Patent ID: 8567525

Claim:
A method comprising: controlling formation pressure during drilling of a borehole through a subterranean formation, comprising: selectively pumping a drilling fluid through a drill string extended into a borehole, out a drill bit at the bottom end of the drill string, and into an annular space between drill string and the borehole; and discharging the drilling fluid from the annular space proximate the Earth's surface; and determining existence of a well control event comprising: determining whether at least one of the following events occurs: (i) a rate of the selective pumping remains substantially constant and pressure in the outlet of the annular space increases, and (ii) the rate of the selective pumping remains substantially constant and the pressure in the outlet of the annular space decreases, as the annular space proximate an upper end of the wellbore is sealed, and the drill fluid from beneath the seal is discharged through a selectable aperture flow control device at a fixed aperture.