Wellbore annular pressure control system and method using accumulator to maintain back pressure in annulus

Abstract

A system for controlling wellbore annulus pressure during the drilling of a subterranean formation includes a drill string extending into a wellbore. A primary pump is configured for selectively pumping a drilling fluid through the drill string and into an annular space created between the drill string and the wellbore. A fluid discharge conduit is in fluid communication with the annular space for discharging the drilling fluid to a reservoir. A fluid back pressure system is connected to the fluid discharge conduit configured to selectively maintain pressure therein. The fluid back pressure system includes a fluid choke, a back pressure source and an accumulator hydraulically coupled between the back pressure source and the discharge conduit.

1 . A system for controlling wellbore annulus pressure during the drilling of a subterranean formation, comprising:
 a drill string extending into a wellbore;   a primary pump for selectively pumping a drilling fluid through the drill string and into an annular space created between the drill string and the wellbore;   a fluid discharge conduit in fluid communication with the annular space for discharging the drilling fluid to a reservoir; and   a fluid back pressure system connected to the fluid discharge conduit configured to selectively maintain pressure therein, the fluid back pressure system including a first controllable fluid choke, a fluid back pressure source and an accumulator hydraulically coupled between the back pressure source and the discharge conduit.   
   
   
       2 . The system of  claim 1  wherein the back pressure source comprises a secondary pump. 
   
   
       3 . The system of  claim 2  further comprising a pressure control switch connected to the secondary pump and configured to switch the secondary pump off when a fluid side of the accumulator is charged to a selected fluid pressure. 
   
   
       4 . The system of  claim 1  wherein the back pressure source comprises the primary pump. 
   
   
       5 . The system of  claim 1  further comprising a second controllable fluid choke disposed hydraulically between the accumulator and the discharge conduit. 
   
   
       6 . A method for controlling fluid pressure in a wellbore annulus, comprising:
 pumping drilling fluid through a drill string extended into a wellbore, out the bottom of the drill string and into the wellbore annulus;   discharging fluid from the annulus through a discharge conduit, the discharge conduit including therein a fluid backpressure system, the backpressure system including a first controllable fluid choke, a fluid backpressure source and an accumulator hydraulically coupled between the fluid backpressure source and the discharge conduit;   controlling the first controllable fluid choke to maintain a selected pressure in the annular space;   operating the fluid backpressure source to charge a fluid side of the accumulator to a selected pressure; and   discharging fluid from the accumulator into the discharge line to maintain the selected pressure in the annulus.   
   
   
       7 . The method of  claim 6  wherein the fluid backpressure source comprises a drilling rig mud pump. 
   
   
       8 . The method of  claim 6  wherein the fluid backpressure source comprises an additional pump. 
   
   
       9 . The method of  claim 6  further comprising controlling a rate of discharge of fluid from the accumulator. 
   
   
       10 . The method of  claim 9  wherein the controlling discharge from the accumulator is performed by operating a second controllable fluid choke hydraulically disposed between the accumulator and the discharge conduit.