Abstract:
The present invention generally relates to a subsurface safety valve configured to control fluid flow through a production tubing string. In one aspect, a safety valve for deployment beneath a surface of a wellbore is provided. The valve includes a control piston and a balance piston. The valve is configured to be connected to a controller at the surface by a control line so that the control piston is actuatable between a first position and a second position in response to receiving pressurized fluid from the controller through the control line. The balance piston is configured to compensate for hydrostatic pressure in the control line. The valve may have a bore therethrough and the control piston may be configured to utilize tubing pressure within the valve bore to urge the control piston towards the second position.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of this invention are generally related to safety valves. More particularly, embodiments of this invention pertain to subsurface safety valves configured to control fluid flow through a production tubing string. 
   2. Description of the Related Art 
   Safety Valves are designed to minimize the loss of reservoir resources or production equipment resulting from catastrophic subsurface events by shutting in the well. The “standard” safety valve achieves this by design with one “active control line”. The normally closed safety valves are controlled from the surface via a hydraulic control line that extends from the valve, through the wellhead to a surface controlled emergency closure system. Hydraulic pressure P C  applied through the control line maintains the valve in the opened position. Removal of control line pressure returns the valve to its normally closed position. Setting depth directly affects the operational characteristics of the valve due to the hydrostatic pressures P H  created from the normal control system. 
   Conventional safety valve design incorporates a hydraulic piston and spring to open and close the valve. The hydraulic chamber housing the piston is connected to the surface by a hydraulic control line. Pressure is applied to this control line to hold the valve in the open position. Hydrostatic or “head” pressure P H  is always present in the control line due to the column of fluid between the safety valve and the surface. 
   Functionally, control line pressure P C  actuates a piston which is mechanically linked to a “flow tube”. The flow tube traverses across a closed flapper thus opening the flow through the safety valve and its tubing. When the surface pressure is released, a return spring returns the valve back to its closed position. The nature of the design is such that the tubing pressure P T , which acts against the active control line piston effect, will assist in valve closure. 
   To open the valve, hydraulic pressure P C  is applied to the upper end of the piston, via the control line, forcing the flow tube downward, opening the flapper. 
   To close the valve, the applied hydraulic pressure P C  is removed from the upper end of the piston. There are two forces available now to force the flow tube upward allowing the flapper to close. The spring now furnishes an upward force F S  sufficient to counteract the downward force due to the hydrostatic pressure P H  of the fluid in the hydraulic control line. This causes the flow tube to move upward allowing the flapper to close. Tubing pressure P T  at the safety valve will also apply an upward force on the hydraulic piston. This will assist the piston in the upward movement of the flow tube allowing the flapper to close. 
   In a deep set application, the active control line hydrostatic pressure P H  is significant, such that a spring may not be able to overcome the hydrostatic pressure, thus not allowing the flapper to close. To compensate for the active control line&#39;s hydrostatic pressure P H , a second “balance” line is used to negate the affect of hydrostatic pressure P H  from active control line. In existing balance line valves, the second line acts on the underside of the piston, to balance the hydrostatic pressure P H . However, in this design, since the underside of the piston is in fluid communication with the balance line, it is no longer in fluid communication with the tubing; thereby the beneficial effect of the tubing pressure P T  is not utilized. 
   Therefore, there is a need for a safety valve that balances the control line hydrostatic pressure P H  while still utilizing the tubing pressure P T  to aid in closure of the valve. 
   SUMMARY OF THE INVENTION 
   The present invention generally relates to a subsurface safety valve configured to control fluid flow through a production tubing string. In one aspect, a safety valve for deployment beneath a surface of a wellbore is provided. The valve includes a control piston and a balance piston. The valve is configured to be connected to a controller at the surface by a control line so that the control piston is actuatable between a first position and a second position in response to receiving pressurized fluid from the controller through the control line. The balance piston is configured to compensate for hydrostatic pressure in the control line. The valve may have a bore therethrough and the control piston may be configured to utilize tubing pressure within the valve bore to urge the control piston towards the second position. 
   In another aspect, a subsurface safety valve is provided. The valve includes a flow tube having a bore therethrough; a control piston having two sides isolated from each other by a seal assembly and coupled to the flow tube; and a balance piston having two sides isolated from each other by a seal assembly and selectively coupled to the flow tube. The valve is configured so that the control piston will receive a control pressure on the first side and the balance piston will receive a hydrostatic pressure on the second side. 
