Traditional well control methods rely on surface measurements to identify a kick or wellbore influx. While most well control events occur during underbalanced conditions, an influx may be prompted by major vugular losses, typically caused by a carbonate vug. During such incidents, overbalanced drilling conditions plunge and the bottomhole pressure equalizes to pore pressure in the vug. An influx at surface is never experienced, as the annulus fluid level drops. Even filling the annulus does not prevent the BHP decrease and the resulting wellbore pressure is then approximately equal to the vug pore pressure minus the hydrostatic head. Since the drilling fluid gradient exceeds the formation pressure gradient, an underbalance is created along the exposed carbonate wellbore section. Hydrocarbons may enter, flowing down towards the vug along with the displacing fluid, until the wellbore pressure is equal to the formation pressure.
This paper describes efficient detection of well control events, both in underbalanced and overbalanced conditions, which is especially relevant for deepwater operations as the reaction time is significantly less than with surface BOPs. A methodology is offered to detect the annulus fluid level in conditions of unknown hydrostatic column height and fluid density through the use of discrete annular pressure acquisition along a networked drillstring. The downhole real time pressure evaluation also provides for monitoring of the gas migration to manage annular filling while losses continue to be experienced. Further, the ‘direct measurement method' is offered for well control based on constant bottomhole pressure through downhole pressure measurements independently from surface measurements.
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