Can a cemented well casing really take 15000 psi ? 4

[docellen]

Seems to me that the casing would break loose from the cement.

Young's modulus for steel is about 29 x 10**6 psi. If the cross-sectional area of the steel is about 10% of the entire pipe, 15,000 psi in the pipe will produce about 150,000 psi in the steel, which will stretch it about 0.5% ( 1mm for every 200mm of pipe length).

Now assume the pipe is embedded in rigid cement and rock. The cement will shear long before the pipe moves even 1mm, probably at the smooth interface between the steel and the cement. The action will start in a thin band at the edge of the cement near the top of the pipe, and move down quickly.

Am I missing something?

 

[DrillerNic]

Docellen I have no idea what the rupture disks are, or if they are in the 16" casing string of the drill pipe work string.

You are correct in that all the inner pipes are intemediate strings of casing. The production casing is the 9-5/8" x 7" tube and the picture shows the 5" drill pipe with 3" cement stinger below it as they were at the time of the blowout.

14SOBM is 14ppg Synthetic Oil Based Mud, and the 14 at the top shows that you have 14" mud behind the 9-5/8" x 7" production casing. I'm not sure if the white portions of the diagram show where the mud is or not.

The reason the drill pipe and the upper part of the production casing have been displaced to seawater is to perform an inflow pressure test- with 14ppg outside the casing and a mixture of 8.7ppg seawater and 14ppg mud inside, the pressure inside the production casing is less than outside. If you shut in the well, and monitor the pressure on the drill pipe, then
no pressure increase = no leak = casing integrity proved under inflow conditions
(they had already proved casing integrity was OK the other way by pumping up the pressure inside, but there are flapper valves in the end of the casing that might hold in a positive test but leak on an inflow test)

Sorry for the use of terminology- casing shoes are simply the bottom of the casing. The casing is suspended either from a casing hanger set in the wellhead (as in the 28", 22" and 9-5/8" x 7" string) or suspended from a liner hanger which is attached to the previous casing string (represented by the black rectangle at the top of the 18", 13-5/8", 11-1/8" and 9-7/8" casings).

 

[docellen]

Excellent discussion, DrillerNic. I wish other oil-industry experts would be as helpful.

There is a much more readable diagram of the well, with essentially the same information as on the DOE site, at

http://en.wikipedia.org/wiki/Offshore_oil_spill_prevention#Well_casings.

You might want to take a look at that and make any corrections you think are appropriate. The diagram is used as an example in an article intended to be more than just a discussion of this one situation, so feel free to modify it, even if your modifications don't match any particular well. What would you do if you were designing a similar set of casings?