Abstract:
The present invention generally provides a method and apparatus for selectively sealing a bore. The tubular valve generally includes a closing member for seating in and closing the bore, and a pressure-actuated, retention member having first and second opposed piston surfaces opening and closing the valve. The tubular valve prevents sudden loss of pressure in the tubular and is controllable from the surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/957,240 filed Oct. 1, 2004. Further, this application claims benefit of U.S. provisional patent application Ser. No. 60/664,487 filed Mar. 23, 2005, which is herein incorporated by reference. Each of the aforementioned related patent applications are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Embodiments of the present invention generally relate to safety valves. More particularly, embodiments of the present invention pertain to subsurface safety valves configured to actuate using wellbore pressure in the event of an unexpected pressure drop. More particularly still, embodiments of the present invention pertain to the further ability to control the safety valves from the surface.  
         [0004]     2. Description of the Related Art  
         [0005]     Subsurface safety valves are commonly used to shut-in oil and gas wells. The safety valves are typically fitted in a string of production tubing installed in a hydrocarbon producing well. The safety valves are configured to selectively seal fluid flow through the production tubing to control the flow of formation fluids upwardly should a failure or hazardous condition occur at the well surface.  
         [0006]     Typically, subsurface safety valves are rigidly connected to the production tubing and may be installed and retrieved by known conveyance methods, such as tubing or wireline. During normal production, safety valves are maintained in an open position by the application of hydraulic fluid pressure transmitted to an actuating mechanism. The actuating mechanism in such embodiments may be charged by application of hydraulic pressure through hydraulic control systems. Hydraulic control systems may comprise a clean oil supplied from a surface fluid reservoir through a control line. A pump at the surface delivers regulated hydraulic fluid under pressure from the surface to the actuating mechanism through the control line. The control line resides within the annular region between the production tubing and the surrounding well casing.  
         [0007]     In the event of a failure or hazardous condition at the well surface, fluid communication between the surface reservoir and the control line is interrupted. This, in turn, breaks the application of hydraulic pressure against the actuating mechanism. The actuating mechanism recedes within the valve, allowing a flapper to quickly and forcefully close against a corresponding annular seat—resulting in shutoff of the flow of production fluid. In many cases, the flapper can be reopened (and production flow resumed) by restoring the hydraulic fluid pressure to the actuating mechanism of the safety valve via the control lines.  
         [0008]     For safety reasons, most surface controlled subsurface safety valves (such as the ones described above) are “normally closed” valves, i.e., the valves are in the closed position when the hydraulic pressure in the control lines is not present. The hydraulic pressure typically works against a powerful spring and/or gas charge acting through a piston. In many commercially available valve systems, the power spring is overcome by hydraulic pressure acting against the piston, producing axial movement of the piston. The piston, in turn, acts against an elongated “flow tube.” In this manner, the actuating mechanism is a hydraulically actuated and axially movable piston that acts against the flow tube to move it downward within the tubing and across the flapper. These safety valves require a control system for operation from the surface in order to open the valve and produce.  
         [0009]     Safety valves employing control lines, as described above, have been implemented successfully for standard depth wells with reservoir pressures that are less than 15,000 psi. However, wells are being drilled deeper, and the operating pressures are increasing correspondingly. For instance, formation pressures within wells developed in some new reservoirs are approaching 30,000 psi. In such downhole environments, conventional safety valves utilizing control lines are not operable because of the pressure limitations of the control line. In other words, high-pressure wells have exceeded the capability of many existing control systems.  
         [0010]     Therefore, a need exists for a subsurface safety valve that is equipped with a self contained control system without control lines conveying hydraulic fluid to an actuating mechanism. A further need exists for a subsurface safety valve that is suitable for use in high pressure environments. There is yet a further need for the ability to reopen the safety valve remotely from the surface of the well. There is a further need for the ability to close the safety valve from the surface.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention generally can be a wireline or a tubing safety valve which can be operated from the surface of the well.  
         [0012]     The present invention generally provides a method and apparatus for selectively sealing a bore. The tubular valve generally includes a closing member for seating in and closing the bore, and a pressure-actuated, retention member having first and second opposed piston surfaces opening and closing the valve. The tubular valve prevents sudden loss of pressure in the tubular and is controllable from the surface.  
