Can a cemented well casing really take 15000 psi ?
Can a cemented well casing really take 15000 psi ?
(OP)
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?
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?





RE: Can a cemented well casing really take 15000 psi ?
A rebar buried to 40 times its diameter in concrete will break before pulling out.
Is there a similar ratio for casing?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can a cemented well casing really take 15000 psi ?
longitudinal stress, Pressure * Area Flow / Area Steel
and
hoop stress Press * Diam / 2 / Wall thickness
On the 21" x 1" wall casing, if pressured to 15000 psia
at 5000 ft depth there's 2000 psi external pressure = 13000 psig.
Longitudinal stress is about 68 ksi
Hoop stress is about 158 ksi
I'll assume 80 ksi steel (no idea if that's right)
Combined stress limit of 90% = 72 ksi
The longitudinal stress is almost at that limit by itself, and you have to add the 158 ksi, which is already 2 times the pipe yield stress.
So, pipe doesn't stretch, it already split before that.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Can a cemented well casing really take 15000 psi ?
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can a cemented well casing really take 15000 psi ?
RE: Can a cemented well casing really take 15000 psi ?
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Can a cemented well casing really take 15000 psi ?
Maybe Bill's rebar analogy is the best we can do. Pull on a rebar, and the cement bond near the surface will crack, but if the depth into the concrete is 40 times the diameter, it will hold.
I wonder if repeated pressurization will make a difference. Maybe each time the sheared zone moves down a little further. Maybe there is a limit to this process, where the expansion of the pipe, even in what must be loose powdered cement will still provide enough grip to keep the pipe from pulling out.
Has anyone seen a BOP under high pressure? Does the wellhead come up out of the ground a few inches?
RE: Can a cemented well casing really take 15000 psi ?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can a cemented well casing really take 15000 psi ?
In a well casing, any upward force from internal pressure on an "endcap" effect of a closed ram or valve would be resisted by the steel pipe immediately below it pulling down with longitudinal stress set up in the casing steel. There would be no net pullout force on the casing itself.
Laterally, internal pressure expands the steel, perhaps increasing contact stress against the cement and soil somewhat, perhaps also cracking the cement around it making contact stress with the soil ineffective, but there's no net lateral forces from internal or external pressures acting left or right, up, or down on the casing anyway. Effectively you have a can of Coke embedded in the sand. Yes it has internal pressure on the lid, on the inside walls and sand pressure on the outside walls, but the can isn't going anywhere until the internal pressure increases and either splits it or blows the lid off.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Can a cemented well casing really take 15000 psi ?
My whole point being that hoop stresses from internal pressure are usually going to control the situation, not longitudinal tensions from end cap effects.
Actually axial tension, according to Mohr's circle analysis, helps the combined stress, as it reduces shear stress; the real mode of failure stress. Axial compression combined with hoop stress increases shear stress, so is the more dangerous of the two.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Can a cemented well casing really take 15000 psi ?
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Can a cemented well casing really take 15000 psi ?
Lets do the maximum burst load case: internal pressure is 15,000psi, external hydrostatic at the wellhead is 2236psi, so resultant stress in casing at wellhead is 15000- 2236 = 12764psi.
Burst rating of 9-5/8" 62.8ppf Q125 casing is 13851psi. we're OK (just).
Tensile stress.... thermal effects: thermal gradient of 1°/ 100ft (a bit high perhaps?), mudline temperature at 5000ft of 40°: reservoir temperature at 18,000ft of about 220°f. Assume casing temperature about 70° when cemented (ie close to surface temperrature) so differential temp of 150°.
F = EaT(OD area - ID area)
E = Youngs mod = 29x10^6,
a = coeff of expansion = 6.9x10^-9
OD area = 3.142 * 0.25 * 9.625 * 9.625 = 72.77
ID area = 3.142 * 0.25 * 8.625 * 8.625 = 58.43
T= 150
Thermal stress = 428,100lbs force
Tensile stress... pressure effects.
F = 2u(ID area* Internal pressure change - OD area* external pressure change)
u= Poisson's ratio = 0.3
No change in external pressure
let's assume the worst case and the change in pressure is 15000psi (ie we're ignoring the pressure due to the mud in the casing when it was run).
F = 2 * 0.3 * (15000 * 58.43 ) = 525,870lbs force
Tensile stress is additive, so total tensile stress form teh as cemented case to the blowout case is 428,100 + 525,870 = 956,970lbs force
Tensile yield of 9-5/8" 62.8ppf Q125 casing is 2,270,000lbs force, so OK.
I can't be bothered to work out triaxial stress using Von Misens, but I think the production casing is string enough to cope with 15000psi!
RE: Can a cemented well casing really take 15000 psi ?
I ignored thermal stress and ... I thought zdas said the casing was 21" diam x 1" wt. No wonder those hoop stresses were so high. That's a relief.
Is the temperature expansion coefficient alpha that low? I thought it might have been 100 times that.
I also thought that a pipe in contact with sufficient soil (typically anything over 250 m long) would develop enough skin friction to not allow low temperature axial expansion, eassentially making longitudinal thermal stress compressive, and also making the axial component of the pressure stress tension, thereby subtracting those axial stresses, not adding.
