Method for circulating a fluid entry out of a subsurface wellbore without shutting in the wellbore
A method for removing a fluid influx from a subsea wellbore drilled using a pump to return fluid from the wellbore to the surface. The method includes detecting the influx when a rate of the return pump increases. Flow from the wellbore is diverted from the return pump to a choke line when the influx reaches the wellhead. A choke in the choke line is operated so that a rate of fluid pumped into the wellbore is substantially equal to a flow rate through the choke line. Fluid flow from the wellbore is redirected to the return pump inlet when the influx has substantially left the well.
1. A method for removing a fluid influx from a subsea drilling wellbore, the wellbore drilled using a pump to return drilling fluid from the wellbore to the sea surface, the method comprising:
observing the fluid influx;
continuing to pump drilling fluid through the drill string and the return pump until the fluid influx reaches the wellhead, the return pumping performed at a rate such that a flow into the wellbore substantially equals a flow out of the wellbore;
hydraulically isolating an intake to the return pump;
diverting flow out of the wellbore to a choke line;
operating the choke so that the flow into the wellbore substantially equals a flow out of the wellbore;
diverting flow out of the wellbore back to the intake of the return pump when an end of the influx reaches the wellhead;
pumping a less dense fluid down a kill line from the surface to proximate a bottom end thereof and thereto proximate a bottom end of the choke line; and
displacing influx fluid from the choke line.
2. The method of claim 1 wherein after the detecting, a rate of the fluid return pump is reduced until the rate of fluid pumped into the wellbore is substantially the same as the rate of fluid flowing out of the wellbore.
3. The method of claim 1 further comprising closing an annular blowout preventer below a riser connected between the wellbore and the surface upon detection of the influx.
4. The method of claim 3 further comprising opening an annular blowout preventer below the riser after the interface level is raised.
5. The method of claim 3 further comprising, pumping drilling fluid down an auxiliary line into a lower end of the riser to raise an interface level between drilling fluid and the less dense fluid such that a fluid pressure at the bottom of the well is at least as much as fluid pressure in rock formations penetrated by the wellbore.
6. The method of claim 1 wherein a riser is open above the wellhead and is substantially filled with fluid less dense than drilling fluid pumped through the drill string used to drill the wellbore.
7. The method of claim 6 wherein the less dense fluid comprises sea water.
8. The method of claim 1 wherein the observing the influx comprises detecting an increase in operating rate of the mud return pump.