Method and System for Drilling with Reduced Surface Pressure

Abstract

A drilling method includes arranging a dual drill string ( 20 ) having an inlet fluid conduit ( 9 ) and a return fluid conduit ( 10 ) in a well ( 14 ) formed in a subsurface formation such that a well annulus ( 22 ) is formed between the dual drill string ( 20 ) and a wall ( 21 ) of the well ( 14 ). A divider element ( 3 ) is disposed in the well annulus ( 22 ) to divide the well annulus ( 22 ) into an upper annular region ( 5 ) extending above the divider element ( 3 ) to a surface of the well annulus ( 22 ) and a lower annular region ( 12 ) extending below the divider element ( 3 ) towards a bottom of the well annulus ( 22 ). The lower annular region ( 12 ) contains a first fluid ( 25 ). A second fluid ( 27 ) is fed into the upper annular region ( 5 ). The method includes configuring the second fluid such that the density of the second fluid ( 27 ) is greater than the density of the first fluid ( 25 ).

1 . A drilling method, comprising:
 arranging a dual drill string having one inlet fluid conduit and one return fluid conduit in a well drilled in a formation such that a well annulus is formed between the dual drill string and a wall of the well;   disposing a divider element in the well annulus to divide the well annulus into an upper annular region , extending above the divider element, to a surface of the well annulus and a lower annular region extending below the divider element towards the bottom of the well annulus, the lower annular region containing a first fluid having a first density; and   feeding a second fluid having a second density into the upper annular region, the feeding comprising configuring the second fluid such that the second density is greater than the first density.   
     
     
         2 . A method according to  claim 1 , wherein at least a portion of the first fluid is fluid from the formation. 
     
     
         3 . A method according to  claim 1 , further comprising providing the second fluid as an unpressurized fluid. 
     
     
         4 . A method according to  claim 1 , further comprising measuring pressure at or near at least one of an upper surface of the divider element exposed to the upper annular region and a lower surface of the divider element exposed to the lower annular region. 
     
     
         5 . A method according to  claim 1 , further comprising using the measured pressure to selectively adjust the second density such that the second density remains greater than the first density during drilling of the well. 
     
     
         6 . A method according to  claim 1 , further comprising arranging a rotating control unit near the surface of the well annulus, wherein pressure acting on the rotating control unit is determined by pressure in the well and density of fluid in the well annulus. 
     
     
         7 . A drilling system comprising:
 a dual drill string arranged in a well formed in a formation such that a well annulus, is formed between the dual drill string and a wall of the well, the dual drill string having one inlet fluid conduit and one return fluid conduit;   a divider element arranged in the well annulus, to divide the well annulus into an upper annular region extending above the divider element to a surface of the well annulus and a lower annular region below the divider element towards a bottom of the well annulus, the lower annular region containing a first fluid having a first density; and   a second fluid having a second density disposed in the upper annular region, the second fluid being configured such that the second density is greater than the first density.   
     
     
         8 . A drilling system according to  claim 7 , further comprising a fluid inlet through which the second fluid is fed into the upper annular region. 
     
     
         9 . A drilling system according to  claim 8 , further comprising a rotating control unit arranged near the surface of the well annulus the rotating control unit being in communication with the fluid inlet. 
     
     
         10 . A drilling system according to  claim 9 , further comprising a blowout preventer arranged near the surface of the well annulus in connection with the rotating control unit, the blowout preventer forming an upper boundary of the upper annular region. 
     
     
         11 . A drilling system according to  claim 7 , further comprising means for measuring pressure at or near a lower surface of the divider element exposed to the lower annular region. 
     
     
         12 . A drilling system according to  claim 7 , further comprising means for measuring pressure at or near an upper surface of the divider element exposed to the upper annular region. 
     
     
         13 . A drilling system according to  claim 7 , wherein the divider element is a piston. 
     
     
         14 . A drilling system according to  claim 7 , wherein the divider element is fixed to the dual drill string. 
     
     
         15 . A drilling system according to  claim 7 , wherein the divider element is movable relative to the dual drill string 
     
     
         16 . A drilling system according to  claim 7 , wherein the two fluid conduits of the dual drill string are concentric. 
     
     
         17 . A drilling system according to  claim 7 , wherein at least a portion of the first fluid is fluid from the formation. 
     
     
         18 . A drilling system according to  claim 7 , wherein the second fluid is unpressurized.