Annular Unloading Efficiencies

[ENGpetro]

We're in an offshore field and annular unloading is a bit of an operational challenge. In case some may be unfamiliar with what we call annular unloading, we utilize gas lift in our field. Our wells sit in overbalanced completion fluid until they are handed over to production/operations. I should mention that the gas lift mandrel is sheared prior to the rig leaving site. This would be a hard sell to bypass due to risk mitigation purposes/intervention efficiency of having the rig on location if we had an issue.

Just prior to start up we need to clear the fluid in the annulus and displace it with gas. Liquid flow rate constraint through the gas lift mandrel is 1bbl/min as per vendor spec. That's not the problem though. System gaslift pressure upstream of the AWV is ~24MPa and downstream is 2MPa. If we were to open the valve or even use the choke to regulate flow we'd experience JT cooling to the point where we'd exceed -18C which is an equipment limitation of our SS tree.

A typical operation would include an intervention vessel equipped with N2. The downline would attach between the AWV and the AMV. Using N2, the pressure would be brought up to an acceptable value and the job would resume controlled with system lift gas. JT cooling would be insignificant.

Given the rough attachment schematic, is there any way we could exclusively use system gas to safely carry out this operation without depressurizing the system lift gas? A system depressurization would take too long and result in lost production from all well on the system. Waiting on the vessel and the cost associated is also not ideal. Am I missing a solution here? What's industry experience?

 

[LittleInch]

This forum isn't wildly active so you might not get many responses here - try the pipelines and piping forum or link to this post to get a few more responses...

Anyway, it is not clear to me exactly what you mean by " Using N2, the pressure would be brought up to an acceptable value and the job would resume controlled with system lift gas. JT cooling would be insignificant"

what is an "acceptable value"??
Which job would resume? - Gas lift?
JT cooling insignificant - Why?
What is lift gas temp?

What sort of flow are you looking at during the high DP operation? - presumably the 1 bbl/min actual volume?? That's not a lot.

Often simple gas temp calculations from a process package don't translate into metal temperature, especially at low gas flow rates. Usually you can get more DP and more flow if you control on actual temperature. Many times I've seen that concerns over low temperature don't actually materialize.

I would be more concerned about hydrates, but if the gas is dry enough maybe that's not an issue?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

24MPa is a very high pressure for lift gas - am used to seeing this range of pressure for injection.

Is there a design pressure concern with 24MPa supply pressure with the annulus ? If there is no design pressure concern, then I'd go start - stop, start - stop.

Where would you unload the annulus liquid to - into the production tubing ?

Am not used to assisting well engineers with these concerns, but I'll try.

 

[zdas04]

If you are lifting water from the bottom of an 8000 ft welbore that is the kind of lift pressure you need. In some of the deep water (which would require nearly 24 MPa just to get from the sea floor) it is much higher.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist