112  between the downhole valve system  120  and the wellbore can contain a fluid such as well fluid. The illustrated well system  100  can be implemented in, for example, a downhole tubing and/or tool system that extends from a terranean surface, that is above sea-level (e.g., or otherwise not extending from a location that is under a body of water), or can be implemented in a well system located in an ocean-based environment or other environment that includes a body of water. Thus, reference to a terranean surface includes surface locations that are above, as well as below, a body of water (e.g., ocean, sea, lake, river, or otherwise). 
     The downhole valve system  120  includes a valve actuator  150  connected to a mandrel  140 . The mandrel  140  is a tubular member that partly defines a bore  122  within the downhole valve system  120 . The valve system  120  includes a housing  124  that is coupled to one or more operating cases  102 . The housing  124  and/or the operating cases  102  may be coupled to the downhole valve actuator  150 . The operating case  102  can at least in part encase and support a retainer  104 , where the uphole side of the retainer  104  is mechanically coupled, directly or indirectly, to machinery or apparatus at the top of the wellbore or well system controlling the well system  100 . The downhole side of the retainer  104  is mechanically coupled to the uphole side of a ball-and-seat valve  106 , which includes a seat  106   a  in which a ball  106   b  is rotatably mounted. Rotation of the ball  106   b  between a first and second position within the seat  106   a  corresponds to the ball-and-seat valve  106  switching between a closed and an open position. As shown in  FIG. 1 , the ball-and-seat valve  106  is in a closed position. Although shown as a ball-and-seat valve  106 , other types of valves may also be used without departing from the scope of the disclosure. For example, any downhole valve that can be actuated mechanically (e.g., by shifting a mandrel coupled to the valve) may be implemented in the present system  120 . 
     The downhole side of the ball-and-seat valve  106  is mechanically coupled to the uphole side of a ball cage  108 . The ball cage  108  is configured to at least in part encase and support the ball-and-seat valve  106 . Similarly, the operating case  102  can at least in part encase and support both the ball-and-seat valve  106  and the ball cage  108 . An operating arm  110  has an uphole end proximate to the ball-and-seat valve  106  that is mechanically coupled to the ball  106   b . The operating arm  110  is coupled to the mandrel  140 . 
     The mandrel  140  can be actuated by a mechanism. For example, the mandrel  140  can be translated uphole or downhole by valve actuator  150 . When the mandrel  140  is translated, the mandrel  140  shifts the operating arm  110 . The shifting operating arm  110  rotates the ball  106   b  within the seat  106   a  between a first and second position, respectively switching the ball-and-seat valve  106  between the closed and the open positions. In some implementations, the mandrel  140  is translated downhole to open the ball-and-seat valve  106  and translated uphole to close the ball-and-seat valve  106 . When the ball-and-seat valve  106  is in an open position, the interior volumes of the retainer  104 , ball-and-seat valve  106 , and ball cage  108  are all in fluid communication with each other. 
       FIG. 2  illustrates a cross-section view of another example embodiment of a valve actuator  200  for downhole valve system  100 .  FIG. 2  illustrates the valve actuator  200  in the closed configuration. The valve actuator  200  includes a mandrel  140  that is configured to translate in response to applied annulus  112  pressure. The valve actuator  200  also includes a pressure chamber (not shown) enclosing a pressurized fluid at the particular pressure. The pressurized fluid can be a fluid such as air, nitrogen, or another fluid. 
     The mandrel  140  in the downhole valve actuator  200  is adjustable to a first position to close the downhole valve  106  based on a particular pressure in a pressure chamber greater than an annulus  112  pressure at a first pressure in the annulus  112 . The first pressure in the annulus  112  can be at or greater than a hydrostatic pressure of the fluid in the wellbore at a depth of the downhole valve system  100 . The difference between the annulus  112  pressure and the particular pressure in the pressure chamber can impart a net force on the mandrel  140 , shifting the mandrel  140  uphole or downhole. In the closed configuration, the mandrel  140  is positioned relatively uphole (as shown); in the open configuration, the mandrel  140  is positioned relatively downhole. 
     The valve actuator  200  includes a housing  124 , a tubular section  204 , and the operating case  102 . The mandrel  140 , housing  124 , and tubular section  204  define a fluid chamber  206 . The fluid chamber  206  is fluidly connected to the annulus  112  but fluidly isolated from the bore  122 . The mandrel  140  includes a shoulder  242  that protrudes radially outward into the fluid chamber  206 . 