         [0013]     In one embodiment the invention is a downhole valve for selectively sealing a bore. The valve includes a closing member for sealing the bore, a retention member having first and second piston surfaces for initially holding the closing member in an open position, a pressure chamber for applying pressure to the second piston surface, and a control line in communication with the pressure chamber.  
         [0014]     In another embodiment the invention is a method of operating a downhole valve. The method includes providing the valve in a downhole tubular, the valve having: a closing member for sealing a bore, a retention member having a first and second piston surface, mechanically biased to interfere with a closing member normally keeping the valve open, a pressure chamber in communication with the second piston surface and a control line in communication with the pressure chamber. The method further includes applying a wellbore pressure to the first piston surface and increasing the pressure in the pressure chamber to a level sufficient to overcome the mechanical bias of the retention member, but insufficient to overcome both the pressure on the first piston surface and the mechanical bias.  
         [0015]     In yet another embodiment the invention is a downhole valve. The valve includes a flapper mechanically biased to seal a bore, a retention member mechanically biased to interfere with the flapper to maintain the bore in the open position, a pressure chamber for controllably moving the retention member out of interference with the flapper, a control line for controlling the pressure chamber, and a bore pressure for applying a force to the retention member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     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.  
         [0017]      FIG. 1  is a cross-sectional view of a wellbore illustrating a string of production tubing having a subsurface safety valve in accordance with one embodiment of the present invention.  
         [0018]      FIG. 2A  is a cross-sectional view of the subsurface safety valve in an open position.  
         [0019]      FIG. 2B  is a cross-sectional view of the subsurface safety valve of  FIG. 2A , shown in the closed position.  
         [0020]      FIGS. 3A and 3B  illustrate cross-sectional views of a subsurface safety valve in accordance with an alternative embodiment of the present invention.  
         [0021]      FIGS. 4A-4C  illustrate cross-sectional views of a subsurface safety valve in accordance with yet another embodiment of the present invention.  
         [0022]      FIG. 5  is a chart illustrating the operation of the subsurface safety valve when an orifice design is used.  
         [0023]      FIG. 6  is a chart illustrating the operation of the subsurface safety valve with no orifice. 
     
    
     DETAILED DESCRIPTION  
       [0024]     The apparatus and methods of the present invention allow for a safety valve for subsurface wells. Embodiments of the present invention provide safety valves that utilize normal wellbore pressure for actuation of the valve, which removes the need for hydraulic systems with control lines extending from the surface to the valve, however, a control system is incorporated into the invention for further control of the valve.  
         [0025]      FIG. 1  is a cross-sectional view of an illustrative wellbore  10 . The wellbore is completed with a string of production tubing  11 . The production tubing  11  defines an elongated bore through which servicing fluid may be pumped downward and production fluid may be pumped upward. The production tubing  11  includes a safety valve  200  in accordance with one embodiment of the present invention. The safety valve  200  controls the upward flow of production fluid through the production tubing  11  in the event of a sudden and unexpected pressure loss (also referred to herein as a “pressure drop”). The pressure drop may coincide with a corresponding increase in flow rate within the production tubing  11 . Such a condition could be due to the loss of flow control (i.e., a blowout) of the production fluid at the wellbore surface. In the event of such a condition, a subsurface safety valve, implemented according to embodiments of the current invention, automatically actuates and shuts off the upward flow of production fluid. Further, when flow control is regained at the surface, the safety valve is remotely reopened to reestablish the flow of production fluid. Further still, the safety valve is remotely closed or opened to shut off or reestablish flow of production fluid at any time through use of a control line  600 . Discussion of the components and operation of embodiments of the safety valve of the present invention are described below with reference to  FIGS. 2A-2B ,  3 A- 3 B,  4 A- 4 C,  5  and  6 .  
         [0026]     It should be understood, that as used herein, the term “production fluid” may represent both gases or liquids or a combination thereof. Those skilled in the art will recognize that production fluid is a generic term used in a number of contexts, but most commonly used to describe any fluid produced from a wellbore that is not a servicing (e.g., treatment) fluid. The characteristics and phase composition of a produced fluid vary and use of the term often implies an inexact or unknown composition.  
