1. n. 
The technology focused on maintaining pressure on open formations (that is, exposed to the wellbore) to prevent or direct the flow of formation fluids into the wellbore. This technology encompasses the estimation of formation fluid pressures, the strength of the subsurface formations and the use of casing and mud density to offset those pressures in a predictable fashion. Also included are operational procedures to safely stop a well from flowing should an influx of formation fluid occur. To conduct well-control procedures, large valves are installed at the top of the well to enable wellsite personnel to close the well if necessary.
See: abnormal pressure, adjustable choke, aquifer, blowout preventer, BOP stack, check valve, crossflow, flapper valve, formation fluid, kill line, lost circulation, mud weight, openhole, ram blowout preventer, swab