     The example valve actuator  200  also includes multiple segmented locking dogs  208  located within the fluid chamber  206 . The dogs  208  are members positioned radially around the mandrel  140 . The dogs  208  are held by one or more garter springs  210  that encircle the dogs  208 . The garter springs  210  bias the dogs  208  radially inward. For example, in the closed configuration shown in  FIG. 2 , the dogs  208  are biased inward and impinge on the shoulder  242 . 
     A spring-actuated piston  212  is located within a recess  213  defined by the housing  124 . One end of the recess  213  is open to the fluid chamber  206 . The end of the spring-actuated piston  212  distal the fluid chamber  206  is coupled to a spring  214 . The spring  214  imparts a force that biases the spring-actuated piston  212  toward the fluid chamber  206 . The housing  124  also defines an air chamber  216 . The air chamber  216  is fluidly isolated from the annulus  112 , the fluid chamber  206 , and the bore  122 . The air chamber  216  can be filled with a fluid such as air, nitrogen, or another fluid. A portion of the air chamber  216  is defined by the spring-actuated piston  212 . The volume of fluid in the air chamber  216  is isolated, and the force from spring  214  is insufficient to shift the spring-actuated piston  212  out of the recess  213 . 
     Thus, the spring-actuated piston  212  is maintained within the recess  213 . The air chamber  216  is isolated from the annulus  112  by a rupture disk  218 . The rupture disk  218  is a breakable member that can be ruptured by sufficiently high annulus  112  pressure, creating a fluid connection between the air chamber  216  and the annulus  112 . 
     By increasing the annulus  112  pressure sufficiently, the pressure in the fluid chamber  206  can overcome the pressure exerted by a separate downhole pressure chamber (not shown) and cause the mandrel  140  to shift downhole. Shifting the mandrel  140  downhole opens the valve  106 . Once the mandrel  140  is fully shifted downhole, the shoulder  242  shifts from beneath the dogs  208 . The garter springs  210  constrict the dogs  208  against a reduced diameter portion of the mandrel  140  next to the shoulder  242 . The valve  106  can be locked open by increasing the annulus  112  pressure to rupture the rupture disk  218 . 
     The annulus  112  pressure necessary to rupture the rupture disk  218  is a second pressure greater than the first pressure that was sufficient to open the valve  106 . With the disk  218  ruptured, the air chamber  216  is fluidly coupled to the annulus  112  and the spring-actuated piston  212  is released. The force exerted by the spring  214  pushes a portion of the spring-actuated piston  212  into the fluid chamber  206  and at least partially covers the dogs  208 . 
     With the spring-actuated piston  212  positioned between the dogs  208  and the tubular section  204 , the dogs  208  are unable to move radially. The shoulder  242  of the mandrel  140  stops against the dogs  208  if the mandrel  140  attempts to translate in the uphole direction, and the valve  106  is prevented from closing. Thus, the mandrel  140  is locked against movement urged by the pressurized fluid in the pressure chamber and the downhole valve  106  is locked in an open position independent of the particular pressure being greater than the annulus  112  pressure. 
       FIG. 3  illustrates a cross-section view of another example implementation  300  of valve actuator  150  for downhole valve system  100 .  FIG. 3  illustrates the valve actuator  300  in the closed configuration. The valve actuator  300  includes a housing  124  and an operating case  102 . The valve actuator  300  includes a mandrel  140  that is able to translate in response to applied annulus  112  pressure. In the closed configuration, the mandrel  140  is positioned relatively uphole (as shown); in the open configuration, the mandrel  140  is positioned relatively downhole. The mandrel  140  includes a shoulder  342  that extends radially outward to the operating case  102 . 
     The mandrel  140 , shoulder  342 , and operating case  102  define a pressure chamber  320  enclosing a pressurized fluid at a particular pressure. The pressurized fluid can be a fluid such as air, nitrogen, or another fluid. The mandrel  140 , shoulder  342 , housing  124 , and operating case  102  define a fluid chamber  306  that is positioned adjacent a portion of the mandrel  140  that is adjacent the pressure chamber  320 . One or more seals  350  between the shoulder  342  and the operating case  102  fluidly isolate the pressure chamber  320  from the fluid chamber  306 . The fluid chamber  306  is fluidly connected to the annulus  112  via one or more ports  324  in the operating case  102 , but the fluid chamber  306  is fluidly isolated from the bore  122 . 
     A spring-actuated piston  312  is located within a recess  313  defined by the housing  124 . One end of the recess  313  is open to the fluid chamber  306 . The end of the spring-actuated piston  312  distal from the fluid chamber  306  is coupled to a spring  314 . The spring  314  imparts a force that biases the spring-actuated piston  312  toward the fluid chamber  306 . The housing  124  also defines an air chamber  316 . The air chamber  316  is fluidly isolated from the annulus  112 , the fluid chamber  306 , and the bore  122 . The air chamber  316  can be filled with a fluid such as air, nitrogen, or another fluid. 
