Article pubs.acs.org/est
Recurrent Oil Sheens at the Deepwater Horizon Disaster Site Fingerprinted with Synthetic Hydrocarbon Drilling Fluids Christoph Aeppli,† Christopher M. Reddy,*,† Robert K. Nelson,† Matthias Y. Kellermann,‡ and David L. Valentine‡ †
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, United States ‡ Department of Earth Science and Marine Science Institute, University of California, Santa Barbara, California 93106, United States S Supporting Information *
ABSTRACT: We used alkenes commonly found in synthetic drilling-fluids to identify sources of oil sheens that were first observed in September 2012 close to the Deepwater Horizon (DWH) disaster site, more than two years after the Macondo well (MW) was sealed. While explorations of the sea floor by BP confirmed that the well was sound, they identified the likely source as leakage from an 80-ton cofferdam, abandoned during the operation to control the MW in May 2010. We acquired sheen samples and cofferdam oil and analyzed them using comprehensive two-dimensional gas chromatography. This allowed for the identification of drilling-fluid C16- to C18-alkenes in sheen samples that were absent in cofferdam oil. Furthermore, the spatial pattern of evaporative losses of sheen oil alkanes indicated that oil surfaced closer to the DWH wreckage than the cofferdam site. Last, ratios of alkenes and oil hydrocarbons pointed to a common source of oil found in sheen samples and recovered from oil-covered DWH debris collected shortly after the explosion. These lines of evidence suggest that the observed sheens do not originate from the MW, cofferdam, or from natural seeps. Rather, the likely source is oil in tanks and pits on the DWH wreckage, representing a finite oil volume for leakage.
■
INTRODUCTION In September 2012, oil sheens were observed by satellite in the Gulf of Mexico near the Deepwater Horizon (DWH) disaster site and reported to the United States Coast Guard by BP, more than two years after the drilling rig had sunk.1 These observations raised environmental and legal concerns related to the quantity and source of oil sustaining these sheens, as well as ethical concerns related to surveying the wreckage of the DWH rig, a grave for 11 individuals. Since the original report, aerial photography has consistently documented sheens at this location, with numerous reports submitted to the United States’ National Response Center.2 The United States Coast Guard collected sheen samples from the sea surface on September 26, 2012. Their Marine Safety Laboratory stated that the sheen “correlated” to oil that originated from the Macondo well (MW),3 which released 4.4 to 6.1 million barrels of crude oil over 87 days in 2010.4 BP also collected sheen samples and reported the presence of alphaolefins.5 Olefins, a common name for monounsaturated alkenes, are used as lubricants in drilling fluids.6 They also have a powerful forensic value, as they are not present in crude oil or natural seeps. There are two main types of olefins used in drilling fluids, linear alpha olefins (LAOs) and internal olefins (IOs).6 LAOs typically contain 14 to 20 carbons atoms with the double bond in the α-position in the hydrocarbon chain (hence 1enes). IOs are synthesized by isomerization of LAOs to migrate © 2013 American Chemical Society
the double bond, leading to multiple isomers per carbon number. So, an IO mixture could contain LAOs. Beside these two types, highly branched poly alpha olefins (PAO) are also used; however, they are less relevant for drilling in deep water, compared to IOs and LAOs.6 In this study, we use “drilling fluid” interchangeably with “drilling mud”, which is terminology frequently used in offshore drilling. Analyzing oiled samples for LAOs and IOs can be challenging, as they often elute in regions of gas chromatograms dominated by a high density of petroleum hydrocarbons. Since olefins are not native petroleum compounds, detecting these compounds in sheen samples would indicate that they originated from drilling operations, wreckage from the explosion, or operations to stop the leaknot from the reservoir directly; their presence would, therefore, also point to an oil source of finite volume with limited potential for discharge. We developed and patented a method using comprehensive two-dimensional gas chromatography (GC × GC) for accurate identification and quantification of drilling fluid olefins in crude oils.7,8 There has been some confusion about the source of the oil sheen. On October 11, 2012, BP offered that the oil could be Received: Revised: Accepted: Published: 8211
May 31, 2013 June 24, 2013 June 25, 2013 June 25, 2013 dx.doi.org/10.1021/es4024139 | Environ. Sci. Technol. 2013, 47, 8211−8219
Environmental Science & Technology
Article
Figure 1. Map of the sampling site. (a) A partial bathymetric chart of the Mississippi Canyon (MC) with the lease block numbers. Figure adapted from BP.11 The contours represent water depth in feet with a contour interval of 50 feet. (b) Map detail, showing the location of the Macondo well, the abandoned cofferdam, the DWH wreckage, and the approximate location of the sunken riser pipe. The red and blue lines respesent the tracks of collected oil sheen in October (sample numbers 102012−1 to 102012−6) and December 2012 (samples 121312−1 to 121312−8), respectively; the circles are the midpoints of the start and end collection-coordinates. The green arcs are calculated oil surfacing-locations based on evaporative losses of the saturated alkanes (see Results and Discussions for details). ENSCO 8502 is a drilling rig operating in our sampling area during the December campaign.
Table 1. Sample Information for Sheens Sampled on October 20 and December 13, 2012, Near the Deepwater Horizon site, and Comparison of GC-FID and GC×GC-FID Olefin Quantificationa,e olefin content sample ID Sheen Samples Oct-20 2012 102012−1 102012−2 102012−3 102012−4 102012−5 102012−6 Sheen Samples Dec-13 2012 121312−01 121312−02 121312−03 121312−04 121312−05 121312−06 121312−07 121312−08 Potential End-Members MW oil Cofferdam oil DWH debris S6g DWH debris S7g HOS Centerline drilling mudh
sampling locationb
GC-FID
GC × GC
C16/C17/C18-LO distributionc
C18-LOs/phytanec
C18-LOs /hopanec
N28.7437° N28.7454° N28.7448° N28.7426° N28.7445° N28.7421°
W88.3652° W88.3632° W88.3633° W88.3665° W88.3669° W88.3669°
0.2%
