A blowout is the uncontrolled release of a formation fluid, usually gas, from a well being drilled, typically for petroleum production. A blowout is caused when a combination of well control systems fail—primarily drilling mud hydrostatics and blow-out preventers (BOPs)—and formation pore pressure is greater than the wellbore pressure at depth. When such an incident occurs, formation fluids begin to flow into the wellbore and up the annulus and/or inside the drill pipe, and is commonly called a kick. If the well is not shut in, a kick can quickly escalate into a blowout when the formation fluids reach the surface, especially when the fluid is a gas, which rapidly expands as it flows up the wellbore and accelerates to near supersonic speeds. Blowouts can cause significant damage to drilling rigs, and injuries or fatalities to rig personnel.
Prior to the development of blow-out preventers, blowouts were common and were referred to as gushers.
The primary means of detecting a kick is the loss of circulation back up to the surface into the mud pits. The mud engineer keeps track of the level in the mud pits, and a drop in this level would indicate lost circulation to a formation. The rate of mud returns is also closely monitored to match the rate that it is being pumped downhole. If the rate of returns is slower than expected, then a certain amount of the mud is being lost to a thief zone. In the case of the overpressured gas pocket, an increase in mud returns would be noticed when the formation gases push the drilling mud to the surface at a rapid rate.
The first response to detecting a kick would be to isolate the well from the surface by activating the BOPs. Once the wellbore is isolated, the drilling crew would attempt to circulate in a heavier kill fluid to increase the hydrostatic pressure (usually with the assistance of a well control company), compress the kick gases, and slowly circulate out the gas in a controlled manner, taking care not to allow the gas to accelerate up the wellbore.
Often, however, companies drill underbalanced for better, faster penetration rates and thus they "drill for kicks" as it is economically sounder to take time to kill a kick than to drill overbalanced (slow penetration rates). Under these circumstances calling in a "well control" specialist is not necessary.
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 and fracture shallower formations typically near the last casing shoe. 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.