   The flow tube may be actuatable between a first position and a second position and the balance piston may be selectively coupled to the flow tube so that the balance piston may urge the flow tube towards the second position but not towards the first position. The second side of the control piston may be in fluid communication with the flow tube bore. The second side of the balance piston may be in fluid communication with the flow tube bore. The valve may further include at least one housing, wherein the flow tube, the control piston, and the balance piston are disposed within the housing and the balance piston may be selectively coupled to the housing. The valve may further include a flapper coupled to the housing and a flapper spring coupled between the flapper and the housing, wherein the flapper may be actuatable by the flow tube between a first position and a second position and the flapper spring biases the flapper in the second position. 
   In another aspect, a subsurface safety valve is provided. The valve includes a control piston configured to open the valve by receiving pressurized fluid from a control line and means for compensating for hydrostatic pressure in a control line to the valve while utilizing tubing pressure within the valve to assist in closure of the valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a view illustrating a production tubing having a safety valve assembly in accordance with an embodiment of the present invention. 
       FIGS. 2 and 2A  are cross-sectional views illustrating the valve assembly  200  in a first closed position, where the balance piston is idle. 
       FIGS. 3 and 3A  are cross-sectional views illustrating the valve in the open position. 
       FIG. 4  is a cross-sectional view illustrating the valve in a closed position, where the balance piston is active. 
       FIGS. 5A-C  are free body diagrams of the valve, which illustrate the three operational positions of the valve: closed, where the balance piston is idle; open; and closed, where the balance piston is active, respectively. 
       FIGS. 6A and 6B  are hydraulic diagrams of alternate embodiments of the valve. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention is generally directed to a subsurface safety valve assembly for controlling fluid flow in a wellbore. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term, as reflected in printed publications and issued patents. In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals. The drawings may be, but are not necessarily, to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the invention. One of normal skill in the art of subsurface safety valves will appreciate that the various embodiments of the invention can and may be used in all types of subsurface safety valves, including but not limited to tubing retrievable or wireline retrievable valves. 
   For ease of explanation, the invention will be described generally in relation to a cased vertical wellbore. It is to be understood; however, that the invention may be employed in an open wellbore, a horizontal wellbore, or a diverging wellbore without departing from principles of the present invention. Furthermore, a land well is shown for the purpose of illustration, however, it is understood that the invention may also be employed in offshore wells. 
     FIG. 1  is a view illustrating a production tubing  120  having a safety valve assembly  200  (hereinafter “valve”) in accordance with an embodiment of the present invention. The valve  200  is used for controlling the flow of fluid in a production tubing  120 . The valve  200  may be moved between an open position and closed position by operating a controller  150 , such as a pump, which may draw from a reservoir  155 , in communication with the valve  200  through a control line  145 A. When actuated, the controller  150  will exert a control pressure P C  through the control line  145 A to the valve  200 . Due to vertical height of the control line  145 A, a hydrostatic pressure P H  will also be exerted on the valve  200  through the control line. A balance line  145 B is also provided to valve  200 . The balance line  145 B provides fluid communication between the reservoir  155  and the valve  200 , thereby maintaining the outlet of the balance line  145 B connected to the valve  200  at the hydrostatic pressure P H . An inside of the valve  200  is also exposed to a tubing pressure P T  which may vary with conditions within the wellbore  100 . The operation of the valve assembly  200  will first be described generally with respect to  FIG. 1 , thereafter more specifically with  FIGS. 2-5 . 
   The wellbore  100  has been lined with a string of casing  105 . A plurality of perforations  110  has been disposed through the casing  105 , thereby establishing fluid communication between a formation  115  and the production tubing  120 . Thereafter, the production tubing  120  with the safety valve  200  disposed therein is deployed in the wellbore  100  to a predetermined depth. Next, the production tubing  120  is secured in the wellbore proximate a desired zone of interest or a formation  115 . Hydrocarbons (illustrated by arrows) flow into the production tubing  120  through the safety valve  200 , through a valve  135 , and out into a flow line  130 . The flow of hydrocarbons may be stopped at any time during the production operation by switching the valve assembly  200  from the open position to the closed position as will be described in more detail in the following paragraphs. 
     FIGS. 2 and 2A  are cross-sectional views illustrating the valve  200  in a closed position, where a balance piston  205 B is idle. A bore  260  in the valve  200  allows hydrocarbons to flow up through the valve assembly  200  during the production operation, as discussed in a previous paragraph. The valve assembly  200  includes a top sub  270  and a bottom sub  275  to sealingly connect the valve  200  to the production tubing (not shown). 