You don't seem to consider end cap stress either. Wouldn't you have that with a closed off valve?
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Can a cemented well casing really take 15000 psi ?
RE: Can a cemented well casing really take 15000 psi ?
Thermal stress can be ignored if the casing is free to move as it just expands. So, for example tubing with expansion devices (PBRs). If the casing is restrained (by cement, or by friction with the soils) the stress due to thermal effects has to be considerd. In the case of the Macondo well, we could actually argue that the production casing was unrestrained for most of it's length and so would expand slightly and actually buckle into the annuli provided by the earlier casing strings. Provided you don't reach critical buckling, the production casing is fine, and the actual stress in the casing is relieved slightly.
I know some pipelines use this strain based (rather than stress based) design methodolgy, but I don't think it's penetrated to casing design (and most drilling engineers probably wouldn't understand it, ha ha!).
Pressure containment is always supposed to be done by the casing. The function of the cement in a casing string isn't to support the casing, but to seal the annulus between the borehole and the casing (and if necessary to seal the bottom of the casing). This seal then allows certain selected zones in the reservoir section to be accessed with perforating guns.
In this case it appears (although so far this is just the most likely scenario) that the cement job didn't seal the annulus completely (either at all, or during the cement job, before the cement set), allowing gas up the backside of the casing. As the top of the casing was sealed, the gas couldn't expand, so you arrive at the surface with the pressure it had from the reservoir, plus the bottom hole pressure (this concept is a standard part of well control theory), which might have caused the casing hanger seal to fail. Whether the flow path from the reservoir to surface is up the annulus (ie the cement sheath has failed) or something else then broke and it's now coming up the inside of the production casing I don't know.... but the flowrates to me suggest up the inside of the casing?
RE: Can a cemented well casing really take 15000 psi ?
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: Can a cemented well casing really take 15000 psi ?
If we're talking about the 21" marine riser, under the requirements of API RP16Q, a riser analysis will have been done for BP for this well, which owuld consider a maximum pressure, bending load, fatigue and so on and combination loads, but I don't know what the design max pressure for this marine riser was. I can dig around and find some rig specs (maybe even the Deepwater Horizon's detailed rig specs) that might have some actual marine riser wall thickenesses, but probably not rated pressures...
RE: Can a cemented well casing really take 15000 psi ?
I see three "16-inch rupture/burst disks" inside the production tubing. I assume they are designed to take any conceivable pressure. It's hard to believe all three have failed.
It looks from this drawing that there is nothing to keep the oil from flowing up the space between the casing and the inner production tubing. How is that supposed to be sealed?
RE: Can a cemented well casing really take 15000 psi ?
I'm not sure exactly, but the two adapters on the well schematic look to me like liner hangers- that is a piece of kit screwed into the top of a piece of casing that has seals and slips that are hydraulically energised bite into the previous casing to support the liner. I'm not sure why the hangers at the top of the 18" and 16" liners are called adapters. It could be that this well did use some expandable casing strings (run the casing, then run a mandrel through it to expand it slightly so the next section can be drilled with a slightly larger bit) which might also explain some of the weird casing sizes: 10-1/8" for example.
A well is drilled in stages- drill a section, case it off and then drill the next section, and each casing or liner has to be able to withstand any pressures it might see from lower down. This proabably explains why they ran the 16" casing all the way from 11585ft MDBRT to just below the wellhead instead of to just above the previous 18" casing shoe- I guess the calculations showed that either the 18" or the majority of the 22" casing couldn't cope with possible pressures coming from below the 16" shoe.
Finally, the skinny tubing in the middle of the pipe isn't a proper production tubing string- it is the drill pipe and cement stinger work string. If the well had been completed, it would have had a production tubing string run in (4-1/2" or 5" tubing to fit inside 7" casing) with a permanent production packer run and set in the 7" casing just above the top of the reservoir to seal the annulus between the production tubing and the prodcution casing. As the well was still being constructed (they were about to set a cement plug somewhere about 7000-8000ft MDBRT to form the second suspension barrier before removing the BOP, installing a wellhead corrosion cap and going home), you need an open annulus between the drill string and the casing to bring returns back to the surface. In an emergency, this annulus is supposed to be sealed with the BOP pipe rams and the annular preventer, and in extremis, the BOP blind/ shear ram.
RE: Can a cemented well casing really take 15000 psi ?
In that case, the two upper cement plugs are not really there, and the drawing is just showing us where they were supposed to go.
The 7" pipe has a notation 14.0 SOMB. I assume that is 14 pounds-per-gallon mud. Also, there is a small number 14 just inside the top of the 16" casing. I assume that means the same. Should I assume all the white spaces in this drawing are supposed to be filled with 14ppg mud?
Why do they have seawater in the upper sections of the inner tubing? Wouldn't mud be a better balance with the mud just outside these tubes?
What are the "shoes" you are referring to? Are the casing sections suspended from above, or resting on these shoes prior to cementing?
RE: Can a cemented well casing really take 15000 psi ?
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).
RE: Can a cemented well casing really take 15000 psi ?
There is a much more readable diagram of the well, with essentially the same information as on the DOE site, at h