     A portion of the air chamber  316  is defined by the spring-actuated piston  312 . The isolated volume of gas in the air chamber  316  overcomes the force from spring  314  and the spring-actuated piston  312  is maintained within the recess  313 . The air chamber  316  is isolated from the annulus  112  by a rupture disk  318 . The rupture disk  318  is a breakable member that can be ruptured by sufficiently high annulus  112  pressure, creating a fluid connection between the air chamber  316  and the annulus  112 . The valve actuator  300  also includes a sleeve  322 . The sleeve  322  is located within fluid chamber  306  and positioned between the spring-actuated piston  312  and the ports  324 . 
     The housing  124  also defines a passage  323 . One end of the passage  323  is open to the fluid chamber  306 , and the other end of the passage  323  is open to the annulus  112 . A volume displacement piston  315  is located within the passage  323 . One or more seals  350  on the volume displacement piston  315  fluidly isolate the fluid chamber  306  end of the passage  323  from the annulus  112  end of the passage  323 . 
     The valve  106  is opened by increasing the annulus  112  pressure to a first pressure. If the annulus  112  pressure is sufficiently high, the pressure in the fluid chamber  306  overcomes the pressure in the pressure chamber  320 . The net force due to the pressure difference shifts the mandrel  140  downhole and thus open the valve  106 . The valve  106  is locked in the open position by increasing the annulus  112  pressure to a second pressure to rupture the rupture disk  318 . The second pressure to rupture the rupture disk  318  is greater than the first pressure to open the valve  106 . 
     With the disk  318  ruptured, the air chamber  316  is fluidly coupled to the annulus  112  and the spring-actuated piston  312  is released. The force exerted by the spring  314  pushes a portion of the spring-actuated piston  312  into the fluid chamber  306 . The spring-actuated piston  312  impinges on the sleeve  322  and urges it downhole, covering the ports  324 . The sleeve  322  includes one or more seals  350  that fluidly isolate the fluid chamber  306  from the annulus  112  when the sleeve  322  covers the ports  324 . The fluid chamber  306  traps the fluid within at the annulus  112  pressure to lock the mandrel  140  against movement urged by the pressurized fluid in the pressure chamber  320 . As such, the valve  106  is locked in the open position independent of the particular pressure in the pressure chamber  320  being greater than the annulus  112  pressure. 
     Once the sleeve  322  covers the ports  324  sufficiently to isolate the fluid chamber  306 , the volume displacement piston  315  can shift within the passage  323  to increase the effective volume of the fluid chamber  306  and allow the spring-actuated piston  312  to fully extend. Thus, the valve  106  is fully locked in the open position. 
       FIG. 4  illustrates a cross-section view of another example implementation  400  of valve actuator  150  for downhole valve system  100 .  FIG. 4  illustrates the valve actuator  400  in the closed configuration. The valve actuator  400  includes upper housing  124 , lower housing  126 , and operating cases  102 . The valve actuator  400  includes a mandrel  140  that is able to translate in response to applied annulus  112  pressure. In the closed configuration, the mandrel  140  is positioned relatively uphole (as shown); in the open configuration, the mandrel  140  is positioned relatively downhole. The mandrel  140  includes a shoulder  442  and an impinging shoulder  444  that extend radially outward. 
     The valve actuator  400  includes an upper fluid chamber  402  defined by the upper housing  124 , operating case  102 , and the shoulder  442  of mandrel  140 . The upper fluid chamber  402  is fluidly connected to the annulus  112  via one or more upper ports  420  in the operating case  102 . The valve actuator  400  includes an upper piston  416  that is located radially between the mandrel  140  and the operating case  102  and axially between the shoulder  442  and the impinging shoulder  444 . A pressure chamber  404  is defined by the shoulder  442 , the mandrel  140 , the operating case  102 , and the upper piston  416 . The pressure chamber  404  can contain nitrogen or air or another gas or fluid. 
     The valve actuator  400  also includes a tubular section  412  that is located between the impinging shoulder  444  and the lower housing  126 . An upper oil chamber  406  is defined by the upper piston  416 , the mandrel  140 , the operating case  102 , and the tubular section  412 . The upper oil chamber  406  can contain oil or another gas or fluid. The upper piston  416  includes one or more seals  450  to fluidly isolate the pressure chamber  404  from the upper oil chamber  406 . A lower piston  418  is located between the tubular section  412  and the lower housing  126 . A lower oil chamber  408  is defined by the tubular section  412 , the operating case  102 , and the lower piston  418 . The tubular section  412  includes a passage  414  that fluidly connects the upper oil chamber  406  to the lower oil chamber  408 . The passage  414  can be a small orifice or can include a metering device such that fluid transfer between the upper oil chamber  406  and the lower oil chamber  408  is a relatively slow process. 
