Oil Plumes and the Fate of the Spill

I'm as reluctant to insert myself into the debate over what happened to all the oil that leaked from BP's Macondo well between April 22 and July 15--when the second cap stopped the flow--as I was concerning the earlier controversy regarding flow-rate estimates. At the same time, I find the coverage of this story lacking in crucial details that could help us to understand how much of the oil evaporated into the warm air of the Gulf or degraded naturally, how much was collected, and how much potentially remains in the sea. The assessment issued by the National Oceanic and Atmospheric Administration (NOAA) on August 4, 2010 has been disputed by some scientists, and reports of lingering oil plumes add to the public's apprehension that the pieces don't quite add up. But although I don't have nearly enough information to conclude which group is closer to being right, I feel much more confident in pointing out where their arguments seem weak.

Let's begin with the estimate of the total quantity of oil leaked into the Gulf, which lately seems to have become cast in stone at 4.9 million barrels (205.8 million gallons.) This is the crucial starting point for any analysis of how much of it remains in the Gulf. This figure appears to be based on the estimate by the Flow Rate Technical Group of an average rate of around 58,000 bbl/day for the 85 days that the well was leaking. NOAA indicates an uncertainty for this figure of +/- 10%, but with all due respect to the scientists who worked on it, that seems excessively precise for something that was never measured directly.

There are only two ways I know of to measure such a flow, as distinct from estimating it. The most accurate involves gathering all the oil flowing during a given interval--say, a day--and gauging the tanks into which it flowed at the beginning and end of the interval. From a quick review of the transcripts of BP's technical briefings, it appears that the largest quantity of oil that was actually collected in a 24-hour period equated to a flow rate of about 24,000 bbl/day, though this represented only a portion of the total flow, with the remainder continuing to leak into the sea due to containment limitations. So we know the rate must have been higher than that figure, but not how much higher. The other way to measure oil flow is with a flow meter. It's a pity that BP's "Lower Marine Riser Package", the second cap and valve assembly installed on the well, didn't include this capability. I don't even know if it would have been feasible, given the pressures and high flows of oil and natural gas involved.

In the absence of direct flow measurements, the Flow Rate Technical Group had to rely on sophisticated techniques for calculating the flow, based on the observed velocity of the fluid leaving the well and a complex set of assumptions--grounded in a limited amount of actual data--concerning the gas:oil ratio of the fluid, the rapid expansion of the gas coming out of solution within the space over which the velocity was determined, as well as the changing pressure and temperature within this regime. Tricky stuff, particularly considering how much of the observed flow was attributable to gas, rather than oil, as I noted in May. I'd also note that since the estimated 58,000 bbl/day flow rate is at the top of the range of flow rates observed from other oil wells in the history of the industry, it's quite possible that the range of uncertainty for the total amount leaked is not only wider than +/- 10%, but also non-symmetrical, with more downside than upside. I'm sure we will hear much more about this in the future, not least because the size of the fine BP would ultimately pay for the leak depends on it. That's not the concern of the moment, however.

The pie chart in NOAA's report indicating the breakdown of the different fates of the oil that leaked has gotten a lot of scrutiny. Some reports have interpreted it as indicating that only a quarter of the oil remains in the marine environment. I wouldn't read it that way. Instead, I'd see three distinct categories for the oil's current status. The first and least ambiguous concerns the oil physically collected directly from the well, skimmed from the surface, or burned off, constituting an estimated--and only partly measured--25% of the uncertain total discussed above. This oil is clearly no longer in the water. The next category is oil that is likely no longer in the water, and that is the portion of the "Evaporated or Dissolved" segment that evaporated. If the oil had all reached the surface, I wouldn't be at all surprised if most of that segment should be attributed to evaporation; this was, after all, light, sweet oil with a high proportion of volatile fractions. The problem is that we don't know how much of the oil that leaked a mile down made it to the surface. The portion that didn't, which in NOAA's parlance was dissolved, naturally dispersed or chemically dispersed--potentially up to 49% of their total estimate--could still be in the water column, along with the 26% "Residual"--less the unknown portion actually broken down by bacteria and other processes. And it's some of this remaining oil that makes up the plumes we've been hearing about.

The undersea oil plume currently in the news was found in June by scientists from the Woods Hole Oceanographic Institute. They describe it as being at least 22 miles long, 1.2 miles wide, and 650 ft. high. The total volume of the plume, assuming it filled that entire rectangular solid, would be about 3.6 trillion gallons. However, the critical data point that I didn't see reported in any of the newspaper accounts I read was the concentration of oil in that water. According to the report on the Woods Hole site, the concentration of specific oil-derived molecules ("BTEX") is "in excess of 50 micrograms per liter". Adjusting for the density of the chemicals in question, that means that they found oil-related concentrations of approximately 57 parts per billion by volume. So by my math, the total volume of these chemicals within the plume is on the order of 200,000 gallons, or under 5,000 bbl. Unless these chemicals are only the tip of the iceberg in terms of oil derivatives in the plume--and Woods Hole hints that there is more--then we're talking about less than 0.1% of the 4.9 million barrels estimated to have leaked into the Gulf. In other words, while a plume like this might be potentially serious for aquatic life, it's not clear how much doubt its existence casts on NOAA's analysis of where all the oil went.

I will be very interested in seeing further refinements of all these estimates in the weeks and months ahead. Perhaps the media will even include more of the details crucial for putting it into perspective.