Ixtoc 1 was an exploratory oil well being drilled by the semi-submersible drilling rig Sedco 135 in the Bay of Campeche of the Gulf of Mexico, about northwest of Ciudad del Carmen, Campeche in waters deep. On 3 June 1979, the well suffered a blowout resulting in the largest oil spill in history at its time. To-date, it remains the second largest oil spill in history after the Deepwater Horizon oil spill.
Accident
Mexico's state-owned oil company Pemex (Petróleos Mexicanos) was drilling a deep oil well when the drilling rig Sedco 135 lost drilling mud circulation.
In modern rotary drilling, mud is circulated down the drillpipe and back up the wellbore 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 135 was extensively burned then scuttled.
At the time of the accident, Sedco 135 was drilling at a depth of about 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 to seal off the fractures that were causing the loss of circulation.
During the removal of the pipe on Sedco 135, the dancing mud suddenly began to flow up towards the surface; by removing the drillstring the well was swabbed (an effect observed when mud must flow down the annulus to replace displaced drill pipe volume below the bit) leading to a kick. Normally, this flow can be stopped by activating shear rams contained in the blowout preventer (BOP). These rams are designed to sever and seal off the well on the ocean floor; however, in this case, the drill collars had been brought in line with the BOP and the BOP rams were not able to sever the thick steel walls of the drill collars leading to a catastrophic blowout.
The drilling mud was followed by a large quantity of oil and gas at a flow rate that was still increasing. The oil and gas fumes exploded on contact with the operating pump motors, starting a fire which led to the collapse of the Sedco 135 drilling rig riser. The collapse caused damage to the BOP stack at the seafloor. The damage to the BOP led to the release of significant quantities of oil into the Gulf.
Volume and extent of spill
In the initial stages of the spill, an estimated of oil per day were flowing from the well. One barrel of oil is equivalent to 159 liters (or 42 gallons) of liquid. In July 1979, the pumping of mud into the well reduced the flow to per day, and early in August the pumping of nearly 100,000 steel, iron, and lead balls into the well reduced the flow to per day. Pemex claimed that half of the released oil burned when it reached the surface, a third of it evaporated, and the rest was contained or dispersed. Mexican authorities also drilled two relief wells into the main well to lower the pressure of the blowout, however, the oil continued to flow for three months following the completion of the first relief well. In total, around were spilled throughout the roughly 10 months it took for the oil to stop leaking.
Pemex contracted Conair Aviation to spray the chemical dispersant Corexit 9527 on the oil. A total of 493 aerial missions were flown, treating of oil slick. Dispersants were not used in the U.S. area of the spill because of the dispersant's inability to treat weathered oil. Eventually the on-scene coordinator (OSC) requested that Mexico stop using dispersants north of 25°N.
In Texas, an emphasis was placed on coastal countermeasures protecting the bays and lagoons formed by the barrier islands. Impacts of oil on the barrier island beaches were ranked as second in importance to protecting inlets to the bays and lagoons. This was done with the placement of skimmers and booms. Efforts were concentrated on the Brazos-Santiago Pass, Port Mansfield Channel, Aransas Pass, and Cedar Bayou which during the spill was sealed with sand. Economically and environmentally sensitive barrier island beaches were cleaned daily. Laborers used rakes and shovels to clean beaches rather than heavier equipment which removed too much sand. Ultimately, of oil impacted of U.S. beaches, and over of oiled material were removed.