     The valve actuator  400  also includes a lower fluid chamber  410  defined by the lower housing  126 , the operating case  102 , and the lower piston  418 . The lower fluid chamber  410  is fluidly connected to the annulus  112  via one or more lower ports  422  in the operating case  102 . Regions can be fluidly isolated by seals  450  as shown in  FIG. 4 . 
     The valve  106  is opened by sufficiently increasing the pressure within the annulus  112 . Increasing the annulus  112  pressure increases the pressure within the fluidly connected upper fluid chamber  402 . When the pressure within the upper fluid chamber  402  is high enough to overcome the pressure in the pressure chamber  404 , the net force shifts the mandrel  140  downhole and opens the valve  106 . The increased pressure in the annulus  112  also increases the pressure in the fluidly connected lower fluid chamber  410 . The increased pressure in the lower fluid chamber  410  shifts the lower piston  418  uphole and compresses the oil in lower oil chamber  408 . The compressed oil in lower oil chamber  408  transfers slowly through passage  414  into upper oil chamber  406 . As the oil pressure in upper oil chamber  406  increases, the upper piston  416  shifts uphole. 
     When the annulus  112  pressure is decreased, as when the tool is removed from the well, the pressure in the pressure chamber  404  imparts a force on the shoulder  442  and the upper piston  416 . The pressure in the pressure chamber  404  shifts the upper piston  416  downhole until it impinges on shoulder  444 . The pressure in the pressure chamber  404  thus imparts an uphole force and a downhole force equally on the mandrel  140 , preventing the mandrel  140  from shifting. Thus, the valve  106  remains in the open position. 
       FIG. 5  illustrates a cross-section view of another example implementation  500  of valve actuator  150  for downhole valve system  100 .  FIG. 5  illustrates the valve actuator  500  in the closed configuration. The example valve actuator  500  is substantially similar to the example valve actuator  400  shown in  FIG. 4 . The valve actuator  500  does not include an impinging shoulder  444 . The valve actuator  500  does include a fluid conduit  546  within the shoulder  442 . One end of the conduit  546  is open to the upper fluid chamber  402  and one end of the conduit  546  is open to the pressure chamber  404 . The upper fluid chamber  402  is fluidly coupled to the annulus  112 , thus the conduit  546  fluidly couples the pressure chamber  404  and the exterior surface of the downhole valve system  100 . A rupture disk  544  is a breakable member positioned within the conduit  546  between the pressure chamber  404  and the upper fluid chamber  402  such that the upper fluid chamber  402  is isolated from the pressure chamber  404 . 
     Similarly to valve actuator  400 , increasing the annulus  112  pressure opens the valve  106  by shifting the mandrel  140 . To lock the valve  106  in the open position, the annulus  112  pressure is increased to rupture the rupture disk  544 . Rupturing the rupture disk  544  fluidly connects the pressure chamber  404  and the upper fluid chamber  402 , and thus the pressurized fluid in the pressure chamber  404  bleeds into the upper fluid chamber  402 . This renders the mandrel  140  lockable against movement, and thus the valve  106  is locked in the open position irrespective of annulus  112  pressure. In some implementations, the rupture disk  544  is a supported rupture disk that can only be ruptured by high pressure in the annulus  112  relative to the pressure in the pressure chamber  404 . 
       FIG. 6  illustrates a cross-section view of another example implementation  600  of valve actuator  150  for downhole valve system  100 .  FIG. 6  illustrates the valve actuator  600  in the closed configuration. The valve actuator  600  includes upper housing  124  and operating case  102 . The valve actuator  600  also includes an upper mandrel  140  and a lower mandrel  142  radially adjacent the upper mandrel  140 . The upper mandrel  140  is coupled to the valve  106 . The lower mandrel  142  is able to translate in response to applied annulus  112  pressure. Components of valve actuator  600  can be fluidly isolated by one or more seals  650 . 
     The valve actuator  600  includes an upper fluid chamber  602  defined by the housing  124 , the operating case  102 , the upper mandrel  140 , and the lower mandrel  142 . The upper fluid chamber  602  is fluidly connected to the annulus  112  by port  620 . A passage  614  is defined by the operating case  102  and the lower mandrel  142 . One end of the passage  614  is fluidly connected to the upper fluid chamber  602 . 
