Scientific basis for safely shutting in the Macondo Well after the April 20, 2010Deepwater Horizonblowout

Abstract

As part of the government response to the Deepwater Horizon blowout, a Well Integrity Team evaluated the geologic hazards of shutting in the Macondo Well at the seafloor and determined the conditions under which it could safely be undertaken. Of particular concern was the possibility that, under the anticipated high shut-in pressures, oil could leak out of the well casing below the seafloor. Such a leak could lead to new geologic pathways for hydrocarbon release to the Gulf of Mexico. Evaluating this hazard required analyses of 2D and 3D seismic surveys, seafloor bathymetry, sediment properties, geophysical well logs, and drilling data to assess the geological, hydrological, and geomechanical conditions around the Macondo Well. After the well was successfully capped and shut in on July 15, 2010, a variety of monitoring activities were used to assess subsurface well integrity. These activities included acquisition of wellhead pressure data, marine multichannel seismic profiles, seafloor and water-column sonar surveys, and wellhead visual/acoustic monitoring. These data showed that the Macondo Well was not leaking after shut in, and therefore, it could remain safely shut until reservoir pressures were suppressed (killed) with heavy drilling mud and the well was sealed with cement. In mid-June, a Well Integrity Team (WIT) was created to make recommendations to the government on whether a shut in of the Macondo Well could be safely undertaken and if so, under what conditions. The WIT consisted of the authors of this paper plus engineers from the Department of Energy National Laboratories: Sandia, Los Alamos, and Lawrence Livermore. Although the WIT analyzed both the geologic environment of the Macondo Well and the hydraulic and mechanical performance of engineered components of the well (wellhead, casing flow paths, rupture disks, etc.), this paper deals only with geologic aspects of well integrity. In this paper, we summarize the WIT's assessment of the geologic risks of shutting in the Macondo Well and provide analyses of wellhead pressure and geophysical monitoring data during shut in. These analyses were essential for determining whether the capping stack, when closed, could remain safely shut until the well was killed. Geologic data analyzed by the WIT came from the Macondo Well and nearby wells (including relief wells) and consisted of in situ stress and fluid pressure measurements, geophysical logs, core, cuttings, and gas analyses, 3D and 2D seismic lines, seafloor bathymetry, water-column imagery, sidescan sonar surveys, and drilling records. Extensive discussions were also carried out with scientists and engineers from BP, industry experts, and other federal agencies and academia on lithologic and structural interpretations, reservoir and geomechanical analyses, oceanographic conditions, and well kill and cementing procedures.

Applications of science and engineering to quantify and control the Deepwater Horizon oil spill

“…The well could not be left shut in unless the well passed an integrity test (10) that showed that all rupture disks in the well casings and casing shoes had remained intact, despite the explosion. If it had not, shutting in the well would risk release of hydrocarbons to surrounding geologic formations and potential blowouts to the seafloor (Fig.…”

“…Government and BP experts were weighing whether to proceed with the well integrity test (WIT) (10). Risks identified included (i) broaching of hydrocarbons to the seafloor through multiple subsea floor vents and (ii) sinking of the BOP into the seafloor through liquefaction of surrounding sediments by broaching oil (Fig.…”

Acoustic mapping of mixed layer depth

“…Not long after the first military applications, acoustic water column mapping with echo sounders was applied to fisheries science, for which the detection and quantification of fish distributions were the primary focus (Kimura, 1929;MacLennan, 1990). The applications of acoustic water column mapping have broadened in recent years to include marine ecosystem acoustics (Benoit-Bird and Lawson, 2016;Godø et al, 2014), observations of gas bubbles and oil droplets associated with natural seeps (Jerram et al, 2015;Merewether et al, 1985), and fossil fuel production (Hickman et al, 2012;. Acoustic imaging of the water column has also been used within the field of physical oceanography; single-beam echo sounders can capture fine-scale oceanographic structures typically attributed to biological scattering or turbulent microstructures (Klymak and Moum, 2003;Pingree and Mardell, 1985;Trevorrow, 1998).…”

Estimating oil concentration and flow rate with calibrated vessel-mounted acoustic echo sounders

“…Acoustic observations from all four NOAA ships made with the lower-frequency echo sounders (12,18, and 38 kHz) often indicated the presence of methane seeps rising from the seafloor (13), a capability that was later exploited during wellhead integrity testing after the well was capped in mid-July (14). Gas bubbles in seawater strongly scatter sound, with scattering cross-sections that are several orders of magnitude higher than a similarly sized oil droplet at frequencies close to the mechanical resonance of the bubble (15).…”

“…Deepwater Horizon | oil plume | Gulf of Mexico | acoustic remote sensing O n April 20, 2010, an explosion on the Deepwater Horizon drilling rig precipitated an oil spill from a damaged wellhead located approximately 1,500-m below the sea surface, releasing an estimated 7.0 × 10 5 AE 20% m 3 (4.4 × 10 6 barrels) (1) to 7.82 × 10 5 AE 10% m 3 (4.92 × 10 6 barrels) (2) of oil and between 9.14 × 10 9 -1.25 × 10 10 mol of methane (3,4) between April 22 and July 15, 2010 when the well was successfully capped. The depth of the damaged wellhead obscured our collective ability to observe what was happening to the oil as it exited the well, but by mid-May observations of underwater oil plumes were reported (e.g., ref.…”