Blowout

When all the controls described above fail, a blowout occurs. Blowouts are dangerous since they can eject the drill string out of the well, and the force of the escaping fluid can be strong enough to damage the drilling rig. Blowouts often ignite due to the presence of an ignition source, from sparks from rocks being ejected along with flammable fluids, or simply from heat generated by friction. (Rarely the flowing gas will contain poisonous hydrogen sulfide and the oil operator might decide to ignite the stream to convert this to less hazardous substances.) A well control company will then need to extinguish the well fire and/or cap the well, and replace the casing head and hangars.

Sometimes, blowouts can be so forceful that they cannot be directly brought under control from the surface, particularly if there is so much energy in the flowing zone that it does not deplete significantly over the course of a blowout. In such cases, other wells (called relief wells) may be drilled to intersect the well or pocket, in order to allow kill-weight fluids to be introduced at depth. (Contrary to what might be inferred from the term, such wells generally are not used to help relieve pressure using multiple outlets from the blowout zone.)

An “underground blowout” is a special situation where fluids from high pressure zones flow uncontrolled to lower pressure zones within the open-hole portion of the wellbore. Usually they come up the wellbore to shallower formations (typically near the last casing shoe) that have been fractured from the overall effect of hydrostatic mud head plus casing pressure imposed at the time of the initial kick. Underground blowouts can be very difficult to bring under control although there is no outward flow at the drill site itself. However, if left unchecked, in time the fluids may find their way to the surface elsewhere in the vicinity (possibly “cratering” the rig), or may pressurize other zones, leading to problems when drilling subsequent wells.