     The valve actuator  600  includes a lower fluid chamber  606  defined by the upper mandrel  140  and the lower mandrel  142 . The lower fluid chamber  606  is fluidly connected to passage  614  via port  612 . The valve actuator  600  also includes an air chamber  604  defined by the upper mandrel  140  and the lower mandrel  142 . The air chamber  604  can be filled with a fluid such as air, nitrogen, or another fluid. The air chamber  604  is fluidly isolated from the passage  614  by rupture disk  610 . The air chamber  604  couples the upper mandrel  140  to the lower mandrel  142  such that the mandrels  140 ,  142  translate together as a single component. The valve actuator  600  also includes a pressure chamber  608  partially defined by the lower mandrel  142  and the operating case  102 . The lower mandrel  142  includes an effective surface  609  adjacent the pressure chamber  608 . The pressure chamber  608  can be filled with nitrogen or another gas or fluid. 
     In the closed configuration, the upper mandrel  140  and the lower mandrel  142  are positioned relatively uphole (as shown); in the open configuration, the upper mandrel  140  is positioned relatively downhole. When the upper mandrel  140  is coupled to the lower mandrel  142 , both mandrels  140 ,  142  are positioned relatively downhole when in the open configuration. The valve  106  is opened by increasing the pressure within the annulus  112  to a first pressure greater than the particular pressure within the pressure chamber  608 . Increasing the annulus  112  pressure increases the pressure within the upper fluid chamber  602 . The pressure within the upper fluid chamber  602  overcomes the particular pressure in the pressure chamber  608 , and the net force shifts the lower mandrel  142  downhole. The upper mandrel  140  is coupled to the lower mandrel  142 , and shifting the upper mandrel  140  downhole opens the valve  106 . 
     To lock the valve  106  in the open position, the annulus  112  pressure is increased to a second pressure to rupture the rupture disk  610 . Rupturing the rupture disk  610  fluidly connects the air chamber  604  and the passage  614 , and thus the air in the air chamber  604  vents into the upper fluid chamber  602  and the air chamber  604  is filled with annulus fluid. This decouples the upper mandrel  140  from the lower mandrel  142 . The pressurized fluid in the pressure chamber  608  acts on the effective surface  609  of the lower mandrel  142  independent of the upper mandrel  140  to maintain the upper mandrel  140  at an open position, locking the downhole valve  106  in the open position. In some implementations, the rupture disk  610  is a supported rupture disk that can only be ruptured by high pressure in the annulus  112  relative to the pressure in the air chamber  604 . 
       FIG. 7  illustrates a cross-section view of another example implementation  700  of valve actuator  150  for downhole valve system  100 .  FIG. 7  illustrates the valve actuator  700  in the closed configuration. The example valve actuator  700  is substantially similar to the example valve actuator  600  shown in  FIG. 6 . The valve actuator  700  does not include a port  612  or a lower fluid chamber  606 . Valve actuator  700  does include a lower air chamber  706  positioned radially between the upper mandrel  140  and lower mandrel  142 . The lower air chamber  706  can be filled with a gas or fluid such as air, nitrogen, or another substance. 
     The upper air chamber  604  and the lower air chamber  706  couple the upper mandrel  140  to the lower mandrel  142  such that the mandrels  140 ,  142  translate together as a single component. The valve  106  can be opened and closed via annulus  112  pressure as described above for  FIG. 6 . 
     The valve  106  is locked in the open position by increasing the annulus  112  pressure sufficiently to rupture the rupture disk  610 . Rupturing the rupture disk  610  fluidly connects the upper air chamber  604  to the fluid chamber  602 , bringing the pressure within the upper air chamber  604  to annulus  112  pressure. In some implementations, the volume of upper air chamber  604  is larger than the volume of lower air chamber  706 . Thus, the annulus  112  pressure in the upper air chamber  604  overcomes the pressure in lower air chamber  706  and imparts a force on the effective surface  605  of the upper mandrel  140 , compressing the volume of air in lower air chamber  706 . This urges upper mandrel  140  to lower mandrel  142  while still maintaining the coupling between mandrels  140 ,  142 . Because the upper mandrel  140  is coupled to lower mandrel  142  but positioned relatively downhole, the upper mandrel  140  is unable to shift sufficiently uphole to close the valve  106  even if both mandrels  140 ,  142  are shifted uphole due to decreased annulus  112  pressure. 
