The committee believes that material improvements to the management and safety systems used by the companies engaged in offshore oil development, along with enhancements to the regulatory regime, can and should be made, and that such efforts will materially improve all aspects of safety offshore.
THE MACONDO WELL–DEEPWATER HORIZON DISASTER
The Macondo well is located approximately 50 miles off the coast of Louisiana in the Mississippi Canyon region of the Gulf of Mexico. It was intended as an exploratory well, drilled to assess the presence of extractable hydrocarbons and to survey the associated reservoir structures. The well was originally planned for a total depth of 19,650 feet. A decision was made in early April 2010 to halt drilling at a total depth of 18,360 feet and prepare the well for temporary abandonment in order to utilize the well later for oil and gas production. According to BP’s accident investigation report, four hydrocarbon zones had been discovered at depths ranging from 17,788 to 18,223 feet (BP 2010, 54). Furthermore, the differences between the highest reservoir pore pressure, which had to be offset by the drilling mud to prevent reservoir fluid flow, and the fracture gradients of the formation were becoming very small, leaving little margin for safe drilling. During March and April 2010, the Deepwater Horizon drilling team had encountered both “kicks” (hydrocarbon flows) and lost circulation events2 (due to formation fracturing). This included a lost circulation event on April 4 at a depth of 18,260 feet (BP 2010, 17; Transocean 2011a, I, 20).
Temporary abandonment of a well intended to be used for production is a standard practice. It provides the operator time to install the substantial infrastructure needed to transport the recovered hydrocarbons to shore while releasing the expensive drilling rig for other activities. Sealing the well to ensure that no hydrocarbon flow occurs is critical to the temporary abandonment process. This is typically done through the use of cemented liners or casings, along with additional cement or mechanical plugs that provide multiple barriers to hydrocarbon flow.
The narrow margins between pore pressure and fracture gradient established a challenging environment for sealing the well. The approach chosen was to use a long-string production casing (9 ⅞ × 7 inches) extending from the seafloor to the bottom of the well, cemented in place with a low-density, foamed cement slurry (BP 2010, 18; Transocean 2011a, I, 27). During the cementing operation, difficulties were encountered, including those associated with converting (closing) check valves on the float collar near the bottom of the casing at 18,115 feet. Nonetheless, the drilling team determined mistakenly that the cementing operation had been completed successfully and proceeded to conduct a negative pressure test to establish the integrity of the cemented production casing. A negative pressure test, conducted by displacing some of the heavy drilling
2Lost circulation is a loss of drilling fluids into the formation.