IN SOME ways, the “static kill” of its Macondo well that BP pulled off on August 3rd changed little. The flow of oil leaking into the Gulf had already halted, and plans were on course for sealing off the well at great depth with a relief well around the middle of August.

But the static kill made the well more secure and an added sense of confidence to the work on it, a sense of being in charge and getting things done right. The well, or at least part of it, can now be sealed with cement pumped down from the surface, a procedure which the government authorised late on Wednesday 4th and which should get under way on Thursday 5th.

Even without this next procedure the well is now far more stable than it was before; after the cementing there will be a whole succession of barriers between the Gulf and the oil. The drillers of the relief well will be able to expect a safe, almost routine encounter when they get there (though they would be ill advised to bank on it). Coupled with a government estimate, released on 4th August, that only about a quarter of the oil that spewed out of the well still poses a threat to the environment, it seems that, for the first time since the loss of the Deepwater Horizon drilling rig in April, some good news is coming from the Gulf.

Before this week's static kill, the capping system fitted to the top of the well three weeks ago had been holding back oil that was under high pressure (about 7,000 pounds a square inch, which is almost 500 times atmospheric pressure). If the seals holding the oil back had given way, this pressure would have ensured that the oil once more spurted out into the Gulf. The static kill got rid of that risk by gently pumping a mineral-laden drilling mud into the top of the well. This gloopy liquid is much heavier than oil, so it was able to push the oil down. The lower the level of the oil, the more mud there was on top of it, and the greater the weight pushing the oil lower still. Once the process was finished, in a matter of hours, the oil was held down in its reservoir by a column of thousands of feet of mud.

The difference this makes is the difference between having the wolf at your door—albeit a pretty well-bolted door—and having it back in its lair on the other side of the hill, unwilling to sally forth. The cementing set to begin on Thursday will go further, by blocking up the door to the lair. That does not, though, finish the story.

The well consists of a production casing—a long steel tube—hung within a wider hole. In a properly-functioning well all the fluids involved, be they mud going down or oil coming up, should be inside the production casing while the space between the casing and the outside of the well (the annulus) is sealed off. The Macondo well is a set of fatal initial mistakes, a blowout and three months of uncontrolled flow removed from being a properly-functional well. It is not clear how much oil there is in the annulus, or how it might or might not be able to get from the annulus into the production casing.

From what BP is saying it seems likely that its surface cementing will put a solid plug into the production casing, but quite possibly not into the annulus, which means there might be a way for the wolf to wiggle out. This is one of the reasons that the relief well is still going ahead: it will pierce the annulus and allow the engineers to flush it with mud and close it definitively with more cement. After that the company will probably add more cement to whatever sealing-off of the production casing has been achieved. Killing the well once is not enough. As Admiral Thad Allen of the coastguard put it “there should be no ambiguity about that. I'm the National Incident Commander, and that's the way this will end. It will be end[ed] with the relief wells being drilled, and the annulus and the casing being filled with mud and cement being poured.” The energy secretary, Steven Chu, made the same point more poetically to Joel Achenbach of the Washington Post: “You want to make sure it's really dead, dead, dead. Don't want anything to rise out of the grave.”

What lies above

Heavy steel, miles of mud and, soon, tonnes of cement should rule out any oily resurrection. Meanwhile, at the surface, a lot of the oil that did get out has done little harm. According to an analysis published by the American government on Wednesday August 4th, roughly a quarter of the oil has evaporated or dissolved, a quarter has dispersed—meaning it's mixed up with water at a very fine scale in a form that microbes will find pretty digestible—and a quarter was dealt with directly in some way by the response. The residual quarter is what has been gathered up from the beaches and what is still out at sea in slicks and tarballs, or buried in sediment. Most of it is still in a position to kill and besmirch things; but it is also subject to further evaporation and dispersal.

Of the oil that dispersed, two-thirds did so naturally, a third did so as a result of the use of chemicals designed to aid the process. Of the oil dealt with before it did harm, about two-thirds was recovered from the well by the "top hat" and other containment systems BP put in place in June; more of the rest was burned from the surface than skimmed off it.

The large amount of dispersal and evaporation is due to this having been a leak of light crude, not the heavy sticky stuff, into a warm sea in which wind and waves stir the water up continuously,, but where circulating currents have kept the oil fairly well contained. In July the government removed its restrictions on fishing in the eastern part of the zone that was originally judged at risk, and now that no more oil is escaping there seems to be no chance of any significant amount getting to the Florida Keys, escaping the Gulf and flowing up the east coast, as some of the more apocalyptic visions suggested it might. No new oil may also mean that other fisheries can be reopened before too long. Perhaps the worst remaining risk is that a storm surge pushes oil deep into Louisiana's fragile wetlands. Recently-updated forecasts insist that there remains a risk of a larger than usual number of big storms in this hurricane season, despite the fact that so far none has shown up.

That most of the oil has done no harm does not alter the fact that even a fraction of this spill is still a very large spill indeed. According to estimates of the total flow from the Macondo well released by the government's Flow Rate Technical Group (FRTG) on August 2nd, the residual unintercepted, undispersed and unevaporated quarter still totals more than 1m barrels of crude—a huge amount by almost any previous standards. The initial flow is now put at 62,000 barrels a day. The flow subsided a little as the gusher aged; but by the time the cap was fitted it was still about 53,000 barrels a day. Estimates of the flow rose as the flow itself fell, from what now seems an extraordinarily low initial estimate of 5,000 barrels a day.

On the basis of the FRTG figures, 4.9m barrels came out of the well and just over 4m leaked into the open ocean (the rest was dealt with by the containment systems). The fines BP faces for the spill will be levied on a per-barrel basis. Under normal circumstances they might be $1,100 a barrel, but could rise to $4,300 a barrel if the company were found to have been grossly negligent. Avoiding such a finding could thus now be worth some $13 billion to the company. BP's own preliminary report into what went wrong—which looks likely to be the first report on the subject published, as well as the one least likely to point to gross negligence on the company's part—is expected towards the end of August, at more or less the same time that the relief well will have shut away the Macondo wolf for good.

Source: http://www.whitehouse.gov/blog/2010/08/04/new-report-74-oil-bp-deepwater-horizon-oil-spill-has-been-contained-or-mitigated