Tubing Pressure Balanced Operating System with Low Operating Pressure

Abstract

A control system for a subsurface safety valve features a control line from the surface leading to one side of an operating piston. The operating piston is exposed to tubing pressure and is in pressure balance from tubing pressure. The opposite end of the operating piston is exposed to a chamber in the housing of the subsurface safety valve and that chamber is open through a port to the surrounding annulus. The operating piston is coupled to the flow tube and a power spring so that pressure in the control line shifts the operating piston and the flow tube against the power spring to rotate the flapper open. Provisions for fail closed mode if certain seals fail are also made though a passage extending from between seals near the upper end of the operating piston to the chamber open to the annulus.

1 . A control system for control of a component of a downhole tool, said tool defining an annulus around it when disposed downhole, comprising:
 an actuating piston, sealingly mounted using at least one seal in a housing, one of said piston or said housing being operably connected to the component;   a control line from said housing to the well surface, said control line in fluid communication with a first end of said piston;   said second end of said piston in fluid communication the annulus.   
   
   
       2 . The system of  claim 1 , wherein:
 said component comprises a flow tube in a subsurface safety valve;   said piston is operably connected to the flow tube;   said seals putting said piston in pressure balance to pressures within said housing.   
   
   
       3 . The system of  claim 2 , further comprising:
 a biasing member acting on said piston;   said piston movable from a first position under the bias of said biasing member, where the SSV is closed, to a second position where the force from said biasing member is overcome by applied pressure in said control line to open the SSV.   
   
   
       4 . The system of  claim 2 , further comprising:
 said housing defines an opening through which said piston is operably connected to the flow tube;   said piston comprising a passage therein to direct leakage flow therethrough in the event of failure of at least one of said seals.   
   
   
       5 . The system of  claim 4 , wherein:
 said piston comprises a pair of spaced seals adjacent said first end and at least one seal adjacent said second end;   said passage extends from between said spaced seals to said second end of said piston.   
   
   
       6 . The system of  claim 5 , wherein:
 said biasing member comprises a spring or other stored energy source which exerts a force on said piston toward said piston's first position which exceeds the hydrostatic force in said control line less the hydrostatic force in the annulus and plus the weight and frictional forces acting on or through said piston.   
   
   
       7 . The system of  claim 5 , wherein:
 said piston comprises a rod piston;   said biasing member comprises a return spring which can be overcome with an applied pressure in said control line of less than the hydrostatic pressure in said control line.   
   
   
       8 . A method of controlling a downhole tool which is disposed in a well to define an annulus around it and having an internal component actuated by a piston, comprising:
 mounting said piston in a housing, said housing having a passage therethrough;   operatively connecting said piston to the internal component disposed in said passage;   providing seals in contact with said piston that are exposed to pressure in said passage;   actuating said piston to move in a first direction by a control line from the surface;   offsetting the hydrostatic forces on said piston from said control line with an opening in said housing putting an opposite end of said piston in fluid communication to the annulus.   
   
   
       9 . The method of  claim 8 , comprising:
 providing a leak path through said piston to the annulus;   directing leakage from said passage and past said seals to said leak path.   
   
   
       10 . The method of  claim 9 , further comprising:
 providing a pair of spaced seals in contact with said piston above an access opening to said passage, with one being uppermost and the other, lowermost;   running said leak path from between said spaced seals to a lower end of said piston in communication with the annulus;   providing a lower seal on said piston below said access opening and above a lower end of said piston.   
   
   
       11 . The method of  claim 10 , further comprising:
 providing a return spring acting on said piston to bias it toward a position to move said internal component;   sizing said return spring to overcome hydrostatic forces in said control line and weight and frictional forces acting on or through said piston minus the pressure in the annulus.   
   
   
       12 . The method of  claim 9 , further comprising:
 installing the downhole tool in a well where the fluids passing through said passage are at a greater pressure than hydrostatic pressure in the annulus;   using said leak path as part of the configuration which allows failure of at least one of said seals to put said piston in a position where the internal component closes said passage.   
   
   
       13 . The method of  claim 9 , further comprising:
 pressure-balancing said piston with respect to pressures in said passage;   forming said piston as a rod piston;   using said leak path to allow said piston to move to a position where the internal component closes said passage in situations where at least one of said seals leaks.   
   
   
       14 . The method of  claim 10 , further comprising:
 allowing control line pressure to equalize to the annulus if the uppermost of said spaced seals fails.   
   
   
       15 . The method of  claim 14 , further comprising:
 allowing said piston to move to a position where the internal component closes said passage if said control line leaks into the surrounding annulus.   
   
   
       16 . The method of  claim 15 , further comprising:
 allowing said piston to move to a position where the internal component closes said passage if fluid from the annulus leaks past at least one seal due to annulus pressure being higher than pressure in said housing.   
   
   
       17 . The method of  claim 14 , further comprising:
 allowing the internal component to close said passage as control line pressure leaks past said uppermost seal through said leak path to equalize pressure on said piston.   
   
   
       18 . The method of  claim 16 , further comprising:
 sizing said spring so that said piston can be shifted to open said passage with said component with a force less than the hydrostatic pressure in said control line.   
   
   
       19 . The method of  claim 8 , comprising:
 making the downhole tool a subsurface safety valve and the internal component an assembly of a flow tube moving a flapper in said passage.