Wellbore annular pressure control system and method using gas lift in drilling fluid return line

摘要

A system and method include pumping drilling fluid through a drill string extended into a wellbore extending below the bottom of a body of water, out the bottom of the drill string and into the wellbore annulus. Fluid is discharged from the annulus into a riser and a discharge conduit. The riser is disposed above the top of the wellbore and extends to the water surface. The discharge conduit couples to the riser and includes a controllable fluid choke. A fluid return line is coupled to an outlet of the choke and extends to the water surface. Gas under pressure is pumped into the return line at a selected depth below the water surface. The controllable fluid choke may be operated to maintain a selected drilling fluid level in the riser, the selected fluid level being a selected distance below the water surface.

主权项

1. A system comprising:
a drill string extending into a wellbore below a bottom of a body of water; 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 riser extending from a top of the wellbore to a platform on a surface of the body of water; a fluid discharge conduit in fluid communication with the riser; a controllable orifice choke coupled to the discharge conduit; a fluid return line extending from the choke to the platform; a source of compressed gas coupled to the fluid return line at a selected depth below the surface of the body of water; a separator coupled to the fluid return line; a flow meter coupled to a gas discharge end of the separator; and a controller configured to receive an input signal from the flow meter and configured to compare a flow rate of gas measured by the flow meter to a flow rate of gas pumped into the fluid return line.