         [0027]      FIG. 2A  illustrates a cross-sectional view of a subsurface safety valve in an open position, in accordance with one embodiment of the present invention. The safety valve  200  comprises an upper housing  201 A threadedly connected to a lower housing  201 B, which, in turn, is threadedly connected to a bottom sub  202 . The upper housing  201 A makes up the top of the safety valve  200  and extends upward. Accordingly, the bottom sub  202  makes up the bottom of the safety valve  200  and extends downward. Both the upper housing  201 A and the bottom sub  202  are configured with threads to facilitate connection to production tubing  11  (or other suitable downhole tubulars) above and below the safety valve  200 , respectively.  
         [0028]     The safety valve  200  comprises a flapper  203  and a flow tube  204 . The flapper  203  is rotationally attached by a pin  203 B to a flapper mount  203 C. The flapper  203  is mechanically or hydraulically biased toward the closed position. The flapper  203  pivots between an open position and a closed position in response to axial movement of the flow tube  204 . As shown in  FIG. 2A , the flapper  203  is in the open position creating a fluid pathway through the bore of the flow tube  204 , thereby allowing the flow of fluid through the safety valve  200 . Conversely, in the closed position, the flapper  203  blocks the fluid pathway through the bore of the flow tube  204 , thereby preventing the flow of fluid through the valve  200 .  
         [0029]     As stated earlier,  FIG. 2A  illustrates the safety valve  200  in the open position. In the open position, the flow tube  204  physically interferes with and restricts the flapper  203  from closing. As will be described with reference to  FIG. 2B , when the safety valve  200  is in the closed position, the flow tube  204  translates sufficiently upward to enable the flapper  203  to close completely and shut off flow of production fluid.  
         [0030]     While production fluid is conveyed to the surface under stable and controlled conditions, the safety valve  200  remains in the open position. Under such conditions, the flow tube  204  remains bottomed out against an upward facing internal shoulder  230  of the bottom sub  202 , thereby restricting the flapper  203  from closing. The flow tube  204  is held in this position due to a net downward force resulting from the force exerted by a spring  211  biased towards the extended position. A gap  231  between the inner diameter of the upper housing  201 A and the outer diameter of the flow tube  204  allows a piston surface  209  to be in fluid communication with the wellbore.  
         [0031]     As shown in  FIG. 2A , a pressure chamber  205  is located in the annular space between the outer diameter of the flow tube  204  and the inner diameter of the lower housing  201 B. The pressure chamber  205  is bound by a piston seal  206  on top and the tube seal  207  on bottom. The pressure chamber  205  contains an opening  605  with a control line  600  attached to it. The control line  600  allows for adjustment of the pressure in the pressure chamber  205  from the surface. A spring  211  is also located in the annular area between lower housing  201 B and the flow tube  204 . The spring is held in place by a spring retainer  212  and surface  213  of the flow tube  204 .  
         [0032]     In one embodiment, during normal operation, while the valve  200  is in the open position, the pressure chamber  205  is filled with production fluid that enters the pressure chamber  205  through an orifice  208 . The orifice  208  meters flow that passes through it, regardless of whether the fluid is entering or exiting the pressure chamber  205 . While the valve  200  is in the open position, the fluid flow through the orifice  208  ensures that the pressure of the fluid inside the pressure chamber  205 , acting on surface  210  eventually equalizes with the pressure of the fluid flowing through the bore of the flow tube  204  and acting on the piston surface  209 .  
         [0033]     In the event of a catastrophic failure at the surface of the wellbore and loss of flow control, the safety valve  200  automatically closes, as seen in  FIG. 2B . The loss of flow control typically means that production fluid is flowing upward at a flow rate that is much higher than normal. In keeping with Bernoulli&#39;s Rule, the pressure of production fluid flowing through the bore of the flow tube  204  is much lower than prior to loss of flow control. However, the pressure in the pressure chamber  205  is not reduced in unison with the production flow pressure. This is because the metering effect of the orifice  208  does not allow the fluid to flow out of the pressure chamber  205  to allow for the equalization process to occur immediately. Accordingly, for a particular time span, the pressure of the fluid flowing through the bore and acting on the piston surface  209  is appreciably lower than the pressure of fluid in the pressure chamber  205  acting on the surface  210 .  
