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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention are generally related to safety valves. More particularly, embodiments of the present invention pertain to subsurface safety valves configured to be actuated using wellbore pressure in the event of an unexpected pressure drop. 
   2. Description of the Related Art 
   Subsurface safety valves are commonly used to shut-in oil and gas wells and 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. 
   Typically, subsurface safety valves are rigidly connected to the production tubing and may be installed and retrieved by conveyance means, 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. The 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. 
   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. 
   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. 
   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 effects of hydrostatic pressure on the hydraulic fluid within the control line. In other words, high-pressure wells have exceeded the capability of many existing control systems, especially hydraulic control systems which rely on control lines, which are susceptible to reliability problems. 
   Therefore, a need exists for a subsurface safety valve that is suitable for use in high pressure environments. There is a further need for a subsurface safety valve that does not rely on a control system that requires the use of control lines conveying hydraulic fluid to an actuating mechanism. There is yet a further need for the ability to reopen the safety valve remotely from the surface of the well. 
   SUMMARY OF THE INVENTION 
   In one respect, the present invention provides a downhole valve for selectively sealing a bore. The downhole 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 for initially holding the valve in an open position but, in the event of a pressure differential between the piston surfaces, permits the closing member to operate and close the valve. 
   In another respect, the present invention provides a method of operating a downhole valve. The method generally includes providing the valve in a down hole tubular, the valve having a closing member and an axially movable retention member having a first piston surface and an opposing piston surface and an interfering member to interfere with the closing member and keep the valve in the open position. A sudden pressure drop in the wellbore, shifts the retention member due to a pressure differential between the first and second piston surfaces, and closing the valve due to the axial movement of the interfering member way from the closing member. 
   In yet another respect, the present invention provides a safety valve for use downhole. The safety valve generally includes a pivotly mounted flapper, biased towards a closed position for sealing a bore, an interfering member to hold the flapper in an open position, a first piston surface in fluid contact with the bore, a second opposing piston surface in fluid communication with a pressure chamber having restricted fluid communication with the bore, wherein the valve is constructed and arranged to close in the event of a pressure difference between the bore and the chamber. 

   
     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 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. 
       FIG. 2A  is a cross-sectional view of the subsurface safety valve in an open position. 
       FIG. 2B  is a cross-sectional view of the subsurface safety valve of  FIG. 2A , shown in the closed position. 
       FIGS. 3A and 3B  illustrate cross-sectional views of a subsurface safety valve in accordance with an alternative embodiment of the present invention. 
       FIGS. 4A–4C  illustrate cross-sectional views of a subsurface safety valve in accordance with yet another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The apparatus and methods of the present invention allow for a subsurface safety valve for use in high pressure 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. 
     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  is used for controlling 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”) of production fluid 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 can be remotely reopened to reestablish the flow of production fluid. 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, and  4 A– 4 C. 
   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. 
     FIG. 2A  illustrates a cross-sectional view of a subsurface safety valve in a 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. 
   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  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 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 . 
   As stated earlier,  FIG. 2A  illustrates the safety valve  200  in the open position. It can be seen that the flow tube  204  is positioned such that it 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  is translated sufficiently upward to enable the flapper  203  to close completely and shut off flow of production fluid. 
   While production fluid is being 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 mandrel  201 A and the outer diameter of the flow tube  204  allows piston surface  209  to be in fluid communication with the wellbore. 
   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. 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 . 
   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 . In this embodiment, the orifice  208  is the only path for fluid to enter and exit the pressure chamber  205 . The orifice is designed to meter 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 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  204 . 
   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 which is positioned between piston surface  209  and piston surface  210  and the tube seal  207  on bottom. 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 . 
   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 and shutting-in the well as seen in  FIG. 2B . 
   After the flapper is closed, the pressure of the production fluid acting on the underside of the flapper  203  (pushing upward) is high enough to forceably keep the flapper  203  in the closed position. In terms of the pressure chamber  205 , it should be noted that starting from the instant of the rapid pressure loss (corresponding to the loss of flow control) the metered flow of fluid through the orifice 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. 
   Embodiments of the present invention also provide functionality to remotely reopen the subsurface safety valve  200 . Obviously, this would be done after flow control apparatus at the surface of the wellbore are 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 ). 
   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 . The spring  211  stays compressed, and the pressure chamber  205  does not return to it&#39;s previous volume because the flow tube  204  is not allowed to move downwards due to the closed flapper. 
   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 to the open position. Thereafter, the flow tube will bottom out against a corresponding interior shoulder of the bottom sub  202 . 
   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 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 does not restrict the flapper  203  from fully closing in the event of a loss of flow control. 
   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. 
   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 . 
   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  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  can pivot to block the upward flow of production fluid. 
   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. 
   The pressure chamber  305  is defined by the annular space between the bellows  306  and the flow tube  304 . 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 . There are two channels by which production fluid can enter the pressure chamber  305 : fluid can go past a packing  309 , or fluid can flow into the pressure chamber  305  via an orifice  308 . 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 . 
   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  provides the only path by which fluid is allowed to flow out of the chamber. The orifice  308  is configured to meter the fluid that flows through at a relatively low flow rate. 
   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. 
   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 . 
   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. 
   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. 
     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. 
   Referring now to  FIG. 4A , a safety valve  400  is provided with 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  is connected to a bottom sub  404 . Both the housing  401  and the bottom sub  404  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. 
   The lower end  422  of the crossover sub  402  seals into the lower housing  403  at position  422 . 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 lower housing  404 . 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 . 
   Referring now to  FIG. 4A , a safety valve  400  is provided with 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  is connected 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  which is rotationally attached by a pin  406 B to a flapper mount  406 C, each of which provides generally the same functionality as with other embodiments described above. 
   The lower end  422  of the crossover sub  402  seals into the lower housing  403  at position  422 . 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  403 , production fluid is not able to flow past the seal between the lower end  422  of the crossover Sub  402  and the lower housing  403 . 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 . 
   In the event of a sudden pressure drop, 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  421  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  403  closes and shuts-in the well. 
   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 sleeve  409 . 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. 
   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. 
   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. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
The present invention generally provides a downhole valve for selectively sealing a bore. The downhole 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 for initially holding the valve in an open position but, in the event of a pressure differential between the piston surfaces, permits the closing member to operate and close the valve.