GOR and pipeline pressure drop

[aalshaiba]

Hello everyone,

I was analyzing a pipeline design from a wellhead tower to a recipient separator. The objective was to see whether the GOR increase can lead to overpressure the downstream facilities as it will see more gas eventually (flare system).

However, the analysis indicated something quite interesting; increasing the GOR lead to increasing in the pressure drop (i.e.: downstream facilities would face lower pressure each time!). This was against my intuition as I thought the opposite may occur. Anyway, when looking at the two-phase flow regime map, it seems that having high GOR pushed the wellfluid beyond the slug flow regime (bubble flow regime). Still I am trying to make sense out of it, but I am having difficulty analyzing this.

Could anyone help me with this?

Thanks

 

[zdas04]

The thing to keep foremost in your mind when trying to puzzle out multi-phase flow is that there is no such thing as steady state multi-phase flow. Ever. The flow regime map is a trap. You look at superficial liquid velocity (often monthly volume divided by the number of seconds in a month) and superficial gas velocity (again we work mostly with averages and daily cums divided by the number of seconds in a day or month) and say "I'm in bubble flow" (or stratified or slug or whatever). Trouble is that at any given point at any given second the flow is very different from you predictions using averages of averages.

It is all about the energy, and the energy at a given point will change many times a second on a random sequence. CFD models do a slightly better job at representing this non-steady-state flow, but they are still +/-50% of actual pressure drops. It is very enlightening to set up a flow loop and inject enough gas and water to give a meaningful GOR. The chaos is inspiring.

The model you ran is possibly sometimes in the right direction, but it is also very possible that there will be periods during a given minute when the model is exactly wrong. Putting any faith an in CFD of multi-phase flow is quite risky.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[aalshaiba]

@zdas04

Thank you for your quick reply. The model I ran was an OLGA model FYI

So what you are trying to say is that there is no general trend with the GOR versus pressure drop relationship?

 

[zdas04]

Pretty much.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MortenA]

David,

For all its worth i think you are being too pesimistic! The company i works for runs several long sub sea multiphase tie-back, and the accuracy of these models we find to be sufficient for pipeline sizing and solving ceratin operational problems. The OLGA model is not SS although a SS version do exists.

So aalshaiba now you have two different opinins from either side of the spectrum. Although, i would never say that the model is pefect and that you can use it for predicting the exact flow and pressure at a given time. But it will give an indication of design parameters and operating envelope.

Best regards

Morten

 

[zdas04]

A lot of people run CFD models in multi-phase flow and find them useful. When I have to review systems that were designed using them I generally find that: (1) when the model would lead to a different decision than the company wanted to make in the first place they ignore it and pretend that they never ran it; (2) if the model supports the decision that they wanted to make then they tout its performance. My kids did the same thing with a Ouija board and my great grandpa did it with a water witch. I find that as long as you are willing to disregard outrageous results, CFD doesn't do that much harm.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[MortenA]

Personally i havnt seen CFD for a pipeline (except for some specific low temperature issues during start-up of a well) and OLGA is a 1D model (correlation). But i still put some value in it for design and for solving operational issues, and we have good experience with it.

Best regards

Morten