Gas Flow at two different pressure injection points

[pyMorty]

Hi,

Let's say there is a gas lift compressor handling 2 mmscfd and it is to inject the gas to two different wells, with different pressures. One well is 3500 psig, and the other one is 2000 psig. I know the well with the lowest pressure will take the most of the flow but, how can I calculate how much this difference will be? There is no regulator valve installed and I would like to know how'd the system behave, how much of the 2 mmscfd would the 2000 psig take letting the physics on its own. I assume I should use both Bernoulli's and Continuity eq's, would like to have a path to develop a solutionm though.


Thanks

 

[LittleInch]

Until the pressure at the relevant point reaches 3500psig, the other well will take 100% of flow.

You need to describe your system much better and look at flow and pressure drops and how the downstream system reacts to gas flow.

From the info provided it is not possible to arrive at a solution.

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

 

[zdas04]

I see this lash up pretty often (usually it is more than two wells supplied from a single compressor). Just relying on the dP across the pipe and the gas lift mandrels is a recipe for failure. The only way to balance the loads is with flow meters (I prefer V-Cones, but people have had some level of success with other measurement technologies), control logic, and automated chokes to the individual wells. That way you decide how much gas you want to send to a given well, not random dP. At your pressures, you are looking at some pretty expensive kit, but at those pressures the payoff tends to be worth it. Just sticking in a manual choke or a choke nipple can appear to work, but at then end of the day there is enough variation in downstream conditions to never give the wells what they need.

Remember, the goal is to provide the energy that each well needs, the goal is NOT to just inject gas. Far too often surface guys lose track of the goal.

David Simpson, PE
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

 

[pyMorty]

Thank you for your replies LittleInch and zdas04.

We are getting the regulator valve for the lowest pressure well to keep a fixed flow rate for it, preventing that way the well from taking all the flow, also measurement system for each one (this time based on orifice plate), the lead time is long, though. I was just trying to figure out how would the gas flow behave on its own. For example, our client is requesting us to inject 1.0 MMscfd to each well, one at 3500 psig, the other one at 2000 psig, I'm curious on whether the whole 2.0 MMscfd would go to the lowest pressure well (assuming the well can accept it, eventhough this could mean the well to decrease its production), or would it be just some more than 1.0 MMscfd, considering there is a single discharge line coming from the compressor and spliting into two equal diameter lines, one for each well. I'm trying to get the escenarios with calculations rather than hunches. Also I started studying Fluid Mechanics and want to apply this in my job. I'm aware dealing with compressible fluids is much more complex.

 

[zdas04]

The lead time really doesn't have to be long for orifice meters. A pair of ASME B16.5 Class 900 flanges, tapped in a machine shop for pressure take off, a paddle plate in the flow. I could have it running next week. Oh yeah, MOC, Critical Drawing Review, HazOp, ad nauseam might slow that up, but it won't add any value. You don't need custody transfer, just good enough.

The problem with the regulator is that the well requirements change second to second and the regulator is not up to the task.

All of the arithmetic you will be looking at for this flow is incompressible, not compressible. As a rule of thumb, as long as the pressure at the foot of a pipe is more than about 90% of upstream pressure we assume that the change in density is immaterial. Trying to do this calc with compressible flow calcs would give you very wrong answers (the correlations for low-velocity compressible flow don't work very well).

David Simpson, PE
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

 

[georgeverghese]

A flow meter on the gaslift supply to one of the wells is unavoidable. As a temporary measure, you could use a regular piping class globe valve till the gaslift choke arrives, since this is non erosive gas service.

For the flow meter, a crude temporary setup would be a restriction orifice and a DP guage - that should keep things going for a while - required flow is constant, and pressure upstream is also constant from the compressor.

 

[pyMorty]

Thank you for all your responses. I appreciate it.

Any of you know the model of equations I should use for branching piping pipes like this?

