Ixtoc I oil spill

Ixtoc I was an exploratory oil well being drilled by the semi-submersible drilling rig Sedco 135-F in the Bay of Campeche of the Gulf of Mexico, about 100 km (62 mi) northwest of Ciudad del Carmen, Campeche in waters 50 m (160 ft) deep. On 3 June 1979, the well suffered a blowout resulting in one of the largest oil spills in history.

Accident

Mexico's state-owned oil company Pemex (Petróleos Mexicanos) was drilling a 3 km (1.9 mi) deep oil well when the drilling rig Sedco 135F lost drilling mud circulation.

In modern rotary drilling, mud is circulated down the drill pipe and back up the well bore to the surface. The goal is to equalize the pressure through the shaft and to monitor the returning mud for gas. Without the counter-pressure provided by the circulating mud, the pressure in the formation allowed oil to fill the well column, blowing out the well. The oil caught fire, and Sedco 135F burned and collapsed into the sea.

At the time of the accident Sedco 135F was drilling at a depth of about 3,600 metres (11,800 ft) below the seafloor. The day before Ixtoc suffered the blowout and resulting fire that caused her to sink, the drill bit hit a region of soft strata. Subsequently, the circulation of drilling mud was lost resulting in a loss of hydrostatic pressure. Rather than returning to the surface, the drilling mud was escaping into fractures that had formed in the rock at the bottom of the hole. Pemex officials decided to remove the bit, run the drill pipe back into the hole and pump materials down this open-ended drill pipe in an effort to seal off the fractures that were causing the loss of circulation.