       FIG. 8  illustrates a cross-section view of another example implementation  800  of valve actuator  150  for downhole valve system  100 .  FIG. 8  illustrates the valve actuator  800  in the closed configuration. The valve actuator  800  includes upper housing  124  and operating case  102 . The valve actuator  800  also includes an upper mandrel  140  and a lower mandrel  142 . The upper mandrel  140  is coupled to the valve  106 . The lower mandrel  142  is able to translate in response to applied annulus  112  pressure. The upper mandrel  140  is coupled to the lower mandrel  142  by a shear pin  810  such that the mandrels  140 ,  142  translate together as a single component. The valve actuator  800  includes a fluid chamber  802  defined by the housing  124 , the operating case  102 , the upper mandrel  140 , and the lower mandrel  142 . The upper fluid chamber  802  is fluidly connected to the annulus  112  by port  820 . Components of valve actuator  800  can be fluidly isolated by one or more seals  850 . 
     The valve  106  is opened by increasing the annulus  112  pressure. If the annulus  112  pressure is sufficiently high, the pressure in the fluid chamber  802  overcomes the pressure in the pressure chamber  804 . The net force due to the pressure difference shifts the mandrels  140 ,  142  downhole and thus opens the valve  106 . The valve  106  is locked open by applying a sufficiently high annulus  112  pressure to shear the shear pin  810 . The downhole transit of upper mandrel  140  is limited by its coupling to the valve  106  or by another mechanism (e.g. an impinging shoulder). Once the downhole transit limit of upper mandrel  140  is reached, increasing the annulus  112  pressure sufficiently high imparts enough force on the lower mandrel  142  to shear the pin  810 . Breaking the shear pin  810  uncouples the lower mandrel  142  from the upper mandrel  140 . The pressurized fluid in the pressure chamber  804  acts on the effective surface  805  of the lower mandrel  142  independent of the upper mandrel  140  to maintain the upper mandrel  140  at an open position, locking the downhole valve  106  in the open position. 
       FIG. 9  illustrates a portion of another example implementation  900  of valve actuator  150  for downhole valve system  100 .  FIG. 9  illustrates the valve actuator  900  in the closed configuration. The valve actuator  900  includes an upper operating case  102   a  and a lower operating case  102   b . The upper operating case  102   a  is affixed to an upper locking member  926 . The upper locking member  926  includes an upper hook-shaped member  927  that extends downhole. The lower operating case  102   b  is affixed to a lower locking member  928 . The lower locking member  928  includes a lower hook-shaped member  929  that complements the upper hook-shaped member  927 . A chamber  904  is located between and defined by the upper hook-shaped member  927  and the lower hook-shaped member  929 . 
     The valve actuator  900  also includes a mandrel  140  affixed to a piston  942 . The mandrel  140  is coupled to the valve  106 , and is able to translate in response to applied annulus  112  pressure. The mandrel  140  is affixed to the piston the mandrel  140  and piston  942  translate together as a single component. The valve actuator  900  includes a fluid chamber  902  defined by the lower locking member  928 , the operating case  102   b , the lower mandrel  140 , and the piston  942 . The upper fluid chamber  902  is fluidly connected to the annulus  112  by port  912 . Components of valve actuator  900  can be fluidly isolated by one or more seals  950 . 
     The valve actuator  900  also includes a pressure chamber  908  partially defined by the piston  942  and the lower operating case  102   b . The pressure chamber  908  can be filled with nitrogen or another gas or fluid. The valve  106  is opened by sufficiently increasing the pressure within the annulus  112 . Increasing the annulus  112  pressure increases the pressure within the fluid chamber  902 . When the pressure within the upper fluid chamber  902  is high enough to overcome the pressure in the pressure chamber  908 , the net force shifts the piston  142  downhole. The mandrel  140  is coupled to the piston  942 , and shifting the mandrel  140  downhole relative to the upper operating case  102   a  opens the valve  106 . 
     As the valve actuator  900  is removed from the well, tension in the string pulls apart the lower locking member  928  and the upper locking member  926 . A downhole-facing surface of the upper hook-shaped member  929  stops against an uphole-facing surface of the lower hook-shaped member  927 , closing the chamber  904 . This extends the total length of the valve actuator  900 , and increases the uphole travel distance required for mandrel  140  to shift in order to close the valve  106 . The allowed uphole travel distance of the mandrel  140  or piston  942  can be limited by position of the uphole end of the fluid chamber  902 , an impinging shoulder, or via another mechanism. As the annulus  112  pressure decreases during tool removal, the pressure in pressure chamber  908  shifts the piston  942  uphole. However, with the valve actuator  900  extended, the shift distance is insufficient to completely close the valve  106 . Thus, the valve  106  remains open during tool removal. 
