I've been badgering a longtime family friend (who has considerabe experience in oilfield geology) for something to publish here, and respect his wishes to remain anonymous. Here's what he sent me tonight:
Attached is from one of several e-mails going around the oil industry at the moment about the BP disaster. It is difficult to know what it's provenance is, but is clearly written by a driller. Keep in mind that an enquiry hasn't been held yet and not a lot of information has been released.One thing we do know is that the Mocando oil-field beneath the burnt out hulk of the DW Horizon was a tough prospect to drill. This is due primarily to extremely high formation pressures. Excessive formation pressures (or overpressures) can be a drillers worst enemy. Maintaining safety and control of the borehole can take a lot of time and money and these wells can literally take many months to drill. They had apparently managed to drill to some 18,000 feet and were suspending the well when things went bad. They had actually managed to keep control of this well during months of drilling, then at the last post, things went awry.
A quick geology lesson to explain the pressures: the main (but not the only) cause of overpressures is when an impermeable rock, like a claystone, is deposited quite rapidly, and as the clay particles compress around the formation fluids (water, gas, oil) the fluid can't escape into a nearby aquifer. Overpressures aren't uncommon in basins, we see them in many of the basins in Australia, and they can be managed by proper well planning (you would not believe the engineering that goes into one of these wells - it is truly staggering).
The Gulf of Mexico (called the GoM in the industry), however is one of a suite of basins around the world, that are very young, and very rapidly deposited (the Niger Delta is another example). Generally, these are basins formed outboard of the worlds major deltas. In these basins, you have overpressures on steroids. We don't have any big deltas in Australia. Another tricky thing about the GOM is the abundance of salt. There are literally hundreds of cubic kilometres of salt that actually act like a lava lamp. We don't see a lot of salt in Australian basins, save for a couple of small diapirs in northern Australia.
As a rule of thumb, the higher the overpressures, the less margin for error. You can read the note below in italics and wading through the geotechnical terms you can get the jist.
The BP Moncado well is one of several wells drilling a frontier "play" over the past decade or so in the GoM. It is a brave new world with wells now sometimes drilling over 10 km deep into the middle of a significantly overpressured delta. Rumours of formation pressures of 15 to 20 thousand psi are going around for the Moncado well (this would be extreme though if it were the case. I am sceptical). BP isn't considered a slouch when it comes to drilling wells. It is regarded as a high-end technical company within the industry. However, the articles and emails I have read such as that below seem to point at significant judgement errors being made during operations. I suppose the question to be asked in the inevitable enquiry is what went wrong and whether these deep high-pressure wells can repeatedly drilled safely. The enquiry will tell whether this is the case or whether there has been a monumental screw-up by BP and or the drilling contractor. One would imagine a deepwater drilling moratorium will be in place in the meantime.
Also of interest is this article, albeit a little inflammatory in the language. The discussion on how a well is drilled is a good summary for a lay person
http://mendocinocountry.com/independent/1issuepdfs/iunit66.pdf [1]
Horizon Incident - Yet Another Angle
Good description of what happened from an interview....
This well had been giving some problems all the way down and was a big discovery. Big pressure, 16ppg+ mud weight. They ran a long string of 7" production casing - not a liner, the confusion arising from the fact that all casing strings on a floating rig are run on drill pipe and hung off on the wellhead on the sea floor, like a "liner". They cemented this casing with lightweight cement containing nitrogen because they were having lost circulation in between the well kicking all the way down.
The calculations and the execution of this kind of a cement job are complex, in order that you neither let the well flow from too little hydrostatic pressure nor break it down and lose the fluid and cement from too much hydrostatic. But you gotta believe BP had 8 or 10 of their best double and triple checking everything.
On the outside of the top joint of casing is a seal assembly - "packoff" - that sets inside the subsea wellhead and seals. This was set and tested to 10,000 psi, OK. Remember they are doing all this from the surface 5,000 feet away. The technology is fascinating, like going to the moon or fishing out the Russian sub, or killing all the fires in Kuwait in 14 months instead of 5 years. We never have had an accident like this before so hubris, the folie d'grandeur, sort of takes over. BP were the leaders in all this stretching the envelope all over the world in deep water.
This was the end of the well until testing was to begin at a later time, so a temporary "bridge plug" was run in on drill pipe to set somewhere near the top of the well below 5,000 ft. This is the second barrier, you always have to have 2, and the casing was the first one. It is not known if this was actually set or not. At the same time they took the 16+ ppg mud out of the riser and replaced it with sea water so that they could pull the riser, lay it down, and move off.
When they did this, they of course took away all the hydrostatic on the well. But this was OK, normally , since the well was plugged both on the inside with the casing and on the outside with the tested packoff. But something turned loose all of a sudden, and the conventional wisdom would be the packoff on the outside of the casing.
Gas and oil rushed up the riser; there was little wind, and a gas cloud got all over the rig. When the main inductions of the engines got a whiff, they ran away and exploded. Blew them right off the rig. This set everything on fire. A similar explosion in the mud pit / mud pump room blew the mud pumps overboard. Another in the mud sack storage room, sited most unfortunately right next to the living quarters, took out all the interior walls where everyone was hanging out having - I am not making this up - a party to celebrate 7 years of accident free work on this rig. 7 BP bigwigs were there visiting from town.
In this sense they were lucky that the only ones lost were the 9 rig crew on the rig floor and 2 mud engineers down on the pits. The furniture and walls trapped some and broke some bones but they all managed to get in the lifeboats with assistance from the others.
The safety shut ins on the BOP were tripped but it is not clear why they did not work. This system has 4 way redundancy; 2 separate hydraulic systems and 2 separate electric systems should be able to operate any of the functions on the stack. They are tested every 14 days, all of them. (there is also a stab on the stack so that an ROV can plug in and operate it, but now it is too late because things are damaged).
The well is flowing through the BOP stack, probably around the outside of the 7" casing. As reported elsewhere, none of the "rams", those being the valves that are suppose to close around the drill pipe and / or shear it right in two and seal on the open hole, are sealing. Up the riser and out some holes in it where it is kinked. A little is coming out of the drill pipe too which is sticking out of the top of the riser and laid out on the ocean floor. The volumes as reported by the media are not correct but who knows exactly how much is coming?
2 relief wells will be drilled but it will take at least 60 days to kill it that way. There is a "deep sea intervention vessel" on the way, I don't know if that means a submarine or not, one would think this is too deep for subs, and it will have special cutting tools to try to cut off the very bottom of the riser on top of the BOP. The area is remarkably free from debris. The rig "Enterprise" is standing by with another BOP stack and a special connector to set down on top of the original one and then close. You saw this sort of thing in Red Adair movies and in Kuwait, a new stack dangling from a crane is just dropped down on the well after all the junk is removed. But that is not 5,000 ft underwater.
One unknown is if they get a new stack on it and close it, will the bitch broach around the outside of all the casing??
In order for a disaster of this magnitude to happen, more than one thing has to go wrong, or fail. First, a shitty cement job. The wellhead packoff / seal assembly, while designed to hold the pressure, is just a backup. And finally, the ability to close the well in with the BOP somehow went away.