10 years after Deepwater Horizon oil spill

The semi-submersible offshore oil drilling rig was located 66 km south-east off the Louisiana coast, free floating 1585m over the Gulf floor. It had 126 crew members on board. According to Doctor Jean-Paul Xavier, specialist of remote sensing techniques applied to oil & gas activities: "The oil platform Deepwater Horizon was located on the slope between the shallow plateau (100m deep) of Mississippi-Alabama shelf to the north and the deep seabed (2000m deep) of Grand Gosier area, to the south. On this slope, the GEBCO bathymetry shows beautiful figures of gravity sliding, called 'Domes' in this sector (Gloria Dome in particular)."

According to a news article released by the Jackson School of Geosciences of the University of Texas at Austin: "The well blowout killed 11 people and spewed oil for three months, spilling about 4 million barrels of oil into the Gulf of Mexico before crews successfully capped the well. Researchers and investigators since then have focused mostly on the engineering decisions and mistakes that led to the blowout and the ecological impacts of the oil spill that became one of the country's worst environmental catastrophes. But researchers from the UT Jackson School of Geosciences, aided by thousands of pages of documents made public during lawsuits and legal proceedings, have pieced together how the geologic conditions more than 2 miles under the Gulf floor made drilling difficult and drove engineering decisions that contributed to the well's failure and the ensuing blowout."

"The study, published May 7 in Scientific Reports, documents, among other things, a significant and steep drop in pore pressure inside the rock near the bottom of the well that influenced the decisions that contributed to the blowout."

"BP planned to temporarily abandon the oil well, the initial well in the Macondo prospect, until it could be produced at a later date, by plugging the base with steel and cement. However, the sharp drop in pore pressure, and an associated decline in stress, drastically narrowed the range of options to seal off the well. This led to the decision to use a controversial low-density foam cement that failed to set properly. This was a key cause of the Macondo well blowout." (see the NAE-NRC report).

Envisat carried on board many instruments, in particular a radar instrument and an optical medium resolution sensor. As a radar sensor, Envisat ASAR is very sensitive to the damping of waves caused by the oil layer, allowing a good estimation of the surface covered by the slick. MERIS instrument allows seeing the oil colour when it is thick enough to differ from its marine environment. Sometimes thinner slicks may show with rainbow patterns or even as sheen, especially if sun glint is present. This allows a rough estimate of the thickness of different parts of the slick. It is also able to highlight the areas on fire. Here, fire has been lit to a part of the oil spill, wind pushed the front to the SE edge of the spill.

In her article Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon published on Proceedings of the National Academy of Sciences (PNAS) on the 3rd January 2017, Sarah C. Bagby wrote: "On 20 April 2010, a blowout from the Macondo Well in the Gulf of Mexico caused an explosion on the Deepwater Horizon mobile offshore drilling unit that ultimately led to its sinking and the deaths of 11 crewmembers. From the time of the blowout until the well was capped on 15 July 2010, petroleum fluids flowed continuously from the Macondo Well, with environmental emission estimates of 4.1 million barrels of oil and 170 kT natural gas." "The Deepwater Horizon event led to an unprecedented discharge of 4.1 million barrels of oil to the Gulf of Mexico. The deposition of ~4–31% of this oil to the seafloor has been quantified previously on a bulk basis."

"The spill was noteworthy not only for its volume but also for its distance offshore and its depth: Oil and gas entered the ocean at a water depth of ~1500m and then partitioned between the deep ocean and the sea surface. This partitioning may have varied over time because of reservoir depressurization and deliberate interventions such as the shearing of the riser pipe and the application of chemical dispersant at the wellhead. In all, approximately half of the oil ascended to the ocean surface, where it was skimmed or flared by response teams, trapped in sinking particles by marine oil snow sedimentation and flocculent accumulation, washed ashore, or left exposed to the canonical weathering processes of evaporation, biodegradation, and photooxidation. The rest remained in the deep ocean."

Top: Source: Persistence and biodegradation of oil at the ocean floor following Deepwater Horizon published on Proceedings of the National Academy of Sciences (PNAS) on the 3rd January 2017, Sarah C. Bagby

Nowadays, Sentinel-1A and 1B CSAR instrument monitor the Gulf more frequently than their predecessor Envisat SAR could 10 years ago. It allows a near-complete covering of the Gulf of Mexico every 12 days, and with a much improved ground sampling distance of 10m.

As a whole, the Gulf of Mexico is now cleaner than it was ten years ago, in particular on the American side. As shown above, things can still improve if authorities decide so. There are however other parts of the World where laws are more permissive or harder to enforce or simply do not benefit from an intense monitoring, where large multiple oil spills happen everyday as it has been shown previously on the West African Coast: SED-018   SED-134   EVT-519   SED-577