         [0034]     The pressure difference between the fluid within the pressure chamber  205  and the production fluid results in the pressure chamber  205  increasing in volume and the flow tube  204  being urged upward. It should be noted that as the flow tube  204  moves upward, it meets resistance as the spring  211  is compressed. Provided that the pressure difference is large enough and the pressure chamber  205  expands sufficiently, the flow tube  204  travels sufficiently upward so that it no longer restricts the flapper  203  from closing as seen in  FIG. 2B .  
         [0035]     Further, the safety valve  200  can close at any time through use of control line  600 . The control line  600  monitors and regulates the pressure in the pressure chamber  205  at the surface. To close the safety valve  200  the control line  600  increases the pressure in the pressure chamber  205  until the pressure acting on surface  210  is large enough to overcome the spring  211  force and the pressure acting on piston surface  209 . The control line  600  can further remove pressure from the pressure chamber  205  allowing the safety valve  200  to remain open if desired. Further, this control line  600  can be used to gather more volume for the pressure chamber  205 . The control line  600  monitors any volume changes in the pressure chamber  205 , allowing for better control of the safety valve  200  from the surface.  
         [0036]     In another embodiment, the orifice  208  is not present. Only the control line  600  can relieve the pressure in pressure chamber  205 . The pressure in the pressure chamber  205  increases from the static wellbore pressure and can be decreased as desired with the control line  600 . With the pressure in the pressure chamber  205  lower than that required to overcome the spring  211  force, the safety valve  200  remains open. In a normal producing well the production fluid pressure acts on surface  209  to act with the spring  211  force in order to keep the safety valve  200  open. An increase in the pressure chamber  205  pressure sufficient to overcome the spring  211  force, but insufficient to overcome the production fluid pressure acting on surface  209  and the spring  211  force will have no effect on the open safety valve  200 . If a sudden loss of production fluid pressure occurs in the production tubular the pressure inside the pressure chamber  205  forces the safety valve  200  closed as described above. In this embodiment, however the pressure chamber  205  will not automatically equalize with the production fluid pressure.  
         [0037]     In yet another embodiment, the orifice  208  described in the preceding paragraph, operates as a one way valve. The orifice  208  allows fluid from the bore to enter the pressure chamber  205 , but not exit. Thus, the pressure in the pressure chamber  205  equalizes with the wellbore pressure, if the control line  600  is not used. In the event of a sudden pressure loss, the flow tube  204  will move upward, allowing the flapper  203  to close, as described above. The pressure chamber  205  is controllable with the control line  600 , but is not necessary in order for operation of the valve  200 .  
         [0038]     After the flapper  203  closed, the pressure of the production fluid acting on the underside of the flapper  203  (pushing upward) is enough to forceably keep the flapper  203  in the closed position. In terms of the pressure chamber  205 , it should be noted if the orifice  208  is present the instant of the rapid pressure loss (corresponding to the loss of flow control) the metered flow of fluid through the orifice  208  allows for the pressure equalization process to resume. However, even after the pressure equalizes again, the pressure of the downhole fluid against the bottom-side of the flapper will keep it shut.  
         [0039]     Embodiments of the present invention also provide functionality to remotely reopen the subsurface safety valve  200 . Obviously, this would be done after the flow control apparatus at the surface of the wellbore is returned to working order. In order to reopen the safety valve  200  from the surface, fluid is pumped down to the safety valve  200  and the pressure is built up so that the pressure above the flapper  203  is the same as the pressure of the production fluid below the flapper  203  (i.e., pressure is equalized across the flapper  203 ).  
         [0040]     It should be noted that by this time, the flow of fluid through the orifice  208  has allowed pressure of fluid within the pressure chamber  205  to again equalize with the pressure of fluid outside the pressure chamber  205 . In an embodiment without the orifice  208  the system equalizes when desired by the operator. The spring  211  stays compressed, and the pressure chamber  205  does not return to its previous volume because the flow tube  204  is not allowed to move downwards due to the closed flapper  203 .  
         [0041]     However, once there is equal pressure on both sides of the flapper  203 , the spring  211 , biased towards the extended position, will urge the flow tube  204  downwards, which in turn will push the flapper  203  to the open position. Thereafter, the flow tube will bottom out against a corresponding internal shoulder  230  of the bottom sub  202 .  
