Wellheads—Exploration Onshore and Offshore

An underground blowout can also occur during a drilling operation. An underground blowout occurs when a loss of mud control occurs and the reservoir fluids begin to flow from one underground zone into a zone of lower pressure. Because the loss flow is below the surface it is considered an underground blowout and are more difficult and complex to evaluate and correct. 

Blowout preventers (BOPs) function is to cut off the flow of potential blowout. In all wells being drilled there are normally three holes or pipes within pipes that are at the surface of the wellhead -conductor pipe, casing pipe, and drill pipe. The drill pipe is the actual hole while the outer two are annulus formed around the inner pipe. Any one of these under varying conditions can be a source of through which oil or gas can escape during drilling. The annular preventer is a valve that appears 

The most common cause of a well to become uncontrolled and develop into a blowout is improper mud control operations and the inability of the blowout prevention system to contain it because of system failures, i.e., lack of testing and maintenance. 

It is also interesting that in 1970 Chevron pleaded no contest to charges of knowingly and willfully failing to install safety devices on 90 oil wells in the Gulf of Mexico. Three other oil companies were similarly charged with the same offense (Shell, Conoco and Humble). At the time Chevron was fined 1 million U S. dollars for 500 violations under the Outer Continental Shelf Lands Act. The violations involved failure to install safety chokes at the wells - i.e., a method to cut off the flow of oil automatically in the event of incident. 

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