   The valve  200  further includes a chamber housing  255  disposed adjacent the top sub  270  and a spring housing  280  coupled to the chamber housing  255 . An annulus  240  is formed between the spring housing and a flow tube  225 . The chamber housing  255  includes a control chamber  245 A and a balance chamber  245 B. An upper end of the control chamber  245 A is in fluid communication with the control line  145 A and a lower end of the balance chamber  245 B in fluid communication with the balance line  145 B (only a port shown for the line, line not shown in this view). Routing of a passage through the chamber housing  255  from the balance line  145 B to the balance chamber  245 B may be accomplished in several ways and is not shown as it would be well within one of ordinary skill in the art. Disposed in the control chamber  245 A is a control piston  205 A. The control piston  205 A is movable between an upper position and a lower position in response to control pressure P C  in the upper end of the control chamber  245 A. A seal assembly  215 A is disposed on an upper end of the control piston  205 A to isolate the upper end of the control chamber  145 A. The lower end of the control piston  205 A is exposed to pressure P T  within the valve assembly  200 . 
   Disposed in the balance chamber  245 B is the balance piston  205 B. The balance piston  205 B is movable between a lower position and an upper position in response to hydrostatic pressure P H  in the balance chamber  245 B. A seal assembly  215 B is disposed on a lower end of the balance piston  205 B to isolate the lower end of the balance chamber  245 B. A cap  211  is coupled to the chamber housing  255  to form a bottom of the balance chamber  245 B. A block  207  is coupled to an upper end of the balance piston  205 B to mate with a shoulder  214  of the chamber housing  255  and a shoulder  209  of the flow tube  225  (see  FIGS. 3 and 4 ). An upper end of the balance piston is exposed to the tubing pressure P T  within the valve  200 . Preferably, the balance chamber  245 B is tangentially located proximate to the control chamber  245 A, however, the balance chamber  245 B may also be located tangentially distal from the control chamber  245 A. 
   As illustrated in  FIG. 2 , the valve  200  includes a biasing member  210 , such as a coil spring, disposed in the annulus  240 . A lower end of the biasing member  210  abuts a spacer bearing  265  that is coupled to the spring housing  280 . An upper end of the biasing member  210  abuts a shoulder of the flow tube  225 , which is coupled to the control piston  205 A. In this respect, the movement of the control piston  205 A from the upper position to the lower position compresses the biasing member  210  against the spacer bearing  265  (see  FIG. 3 ). 
   Disposed below the spacer bearing  265  is a flapper  220 . The flapper  220  is rotationally attached by a pin  230  to a flapper mount  290 . The flapper  220  may move between an open position and a closed position in response to movement of the flow tube  225 . In the open position (see  FIG. 3 ), a fluid pathway is created through the bore  260 , thereby allowing the flow of fluid through the valve assembly  200 . Conversely, in the closed position, the flapper  220  blocks the fluid pathway through the bore  260 , thereby preventing the flow of fluid through the valve assembly  200 . The flapper  220  is biased towards a closed position by a flapper spring (not shown). For the sake of simplicity and brevity, the flapper spring will not be further discussed. 
   Further illustrated in  FIG. 2 , the flow tube  225  is disposed adjacent the flapper  220 . As discussed above, the flow tube  225  is coupled to the control piston  205 A. In this respect, the movement of the control piston  205 A in response to the control pressure P C  in the control chamber  245 A also causes the flow tube  225  to move. The functions of the flow tube  225  are to hold the flapper  220  open and to minimize the potential of contaminants, such as solid particulates, from eroding critical workings of the valve assembly  200 , such as the flapper seat. As with the control piston  205 A, the flow tube  225  is movable between an open position and a closed position. In the open position, the flow tube  225  blocks the movement of the flapper  220 , thereby causing the flapper  220  to be maintained in the open position. The flow tube  225  in the closed position on the other hand allows the flapper  220  to rotate on the pin  230  and move to the closed position. 
     FIGS. 3 and 3A  are cross-sectional views illustrating the valve  200  in the open position. Typically, the flow tube  225  remains in the open position throughout the completion operation and the production. The flow tube  225  moves to the open position as the control piston  205 A moves to the lower position and compresses the biasing member  210  against the spacer bearing  265 . Neglecting pressure P T  within the valve  200  and hydrostatic pressure P H  in the lines  145 A,B, controller  150  causes fluid from the control line  145 A to enter the control chamber  245 A, thereby creating the control pressure P C  on the control piston  205 A. As more fluid enters the control chamber  245 A, the hydraulic pressure continues to increase until the force exerted by the hydraulic pressure on the upper end of the control piston  205 A becomes greater than an opposite force on the lower end of the piston assembly  205  created by the biasing member  210 . At that point, the force exerted by the hydraulic pressure in the control chamber  245 A causes the control piston  205 A to move to the lower position. Since the flow tube  225  is coupled to the control piston  205 A, the movement of the control piston  205 A causes the movement of the flow tube  225 . In this manner, the flow tube  225  is moved to the open position. 
