Hydrate inhibitors selection and injection rate 2

[oilpmp]

We have been recently confronted with gas wells prone to some hydrate problems. In terms of pipes, we do not expect plugging to occur any time soon, but we need to look at the problem upfront and evaluate which preventive solution could be most adequate. For instance, we are considering an investment on the project based on injection of alcohol or glycol, probably it would be a methanol-based solution.
I found some interesting resources online on the matter, for example:

[URL unfurl="true"]https://hrcak.srce.hr/file/106473[/url]

Could someone point me to a calculator or spreadsheet that would help estimate the injection rate based on the chosen hydrate inhibitor(s) and the specific well data at hand (PT conditions and samples composition) ? could you share your experience and recommend some suppliers too?

Thanks

 

[SJones]

This is a good starting point for those not overly specialised in flow assurance:

https://www.scribd.com/doc/313888526/Offshore-Hydrate-Engineering-Handbook

The usual suspects of the chemical supply world, such as BHGE, Nalco Champion, etc., will happily call round for a visit. Hydrates are indicative of wet gas, so there may be a corrosion issue to think about that could also require a chemical treatment. Mutual chemical interference could then become a problem. You might think about getting an appropriate flow assurance consultant on board.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[georgeverghese]

The GPSA has a whole chapter dedicated to gas dehydration related topics including hydrate inhibition. If you are looking at intermittent inhibition, also talk to vendors who supply Low Dosage Hydrate inhibitors(LDHI) and or LDKHI - low dosage kinetic hydrate inhibitors - these cost considerably more than conventional inhibitors. If the downstream point is on the borderline of hydrate formation, I would reckon you dont need it - this is my experience with operating at these pressures / temp combinations.

 

[LittleInch]

There are many options and previous studies which will help point you to the correct one for your operation.

Things like how often does the flow start / stop (the kinetic ones work when there is flow - not great if it starts and stops)

How much water do you have?
How does temperature vary?
Can you lower the operating pressure?
Where is injection feasible (U/S of choke usually best)
What are the issues with methanol storage? This is a dangerous chemical and has many downsides in terms of toxicity, flammability and handling)
What is the impact on downstream equipment / user?

For injection rates you really need to understand the phase envelope of you fluid and how much you want to be in the safe zone.

Flow assurance engineers love showing the hydrate curve as a line where reality is that it is in fact a fuzzy boundary. some system will continue to work inside the hydrate curve due to flow rates, velocities and water content. Others start plugging up with only a small excursion inot it. Some work happily when flowing but have issues on start up.

I would employ a good flow assurance / engineering consultant and not fiddle about the edges, especially with methanol. Horrible stuff.


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[apetri]

in this field, there are not simple and accurate solutions
you need complex models to predict accurately hydrate formation, the influence of inhibitors etc.

you may also need a good flow assurance / engineering consultant as already suggested...

anyway, I have tested the hydrate formation conditions predicted by Prode and found the CPA based model accurate with methanol,

https://www.prode.com/en/hydrate.htm

 

[oilpmp]

Hi sorry for delay in answering back. Thanks for your help so far.

Sjones: yes and we are shortlisting one or two consultants via our procurement department. we'll how it goes.
georgeverghese: forgot about the GPSA. Good reference indeed, thanks.
Littleinch: thanks for triggering these questions. Regarding the phase envelope, we have made some simulations (and sensitivity analysis) with Hysys so far (Platteeuw model). How reliable is that model (<1%?)... that is also a concern.
apetri: noted. I will look into your link.

 

[apetri]

almost all complex models for hydrates are based on van der Waals-Platteeuw model,
there are many variants, you may read the Sloan's book for details,
accuracy (comparing predicted hydrate formation pressure vs. experimental data poits) varies depending fron structures (I, II ...) formers, conditions etc. to give you an idea, the hydrate van der Waals-Platteeuw model included in Prode should predict hydrate formation pressures for SI with errors < 7% (average) and < 12% for SII (average), but these numbers are for std. compositions and conditions, you may observe larger erros at high pressures (> 300-500 Bar) etc.
I have not Hysys however I would expect similar numbers,
note that with inhibitors the uncertainty about predicted formation pressure is much larger, I have observer errors of 50% and above for mixtures,
while, if you are interested to formation temperature, the relative error is considerably smaller (Tpredicted-Tmeasured)/Tmeasured

 

[LittleInch]

Nothing about hydrates or inhibition is that accurate (1%). Normally the consequences of a hydrate blockage out weigh pouring a bit more methanol or MEG into the system

The incoming fluid also changes with time, water content, etc and hence getting it precise is very difficult.

There is often a big difference between start-up / shutdown and continuous operation. Problem often is that if you don't overdose and then the flow line shuts down suddenly, you can easily hydrate up on cold re-start.

It's a complex issue and highly dependent on many issues which are particular to your field / design / operation.



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

 

[MortenA]

I agree with Littleinch wrt accuracy. Normally i would assume a 5ºC margin to HFT and then assume "pure water" (no allowance for any salts) for design. You may the want to experiment when you go into production.

Best regards, Morten

 

[gaswell]

in my opinion, there are so many different conditions, formers, structures and compositions (say as example hydrate formation at sea, GHSZ) and probably errors differ from case to case,
anyway for the mentioned gas well composition I think the values provided by apetri are correct (tested several calc's with CSMGEM , Prode and other tools)
also the mentioned van der Waals-Platteeuw model is required to obtain accurate results,
simpler methods can produce much larger errors.