         [0042]     With reference to the discussion above, it can be understood that the amount of upward movement of the flow tube  204  is dependent on the difference in pressure (i.e., “pressure drop”) between the fluid in the pressure chamber  205  and the pressure of the fluid flowing through the bore of the flow tube  204  at the moment of loss of flow control. In other words, the higher the difference in pressure between the fluid in the pressure chamber  205  and the fluid flowing through the bore of the flow tube  204 , the greater the amount of upward movement of the flow tube  204 . Maximizing upward movement of the flow tube  204  is important because it ensures that the flow tube  204  does not restrict the flapper  203  from fully closing in the event of a loss of flow control.  
         [0043]     Other embodiments of the present invention are envisioned for providing more upward movement of the flow tube for a given pressure drop.  FIG. 3A , for instance, illustrates a cross-sectional view of a subsurface safety valve configured with bellows according to an alternative embodiment of the present invention. As will be described below, use of bellows for creating a pressure chamber is beneficial because bellows provide a large change in volume between the compressed and uncompressed position. Greater variance in the volume of the pressure chamber while the safety valve is in the open position versus closed position translates into more axial movement of the flow tube, which ensures complete closure of the flapper.  
         [0044]     Referring now to  FIG. 3A , a safety valve  300  is provided with a housing  301  that is threadedly connected to a bottom sub  302 . Both the housing  301  and the bottom sub  302  are configured with threaded connections to allow for installing the safety valve  300  in a string of production tubing  11 .  
         [0045]     As with the embodiment described earlier, safety valve  300  comprises a flapper  303  and a flow tube  304 . The flapper  303  is rotationally attached by a pin  303 B to a flapper mount  303 C. The flapper  203  is mechanically or hydraulically biased toward the closed position. The flapper  303  pivots between an open position and a closed position in response to axial movement of the flow tube  304 . As shown in  FIG. 3A , the safety valve  300  is in the open position; the flow tube  304  restricts the flapper  303  from pivoting. However, with sufficient upward movement of the flow tube  304 , the flapper  303  pivots to block the upward flow of production fluid.  
         [0046]     An important component of this embodiment is the use of bellows  306  for creating an expandable pressure chamber  305 . The bellows  306  may be made of a variety of materials, including, but not limited to metals. For one embodiment, the bellows  306  are configured with pleated metal to facilitate a volumetric variance between its compressed and uncompressed positions.  
         [0047]     The annular space between the bellows  306  and the flow tube  304  define the pressure chamber  305 . The pressure chamber  305  is bound on the top by the connection between the bellows  305  and the bellows retainer  307 . The lower end of the pressure chamber  305  is bound by a cap  320 . In one embodiment, there are two or more channels by which production fluid can enter the pressure chamber  305 : fluid can enter through opening  605  through which control line  600  passes, fluid can go past a packing  309 , or fluid can flow into the pressure chamber  305  via an orifice  308 . The control line  600  operates in the same manner as described above and can go through any part of the housing  301  so long as it is in fluid communication with the pressure chamber  305 . While the valve  300  is in the open position, the fluid flow through the orifice  308  and the packing  309  ensures that the pressure of the fluid inside the pressure chamber  305  is equalized with the pressure of the fluid flowing through the bore of the flow tube  304 .  FIG. 3B  provides a detailed view of the orifice  308  and the packing  309 .  
         [0048]     In the context of the current application, the packing  309  can be thought of as a one-way valve. As seen in  FIG. 3A , the packing  309  is configured to allow fluid to flow into the pressure chamber  305 , but not out of it. An orifice  308  is also provided to allow for fluid to flow into the pressure chamber  305 . It should be noted that the orifice  308  and control line  600  provide the only paths by which fluid is allowed to flow out of the pressure chamber  305 . The orifice  308  meters the fluid that flows through at a relatively low flow rate.  
         [0049]     A pressure equalization port  321  extending through the cap  320  is provided to ensure that the pressure on either side of the cap  320  is equalized. Further, the port  321  provides a secondary path for production fluid to reach the packing  309  in the event that the path formed around the bottom end of the flow tube  304  and through the area adjacent to the flapper  303  is plugged.  