   For the sake of simplicity, and for further discussion of the operation of the valve  200 , the tubing pressure P T  within the valve  200  will be assumed to be equal to the pressure on an underside of the flapper  220  when the flapper  220  is closed so that there is no pressure difference across the flapper  220 . 
     FIG. 4  is a cross-sectional view illustrating the valve assembly  200  in a closed position, where the balance piston  205 B is active. Neglecting pressure P T  within the valve assembly  200  and hydrostatic pressure P H  in the lines  145 A,B, when controller  150  is shut off or bypassed, fluid in the control chamber  245 A exits into the control line  145 A, thereby decreasing the hydraulic pressure on the control piston  205 A. As more fluid exits the control chamber  245 A, the hydraulic pressure continues to decrease until the force exerted by the hydraulic pressure on the upper end of the control piston  205 A becomes less than the opposite force on the lower end of the control piston  205 A. At this point, the force created by the biasing member  210  causes the flow tube  225  to move to the closed position. Since the control piston  205 A is coupled to the flow tube  225 , the movement of the flow tube  225  also causes the movement of control piston  205 A to the upper position. 
     FIGS. 5A-C  are free body diagrams of the valve assembly  200 , which have been greatly simplified for illustrational purposes.  FIGS. 5A-C  illustrate the three operational positions of the valve assembly  200 : closed, where the balance piston  205 B is idle; open; and closed, where the balance piston  205 B is active, respectively. Operation of the valve assembly  200  among these three positions will now be discussed for situations where P T  and/or P H  are substantial. It is preferred that an area A A1  of the control piston  205 A on which the control line pressure P C  acts is substantially equal to an area A B1  of the balance piston  205 B on which the hydrostatic pressure P H  acts; however, A B1  may be substantially greater than A A1  or the entire cross sectional area of the balance piston  205 B may be larger than that of the control piston  205 A. It is also preferred that an area A A2  of the control piston  205 A on which the tubing pressure P T  acts be substantially equal to A A1  and an area A B2  on which the tubing pressure P T  acts be substantially equal to A B1 . For the following analysis, it will be assumed that these four areas are equal. 
     FIG. 5A  is a free body diagram of the valve assembly  200  in the closed position, where the balance piston  205 B is idle (P T &gt;P H , see also  FIG. 2 ). As discussed above, when the hydrostatic pressure P H  is substantial, it will place a downward force on the control piston  205 A, thereby tending to open the valve assembly  200 . When the tubing pressure P T  is substantial, it, along with the biasing member  210  (the force of which is denoted by F S ), will place an upward force on the control piston  205 A, thereby tending to close the valve assembly  200 . Conversely, the hydrostatic pressure P H  will exert an upward force on the balance piston  205 B, thereby tending to close the valve  200 . Additionally, the tubing pressure P T  will exert a downward force on the balance piston  205 B, however, this force does not tend to open the valve assembly  200  because the balance piston  205 B is structurally isolated from the flow tube  225  (and the biasing member  210 ) by interaction of the block  207  with the shoulder  214  of the chamber housing  255 . Thus, in this situation, the balance piston  205 B can never aid in opening the valve assembly  200 . Since the tubing pressure P T  is greater than P H  in this Figure, the balance piston  205 B is idle as it exerts no force on the flow tube  225  because a net downward force exerted by the tubing pressure P T  keeps the balance piston  205 B resting on the shoulder  214 . 
     FIG. 5B  is a free body diagram of the valve  200  in an open position (see also  FIG. 3 ). To open the valve from the closed position, where the balance piston  205 B is idle, the control pressure P C  is exerted on the control piston  205 A as discussed above. However, additional consideration of the tubing pressure P T  and the hydrostatic pressure P H  changes the analysis from the simplified analysis discussed above. The force exerted by the control pressure P C  that will be applied to open the valve will now have to overcome the force generated by the tubing pressure P T  as well as the force F S  generated by the biasing member  210  to open the valve but will be supplemented by the force exerted by the hydrostatic pressure P H  when the balance piston  205 B is idle (P T &gt;P H ). 
