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A few of weeks ago this TOD comment was linked to at least three times on ET. Here's asdf:
What's really going on out there:

Probably the scariest well I've been on in the GOM DW was about 6 years ago. They had set csg and were drilling ahead at 22,000'. And then they started to loss circ. They weren't sure where but it might have been at the previous csg shoe. They lost 60,000 bbls of OBM while drilling. No mud ever returned to the surface. So no mud log telling them if they had drilled oil/NG, no LWD to estimate pore pressure, no mud parameters to tell if the MW was being cut by oil, NG or water. And most importantly, no way to tell if the well was kicking. They put very heavy drill mud on the outside of the drill pipe but that would have not stopped a blow out coming up the inside of the DP. Took me 6 days to log that 2,500' of open hole. I ran pressure logs in the wet reservoir they cut: 19,000 psi bottom hole pressure. They were probably very lucky they didn't find oil/NG in that sand: a blow out could have easily happened. How scary was it? Some of the hands were sleeping in the escape capsules when they were off tower. And this insane risk was taken by a well known and very experience operator. Needless to say someone very high up in the company was willing to risk the 130 souls onboard that drillship to get this well down. Equally needless to point out: that person never set foot on that rig. We just finished the job, went home, cashed our pay checks and then tried to forget about it.


In that thread, TOD commenter Rockman /the same as quoted by asdf) also writes
Prior to drilling a DW well there's needs to be an estimate of the pressure gradients Euan has described.


Even when there isn't a dramatic change in rock pressure there are limits to the range of mud weights used to drill a hole. That's why we see so many csg sets in the RW's. Too light a MW and the well flows. Too high a MW and you fracture the rocks and can easily lose the hole.


Consider drilling the same rocks at this location but assume the is no oil/NG in the reservoir. The MW needed to stop the flow of water out of the reservoir might be 14.0 ppg. The rock might be fracture at a MW of 15.0 ppg. That's a fairly wide margin and should not be a problem. But now put a 400' tall column of oil/NG in the reservoir. One big reason the DW plays have attracted so much attention is the very tall hydrocarbon columns encountered out there. This tall hydr. column will raise the pressure in the reservoir to the point that a 14.7 ppg MW is needed to contain it. Now you have only a 0.3 ppg margin before rock failure. And this is where DW drilling offers a challenge seldom seen elsewhere: the ECD factor. ECD is the effective circulating density. The mud might weigh 14.7 ppg but when the mud pumps are running the effective mud weight (ECD) at the bottom of the hole maybe several tenths of a ppg higher. So in order to not fracture the rocks you might pump a 14.5 ppg (with an ECD of 14.9 ppg...less than the 15.0 ppg that would fracture the rocks). But when you turn the pumps off to add drill pipe or pull out of the hole the ECD drops to 14.5 ppg....less than the 14.7 ppg needed to keep the reservoir from flowing oil/NG to the surface. I've seen operators drill into such a situation: you can't raise the MW to stop the well from kicking, less you fracture the rocks, and you can't turn the pumps off to pull out of the hole because the well will kick. I've seen operators pump cement down such a well to kill it and then plug and abandon the well.

This is probably what happened at the Macondo well that Deepwater Horizon was drilling. The structural integrity of the rock has been compromised.

By laying out pros and cons we risk inducing people to join the debate, and losing control of a process that only we fully understand. - Alan Greenspan
by Migeru (migeru at eurotrib dot com) on Thu Jun 17th, 2010 at 04:32:37 AM EST

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