         [0050]     The safety valve  300  comprises a spring  311  that resists the upward movement of the bellows retainer  307  and the flow tube  304 . The bottom of the spring  311  rests against the bellows retainer  307 . The top portion of the spring  311  interfaces with a downward-facing internal shoulder of the housing  301 . In the open position of the safety valve  300 , with the flow tube  304  bottomed out, the spring  311  is fully extended. In the closed position of the safety valve  300 , with the flow tube  304  all the way up, the spring  311  is compressed and it exerts a downward force against the bellows retainer  307 .  
         [0051]     This embodiment operates the same as the previous embodiment. In the event of a loss of flow control at the surface of the wellbore, there would be a pressure drop between the fluid flowing through the bore of the flow tube  304  and the fluid in the pressure chamber  305 . As with the previous embodiment, the pressure in the pressure chamber  305  is not reduced in concert with the pressure of the production flow because the metering effect of the orifice  308  does not allow the fluid to flow out of the pressure chamber  305  to allow for pressure equalization to occur immediately. As a result, the pressure chamber  305  expands by extending the bellows  306  axially, which, in turn, urges the bellows retainer  307  and flow tube  304  to move upward, compressing the spring  311 . Upon sufficient upward movement of the flow tube  304 , the flapper  303  will close to shut-in the wellbore.  
         [0052]     Further, the safety valve  300  can be closed at any time through use of the control line  600 . The control line  600  monitors and regulates the pressure in the pressure chamber  305  at the surface. To close the safety valve  300  the control line  600  increases the pressure in the pressure chamber  305  which expands the bellows axially until the force acting on a bellow retainer  307  is large enough to overcome the spring  311  force and the pressure acting on a surface  319 . The control line  600  can further remove pressure from the pressure chamber  305  allowing the safety valve  300  to remain open if desired. Further, the control line  600  can be used to gather more volume for the pressure chamber  305 . The control line  600  can be used to monitor any volume changes in the pressure chamber  305 , allowing for better control of the safety valve  300  from the surface.  
         [0053]     In another embodiment, the orifice  308  is not present. The flow path past the packing  309  is optional. Without the flow path only the control line  600  controls the pressure in the pressure chamber  305  (described above). The pressure in the pressure chamber  305  increases and decreases as desired with the control line  600 . When the pressure in the pressure chamber  305  is lower than that required to overcome the spring  311  force, the safety valve  300  remains open. In a normal producing well the production fluid acts on surface  319  to act with the spring  311  force in order to keep the safety valve  300  open. An increase in the pressure chamber  305  pressure sufficient to overcome the spring  311  force but insufficient to overcome the production fluid pressure acting on surface  319  and the spring  311  force has no effect on the open safety valve. If a sudden loss of fluid pressure occurs in the production tubular the pressure inside the pressure chamber  305  forces the safety valve  300  closed as described above. In this embodiment, however the pressure chamber  305  will not automatically equalize with the production fluid pressure.  
         [0054]     In yet another embodiment the flow path past packing  309  is present without the orifice  308 . This allows fluid from the bore to enter the pressure chamber  305 , but not exit. Thus, the pressure in the pressure chamber  305  equalizes with the wellbore pressure, if the control line  600  is not used. In the event of a sudden pressure loss, the flow tube  304  will move upward allowing the flapper  303  to close, as described above. The pressure chamber  305  is controllable with the control line  600 , but it is not necessary in order for operation of the valve  300 .  
         [0055]     As with the embodiment described earlier with reference to  FIGS. 2A and 2B , the valve can be reopened by equalizing pressure on both sides of the flapper  303  and allowing the spring  311  to urge the flow tube  304  downwards. This, in turn, would return the flapper  303  to the open position.  
         [0056]      FIG. 4A  illustrates yet another embodiment of the present invention that is designed to provide additional axial movement of the flow tube for a given pressure drop. A cross-sectional view of a subsurface safety valve configured with extension rods sliding in their corresponding cylinders is provided. As will be described below, the axial movement of rods for expanding a pressure chamber is beneficial because the process of displacing rods in cylinders with fluid can yield a tremendous amount of axial movement of a flow tube for a given pressure drop. As stated earlier, complete upward movement of the flow tube ensures complete closure of the flapper.  