     In an example implementation, a method of adjusting a downhole valve includes positioning a downhole valve assembly in a wellbore. The downhole valve assembly includes a downhole valve coupled to a downhole valve actuator that includes a pressure chamber enclosing a pressurized fluid at a closing pressure. The downhole valve actuator maintains the valve in a closed position as long as the closing pressure is greater than an annulus pressure in an annulus between the downhole valve assembly and the wellbore. The method includes circulating a fluid in the annulus to set the annulus pressure at a first fluid pressure; based on the annulus pressure greater than the closing pressure of the pressurized fluid, activating the downhole valve actuator to adjust the valve to an open position from the closed position; circulating the fluid in the annulus to set the annulus pressure at a second fluid pressure greater than the first fluid pressure to break a breakable member of the downhole valve actuator; and upon breaking of the breakable member, locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure. 
     In a first aspect combinable with the general implementation, the first set pressure is at or greater than a hydrostatic pressure of the fluid in the wellbore at a depth of the downhole valve assembly. 
     In a second aspect combinable with any of the previous aspects, the pressurized fluid includes nitrogen. 
     In a third aspect combinable with any of the previous aspects, the breakable member includes a rupture disk of the downhole valve actuator. Locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure includes breaking the rupture disk to release a spring-actuated piston to at least partially cover dogs that ride on a reduced diameter portion of a mandrel of the downhole valve actuator; covering, at least partially the dogs by the piston to lock the mandrel against movement urged by the pressurized fluid in the pressure chamber; and locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure. 
     In a fourth aspect combinable with any of the previous aspects, the breakable member includes a rupture disk of the downhole valve actuator. Locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure includes breaking the rupture disk to release a spring-actuated piston to urge a sleeve to cover a port that fluidly connects the annulus and a chamber positioned adjacent a portion of a mandrel of the downhole valve actuator that is adjacent the pressure chamber; trapping the fluid at the annulus pressure set at the second fluid pressure in the chamber; and locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure. 
     In a fifth aspect combinable with any of the previous aspects, the breakable member includes a supportable rupture disk of the downhole valve actuator. Locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure includes breaking the supportable rupture disk to bleed the pressurized fluid from the pressure chamber into the annulus; reducing a pressure, with the bleeding, in the pressurized chamber from the closing pressure to less than the annulus pressure; and locking the valve in the open position based on the pressure in the pressurized chamber being less than the annulus pressure. 
     In a sixth aspect combinable with any of the previous aspects, the breakable member includes a rupture disk of the downhole valve actuator. Locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure includes breaking the rupture disk to fill a chamber with the fluid from the annulus, the chamber adjacent an effective surface of an upper mandrel of the downhole valve actuator, the upper mandrel radially adjacent a lower mandrel of the downhole valve actuator that includes an effective surface adjacent the pressurized chamber decoupling the upper mandrel from the lower mandrel such that the pressurized fluid acts on the effective surface of the lower mandrel independent of the upper mandrel; and maintaining the upper mandrel at a position to lock the downhole valve in the open position. 
     In a seventh aspect combinable with any of the previous aspects, the breakable member includes a rupture disk of the downhole valve actuator. Locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure includes breaking the rupture disk to fill a first chamber with the fluid from the annulus, the first chamber adjacent an effective surface of an upper mandrel of the downhole valve actuator, the upper mandrel radially adjacent a lower mandrel of the downhole valve actuator that includes an effective surface adjacent the pressurized chamber; and based on filling the first chamber with the fluid, urging the upper mandrel to a position to lock the downhole valve in the open position through a second chamber positioned between the upper and lower mandrels. 
     In an eighth aspect combinable with any of the previous aspects, the breakable member includes a shear pin of the downhole valve actuator. Locking the valve in the open position independent of the relative values of the closing pressure and the annulus pressure includes breaking the shear pin to decouple an upper mandrel of the downhole valve actuator from a lower mandrel of the downhole valve actuator such that the pressurized fluid acts on an effective surface of the lower mandrel independent of the upper mandrel; and maintaining the upper mandrel at a position to lock the downhole valve in the open position. 
     In a ninth aspect combinable with any of the previous aspects, the valve includes a downhole tester valve. 
     In another example implementation, a downhole valve system includes a downhole valve positionable in a wellbore and a downhole valve actuator coupled to the downhole valve. An annulus is defined between the downhole valve and the wellbore. The downhole valve actuator includes a pressure chamber that encloses a pressurized fluid at a particular pressure and a breakable member, where the downhole valve actuator is adjustable to a first position to close the downhole valve when the particular pressure is greater than an annulus pressure at a first pressure in the annulus, and the downhole valve actuator is adjustable to a second position to lock the downhole valve in an open position independent of the relative values of the particular pressure and the annulus pressure, with the annulus pressure set at a second pressure greater than the first pressure to break the breakable member. 