 

[georgeverghese]

A DN50 line should be adequate for 1mmscfd of injection gas to each well - and line velocity / frictional pressure drop is low.

Ask an instrumentation engineer to size and set up the RO for you; keep the beta ratio on the RO at higher than 0.2, and select a DP guage to suit the dp produced at the RO. A DN50 globe valve should be okay for now.

 

[georgeverghese]

An even more basic setup is to do away with the RO, and use the vendor info on valve opening vs Cv ( or Cg) to calculate what the flow is. In this case, you would need the DP taps across the globe valve and an approx estimate on valve stem position vs valve opening.

 

[LittleInch]

I don't understand what "model of equations" means.

If the pressures you quote are the wellhead pressures, then until you get to 3,500psig at that point, you won't have any flow in that branch.

If the 2000 tree can accept your 2mmscfd without getting to a point where the friction losses amount to 1500 psig (seems unlikely) ALL flow will go that route.

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

 

[pyMorty]

Thank you for the suggestions georgeverghese.

And thanks LittleInch, your explanations makes the point clear to me. I was uncertain on whether it would be a Zero/All flow to each branch or Most/Less. Now it is clear that ALL the flow will go to the lower pressure branch, not just a significant part.

 

[zdas04]

Not quite so. It is very unlikely that you could eat up 150 psi in fluid friction, but gas lift valve have a pretty small orifice in them. I would expect most of your gas to go to the LP well, but not all of it. Some proportion of the flow (7/8 maybe?) would be able to cram through the gas lift valves, but that would raise the pressure until the other well took some to. If that didn't happen then you likely have too many gas lift valves.

David Simpson, PE
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

 

[georgeverghese]

What David says here is true - however, it is not clear if this application is for gas injection or for gas lift. I got the impression that an "injection" pressure of 2000-3500psig would be too high for gaslift, so this can only be for gas injection ?

Compressor stated to be for "gas lift"?

 

[zdas04]

What is unclear about

Let's say there is a gas lift compressor handling 2 mmscfd

David Simpson, PE
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

 

[rotw]

Can this be done with two compressors configured with LP/MP stages ?

So the LP pressure stage would deliver the flow (2000psig @ 2+2 MMSCFD) to the 1st well and then the MP stage would feed the next well (3500 psig @ 2 MMSCFD). So it is basically an extraction.

Or does the compression unit already exist and the delivery pressure fixed ?

With gas lift I've often seen two stages compressors LP/MP but with dehydration unit in the interstage.
Problem is your pressure is quite high and I would tend to call this service injection instead of gas lift...just a thought.

 

[zdas04]

Rotw,
I've seen a lot of gas lift, but people only tend to use dehydrated gas when they are doing off-site compression. When the compressor is on the same site (or relatively close) then there are some major improvements in economics (with no corresponding reduction in performance) by using raw gas.

This situation (multiple injection pressures) is really common. I saw one stream used for a 23 wells with over 2000 psi spread from the lowest to the highest injection pressure. The only way we were able to get it to work is with high quality pressure regulating valves (we used Fisher V-Cones) and measuring each stream. My client took a half dozen shortcuts before they gave up and hired me to make it work. I got rid of the short cuts and we got control of the process. We were able to inject exactly what we needed into each well (and the injection pressures changed dramatically from hour to hour). After about 3 months of hitting all of our injection targets we shut the gas lift down because with high quality data it became clear that the underlying technology was the wrong way to operate these wells. We couldn't determine that until we got reliable, repeatable performance.

With the extreme variability that any natural reservoir will present you with, the idea of taking interstage gas for some wells and final gas for the rest has failed repeatedly. The volume requirements just swing too much to allow the final (usually third) stage to perform properly. Starving or flooding the third stage on a 1500 hp compressor is a really good way to break something expensive.

David Simpson, PE
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

 

[pyMorty]

The compressor already exits, handling 2 mmscfd at 3500 psig max. Thank you guys for all your comments, have been very handy. Appreciate it.