         [0057]     Referring now to  FIG. 4A , a safety valve  400  has a housing  401  that is threadedly connected to a crossover sub  402 , which is threadedly connected to a lower housing  403 . The lower housing  403  connects to a bottom sub  423 . Both the housing  401  and the bottom sub  423  are configured with threaded connections to allow for installing the safety valve  400  in a string of production tubing  11 . As with previously described embodiments, the safety valve  400  includes a flow tube  404 , spring  411  and flapper  406 , each of which provides generally the same functionality as with other embodiments described above.  
         [0058]     The lower end  422  of the crossover sub  402  seals into the lower housing  403 . It should be understood that because the lower end  422  of the crossover sub  402  is sealingly connected (e.g., press fit, static seal, etc.) to the lower housing  404 , production fluid is not able to flow past the seal between the lower end  422  of the crossover sub  402  and the bottom sub  423 . However, the lower end  422  of the crossover sub  402  does contain an orifice  408  that allows fluid to flow into and out of a pressure chamber  405 . Fluid arrives at the orifice  408  by flowing around the top or bottom of the flow tube  404  and within the annular space between the lower end  422  of the crossover sub  402  and flow tube  404 .  
         [0059]     The pressure chamber  405  is defined by the annular space between the lower housing  403  and the lower end of the crossover sub  402 . The pressure chamber  405  also includes the bores within the crossover sub  402  in which rods  420  are located. The pressure chamber  405  contains an opening  605  with a control line  600  attached to it. The control line  600  allows for adjustment of the pressure in the pressure chamber  405  from the surface. Fluid can flow into the pressure chamber  405  one or more ways: via the orifice  408 , and/or by flowing past rod packings  421  and through the control line  600  as described above. As with the packing  309  described with reference to the previous embodiment, rod packings  421  function as one-way valves, wherein fluid is allowed to flow into the pressure chamber  405  (downwards) past the rods  420 , but the fluid is not allowed to flow out from the pressure chamber  405  (upward) past the interface between the rods  420  and the rod packings  421 .  FIG. 4B  provides a detailed view of the interface between a rod  420  and a rod packing  421 .  
         [0060]     During normal operation, while the valve  400  is in the open position, the pressure chamber  405  is filled with the production fluid. While the valve  400  is in the open position, the fluid flow into the pressure chamber  405  ensures that the pressure of the fluid inside the chamber is equalized with the pressure of the fluid flowing through the bore of the flow tube  404 .  
         [0061]     In the event of a sudden pressure drop, as described in the previous embodiments, the fluid is not capable of immediately exiting the pressure chamber via the orifice  408  (for purposes of pressure equalization), so the pressure in pressure chamber  405  is higher than the pressure of the flowing production fluid. Consequently, the pressure chamber  405  expands and displaces the rods  420  upward from the cylinders. The rods  420  move the flow tube  404  upward against the spring  411 . After the flow tube  404  has moved sufficiently upward, the flapper  406  closes and shuts-in the well.  
         [0062]     Further, the safety valve  400  can close at any time through use of control line  600 . The control line  600  monitors and regulates the pressure in the pressure chamber  405  at the surface. To close the safety valve  400  the control line  600  increases the pressure in the pressure chamber  405  until the pressure acting on a surface  410  of the piston  407  is large enough to overcome the spring  411  force and the pressure acting on a surface  409 . The control line  600  can further remove pressure from the pressure chamber  405  allowing the safety valve  400  to remain open if desired. Further, this control line  600  can be used to gather more volume for the pressure chamber  405 . The control line  600  monitors any volume changes in the pressure chamber  405 , allowing for better control of the safety valve  400  from the surface.  
         [0063]     In another embodiment, the orifice  408  is not present. The flow path past the packing rods  421  is optional. Without the flow path only the control line  600  controls the pressure in the pressure chamber  405  (described above). The pressure in the pressure chamber  405  increases and decreases as desired with the control line  600 . With the pressure in the pressure chamber  405  is lower than that required to overcome the spring  411  force, the safety valve  400  remains open. In a normal producing well the production fluid acts on surface  409  to act with the spring  411  force in order to keep the safety valve  400  open. An increase in the pressure chamber  405  pressure by the control line  600  sufficient to overcome the spring  411  but insufficient to overcome the production fluid pressure acting on surface  409  and the spring  411  force has no effect on the open safety valve  400 . If a sudden loss of fluid pressure occurs in the production tubular, the pressure inside the pressure chamber  405  forces the safety valve  400  to close as described above. In this embodiment, however, the pressure chamber  405  will not automatically equalize with the production fluid pressure.  