     FIG. 5C  is a free body diagram of the valve assembly  200  in a closed position where the balance piston  205 B is active (P T &lt;P H , see also  FIG. 4 ). Since the tubing pressure P T  is less than the hydrostatic pressure P H , the balance piston  205 B is active as a net (the upward force exerted on the balance piston  205 B by P H  less the downward force exerted by P T ) upward force on the balance piston  205 B will unseat the balance piston  205 B from the shoulder  214  of chamber housing  255  and mate with the shoulder  209  of the flow tube  225 , thereby tending to close the valve assembly  200 . Summation of the external forces acting on the flow tube  225  and cancellation of redundant terms will conclude that the only net force acting on the flow tube  225  is the force F S  generated by the biasing member  210 . Therefore, the undesirable effect of the hydrostatic pressure P H  exerting a downward force on the control piston  205 A, thereby tending to open the valve, is removed or negated. 
   To open the valve from the closed position, where the balance piston  205 B is active, the control pressure P C  is exerted on the control piston  205 A as discussed above. The force exerted by the control pressure P C  that will be applied will now have to overcome only F S  to open the valve but without the aid of the hydrostatic pressure P H  (since it is effectively cancelled by the activity of the balance piston  205 B). 
     FIGS. 6A and 6B  are hydraulic diagrams of alternate embodiments of the valve  200 . In both figures, a device  305  enabling manual override of the valve  200 , such as a rupture disc or rupture pin has been added to the valve. In the embodiment illustrated in  FIG. 6A , the override device  305  is disposed between the control line  145 A and a port (not shown) in fluid communication with the bore  260  of the valve. In the embodiment illustrated in  FIG. 6B , the override device  305  is disposed between the control line  145 A and the balance line  145 B. In both embodiments, the inlet side of the override device  305  is in fluid communication with the control line  145 A. Both embodiments address the contingency of failure of the balance piston seal assembly  215 B. The actuation pressure of the override device  305  may be set significantly above the operating pressure of the control line  145 A, to avoid unintentional actuation. In the event of balance seal assembly  215 B failure, the control line pressure P C  may be increased to actuate the override device  305 . 
   In the embodiment of  FIG. 6A , actuation of the device  305  will cause the control line  145 A to be in fluid communication with the bore  260  of the valve  200 . Once the device  305  has actuated, the control pressure P C  may be removed. The column of fluid in control line  145 A will then flow into the bore  260  of the valve  200  until the pressure in the control line  145 A is equal to the tubing pressure P T , thereby closing the valve. Similarly, in the embodiment of  FIG. 6B , actuation of the device  305  will cause the control line  145 A to be in fluid communication with the balance line  145 B. The column of fluid in control line  145 A will then flow around the balance piston  205 B into the bore  260  until the pressure in the control line  145 A is equal to the tubing pressure P T , thereby closing the valve. 
   In another alternative embodiment of the valve  200 , the balance piston  205 B would be modified to receive a second seal assembly between the balance seal assembly  215 B and the block  207 . This would create an intermediate pressure chamber between the two seal assemblies. A port would be provided to this pressure chamber and the port would be connected to the control line  145 A. This would create a “fail safe” valve. The failure of balance seal assembly  215 B would then be of little consequence to valve closure since the intermediate pressure chamber would be at the hydrostatic pressure P H  when attempting to close the valve  200 . Failure of the second seal assembly would have a similar result to actuation of the override device  305  in the embodiment of  FIG. 6A . Failure of both seal assemblies would have a similar result to actuation of the override device  305  in the embodiment of  FIG. 6B . 
   In yet another alternative embodiment of the valve  200 , a plurality of balance pistons would be included in the event of failure of one of the balance pistons. Additional balance lines could be run in with the valve or the additional balance pistons could be connected to the single balance line with bypass valves. 
   In yet another alternative embodiment of the valve  200 , the cross sectional area of the balance piston  205 B is larger than that of the control piston  205 A and the biasing member  210  is removed. The greater closing force of the larger balance piston compensates for the missing force generated by the biasing member  210 . 
   Although the invention has been described in part by making detailed reference to specific embodiments, such detail is intended to be and will be understood to be instructional rather than restrictive. It should be noted that while embodiments of the invention disclosed herein are described in connection with a subsurface safety valve assembly, the embodiments described herein may be used with any well completion equipment, such as a packer, a sliding sleeve, a landing nipple and the like. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.