r/petroleumengineers • u/tradikal2 • 12d ago

Gas Cavern Conundrum

So in a recent HAZOP it was decided to add a second layer of pressure protection on the wellbores connecting the salt caverns to the wellheads. On the first (and only thus far) well we ran plugs removed the wellhead, added a hangar and new production string inside the wellbore casing which was already cemented in place (for roughly 40 years now). A cement job was done to cement the new tubing/production string.

When the wellhead was finally in place we found that the 2500m of annulus cement was not holding pressure. Seemed like a bad cement job. The curious thing is that we were seeing higher pressure buildup in the new tubing hangar (communicating with th ecemented annullus) than was in the cavern. we were seeing slow buildups to 5000psi with only 4000 psi in the production string.

My personal opinion was a hydraulic multiplier effect inside the cement - but no way to prove it obviously. Bond logs do not show anything out of the ordinary (but I'm a surface guy so I wouldn't know what to look at.

Anybody here ever see a similiar phenomena? Anyone have any ideas how the hanger could buildup more pressure than what is in the cavern? Thanks in advance. Note: I am not the responsible engineer in this case, just a casual observer doing work on the surface facilities.

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u/HoleDiggerDan 12d ago edited 11d ago

Thermal expansion during cement setting? Does that pressure remain after bleed off?

Casing integrity issue? Is the pressure you're seeing match the reservoir pressure behind the casing?

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u/tradikal2 12d ago

I can see some expansion of the cement during curing - due to the additives to prevent shrinking. This would be chemical not thermal expansion. Pressure is routinely bled to zero, then begins to build up slowly over days or now weeks. We added a pressure alarm to the Operators console. The pressure builds up past the reservoir pressure as I described. Close to the API rating of the wellhead itself. Casing Integrity of the wellbore string was raised by the downhole consultant -speculating an external source of gas higher up the string getting into the cemented annulus. Seems unlikely to me.

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u/HoleDiggerDan 12d ago edited 10d ago

By bleeding off the initial annulus pressure and seeing the pressure return, we can agree it's not related to the initial cement job.

So it goes back to the second point: casing integrity. What are the various formation pressures behind the casing at their respective depths? Does 5000 PSI WHP correlate to geology zone Y? (Nothing related to the gas storage itself, you have already stated that storage pressure is lower). Does that 5000 PSI pressure related to a nearby storage cavern... do the well bores have communication?

Pressure has to come from somewhere and I am leaning toward agreement with your downhole consultant... Start with a diagram and draw your boundaries (like a basic thermodynamics problem). Work outward from the wellhead.

Next step is to attempt to locate the pressure influx, that could be done with sonic tools that are listening for flow in the annulus.

(Root cause question, why are they cementing their production tubing? How are they going to change it out in a few years when needed for corrosion.)

Edit: the more I think about it , the more likely it is communication from somewhere else. I mean, that would be the impetus for the entire "We need a second barrier" plan in the first place.

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u/dirtyjon7 12d ago

Annular gas sample may narrow some things down fairly quick for cheap. Noise/temp log (the expensive kind) are pretty good now at finding flow behind pipe (works better if you can flow/vent the backside too). I'd first be focused on the area around the new casing shoe (was a shoe test done to the max storage gradient before drilling out the OH plug?

Bond logs in these can be difficult. Usually caverns have some pretty big tubular's, (was a cbl or sbl ran? what size pipe?) and it seems like you're describing a liner that tied back to surface. You may have too thin of a cement sheath to really determine bonding depending on the old production string ID.

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u/tradikal2 9d ago

They claim to have run centralizers with turbulators.

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u/tradikal2 10d ago

Yes that is the drilling consultants working theory. I don't recall if they tried sonic logs. With the slow pressure buildup it would be hard to detect.

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u/tradikal2 10d ago

Agree there was no reason to cement the production string. It was fine as a second barrier with just an annular packer in place. The they should also have installed an SSSV as well.

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u/tradikal2 10d ago

Annular gas sample and analysis is a great idea. I don't think that was done. I don't have access to the original drilling logs to know if that well had hit a gas zone on the way down. The consultant would have though and didn't point it out in their report. As to the shoe condition or pressure tests nothing out of the ordinary was reported - but who knows right?

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u/oilboomer83 8d ago

Higher pressure than expected reservoir pressure? Are you on injection now?