         [0064]     In yet another embodiment the flow path past rod packings  421  is present without the orifice  408 . This allows fluid from the bore to enter the pressure chamber  405 , but not exit. Thus, the pressure in the pressure chamber  405  equalizes with the wellbore pressure, if the control line  600  is not used. In the event of a sudden pressure loss the flow tube  404  will move upward, allowing the flapper  406  to close, as described above. The pressure chamber  405  is controllable with the control line  600 , but it is not necessary in order for operation of the valve  400 .  
         [0065]     It can be seen from  FIG. 4C  that the collective cross-sectional area of rods  420  is considerably less than the annular area between the inner diameter of the lower housing  403  and the lower end of the crossover sub  402 . Accordingly, the use of rods  420  in this manner requires less expansion of pressure chamber  405  to achieve the required amount of axial movement of the flow tube  404  to allow the flapper  403  to close. This is because the volumetric change of the pressure chamber  405  need only be enough to displace the volume of the rods  420 , rather than the entire annular area between the lower mandrel and the flow tube  404 . While three rods  420  are shown for the current embodiment, it should be understood that the number of rods can vary based on the requirements of a particular implementation.  
         [0066]     Those skilled in the art will recognize that safety valves according to embodiments of the present invention may be utilized in any wellbore implementation where a pressure differential (i.e. pressure drop) may arise. For instance, the safety valves described herein are fully functional if there is a pressure differential between fluid in the pressure chamber and fluid flowing through the bore of the safety valve, regardless of the absolute pressures of the respective fluids. Therefore, safety valves according to embodiments of the present invention may be utilized in low pressure wellbores as well as high pressure wellbores.  
         [0067]     While the exemplary safety valves described herein are configured for use with production tubing, those skilled in the art will acknowledge that embodiments of the present invention may be configured for use in a variety of wellbore implementations. For example, some embodiments of the present invention may be implemented as safety valves configured for use with wireline. Yet other embodiments may be configured for use with drill pipe or coiled tubing.  
         [0068]      FIG. 5  illustrates a chart for the operation of the safety valve  200 ,  300  and  400  with use of the orifice  208 ,  308  and  408 . As shown the solid line  700  represents the flowing wellbore pressure. The upper dashed line  710  represents the pressure in the pressure chamber  205 ,  305  and  405 , and the distance between the upper dashed line  710  and the lower dashed line  720  represents the pressure drop required in the wellbore to close the valve  200 ,  300  and  400 . As can be seen as the wellbore pressure decreases naturally the pressure in the pressure chamber  205 ,  305  and  405  also decreases, which enables the valve  200 ,  300  and  400  to remain open. If a sudden drop in wellbore pressure occurs as shown by the solid line branch  730  the valve  200 ,  300  and  400  closes upon the line reaching the pressure of the lower dashed line  720 . If need be, the pressure in the pressure chamber can increase or decrease with the control line and the valve  200 ,  300  and  400  could be closed or remain open.  
         [0069]      FIG. 6  illustrates a chart for the operation of the safety valve  200 ,  300 , and  400  without use of the orifice  208 ,  308 , and  408 . As shown the solid line  800  represents the natural wellbore pressure. The upper dashed line  810  represents the pressure in the pressure chamber  205 ,  305 , and  405 , and the distance between the upper dashed line  810  and the lower dashed line  820  represents the pressure drop required in the wellbore to close the valve  200 ,  300 , and  400 . As can be seen as the wellbore pressure decreases naturally the pressure in the pressure chamber  205 ,  305 , and  405  remains constant. Therefore as the wellbore pressure naturally decreases the pressure required to overcome the spring  211 ,  311 , and  411  and wellbore pressure decreases. In this case, a stronger spring  211 ,  311 , and  411  may be required in order to ensure the valve  200 ,  300 , and  400  does not close during normal operation. If a sudden drop in wellbore pressure occurs as shown by the solid line branch  830  the valve  200 ,  300 , and  400  closes upon the line  830  reaching the pressure of the lower dashed line  820 . If need be, the pressure in the pressure chamber  205 ,  305 , and  405  can increase or decrease with the control line  600  and the valve  200 ,  300 , and  400  could be closed or remain open.  
         [0070]     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.