     In a first aspect combinable with the general implementation, the first pressure is at or greater than a hydrostatic pressure of the fluid in the wellbore at a depth of the downhole valve system. 
     In a second aspect combinable with any of the previous aspects, the pressurized fluid includes nitrogen. 
     In a third aspect combinable with any of the previous aspects, the breakable member includes a rupture disk. The downhole valve actuator further includes a spring-actuated piston; and a mandrel. The spring-actuated piston is released upon breaking the rupture disk to at least partially cover dogs that ride on a reduced diameter portion of the mandrel. The mandrel is locked, when the dogs are covered, against movement urged by the pressurized fluid in the pressure chamber such that the valve is locked in the open position independent of the particular pressure being greater than the annulus pressure. 
     In a fourth aspect combinable with any of the previous aspects, the breakable member includes a rupture disk. The downhole valve actuator further includes a spring-actuated piston; a sleeve; and a mandrel. The spring-actuated piston is released upon breaking the rupture disk to urge the sleeve to cover a port that fluidly connects the annulus and a chamber positioned adjacent a portion of the mandrel that is adjacent the pressure chamber. The chamber is fillable to trap the fluid at the annulus pressure set at the second pressure to lock the mandrel against movement urged by the pressurized fluid in the pressure chamber such that the valve is locked in the open position independent of the relative values of the particular pressure and the annulus pressure. 
     In a fifth aspect combinable with any of the previous aspects, the breakable member includes a supportable rupture disk. The downhole valve actuator further includes a fluid conduit positioned to fluidly couple the pressure chamber and an exterior surface of the downhole valve system, the supportable rupture disk positioned in the fluid conduit between the pressure chamber and the exterior surface; and a mandrel locked against movement, when the valve actuator is in the second position, by breaking the supportable rupture disk to bleed the pressurized fluid from the pressure chamber, such that the valve is locked in the open position. 
     In a sixth aspect combinable with any of the previous aspects, the breakable member includes a rupture disk. The downhole valve actuator further includes a lower mandrel; an upper mandrel; and an air chamber adjacent an effective surface of the lower mandrel. The lower mandrel is radially adjacent the lower mandrel that includes an effective surface adjacent the pressurized chamber. The upper mandrel is uncoupled from the lower mandrel when the air chamber is filled with the annulus fluid based on breaking the rupture disk such that the pressurized fluid acts on the effective surface of the lower mandrel independent of the upper mandrel to maintain the upper mandrel at a position, when the downhole valve actuator is in the second position, to lock the downhole valve in the open position. 
     In a seventh aspect combinable with any of the previous aspects, the breakable member includes a rupture disk. The downhole valve actuator further includes a lower mandrel; an upper mandrel; a first air chamber adjacent an effective surface of the upper mandrel. The upper mandrel is radially adjacent the lower mandrel that includes an effective surface adjacent the pressurized chamber. A second air chamber is positioned radially between the upper and lower mandrels, the upper mandrel urged to the lower mandrel based on filling the first air chamber with the annulus fluid upon breaking the rupture disk such that the pressurized fluid acts on the effective surface of the lower mandrel independent of the upper mandrel to limit the upper mandrel to positions such that downhole valve actuator is not adjustable to the first position, to lock the downhole valve in the open position. 
     In an eighth aspect combinable with any of the previous aspects, the breakable member includes a shear pin. The downhole valve actuator further includes an upper mandrel; and a lower mandrel coupled to the upper mandrel with the shear pin. The upper mandrel is uncoupled from the lower mandrel based on breaking the shear pin such that the pressurized fluid acts on an effective surface of the lower mandrel independent of the upper mandrel to maintain the upper mandrel at a position, when the downhole valve actuator is in the second position, to lock the downhole valve in the open position. 
     In a ninth aspect combinable with any of the previous aspects, the valve includes a downhole tester valve. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, example operations, methods, and/or processes described herein may include more steps or fewer steps than those described. Further, the steps in such example operations, methods, and/or processes may be performed in different successions than that described or illustrated in the figures. As another example, although certain implementations described herein may be applicable to tubular systems (e.g., drillpipe and/or coiled tubing), implementations may also utilize other systems, such as wireline, slickline, e-line, wired drillpipe, wired coiled tubing, and otherwise, as appropriate. For instance, some implementations may utilize a wireline system for certain communications and a casing tubular system for other communications, in combination with a fluid system. Accordingly, other implementations are within the scope of the following claims.