Without disturbance, Earth usually does a good job at containing fluids underground. However, in exploration and production, drilling introduces pathways between the subsurface and the surface. Subsurface containment during drilling operations can be achieved through three broad control strategies.
Primary Well Control
Under normal drilling operations, drillers maintain hydrostatic pressure in the wellbore between the (lower) formation fluid pressure and the (higher) formation fracture pressure. This pressure is maintained using dense drilling mud. Drill ‘underweight’ and the mud cannot hold back the formation fluid pressure and fluids enter the wellbore; drill ‘overweight’ and there is a risk of fracturing the formation.
Secondary Well Control
In the event that a miscalculation leads to a misalignment between hydrostatic pressure within the wellbore and the pressure of the surrounding formation, several technologies exist to prevent a containment loss characterized by migration of fluids to the surface. Blow out preventers at the surface prevent fluid flow from the annulus, drill string, or open wellbore. Additional blowout preventers inside the drill pipe prevent fluid flow up the drill string.
Tertiary Well Control
In the event that both pressure and equipment fail to contain migrating fluids, such as in the case of a subsurface blowout, drillers have several options. They can drill a relief well into the same formation then use heavy mud to stop the flow. They can seek to regain control of the well by rapidly pumping heavy mud to create a hydrostatic balance against formation pressure. They can also pump cement or barite to plug the wellbore.
Images: “Blowout Preventer” by Michael Black