Gas Lift - Does the injected gas add the initial gas flow?

[pyMorty]

Let´s say the well is producing (after a 2-phase separation process) 0.5 mmscfd of gas, we compress that and reinject it to the well. What would it be the gas flow now produced for the well? assuming there was no oil production increase (since the optimal parameters have to be found out still).

A coworker claims it will be added and then the compressor would be able to inject, roughlfy, 1.0 mmcsfd. I say it is not that way since provided we don´t increase the oil production we cannot get any more gas from the same liquids flow, it is a closed-loop that needs to be in balance, until a variable is changed, in this case the oil production (that can be achieved by fisrt inducing the well with nitrogen injection, to increase the production and then being able to operate our system of separation-injection).

I work with reciprocating compressors, and barely know about the well behaviour.

Thanks.

 

[MortenA]

draw a box around your problem - then well being the feed and the discharge from your compressor and the oil stream being the discharges.

If you use all of your gas as LG you got an infinite recycle and the gas volume will continue to increase towards infinity.

If you bleed off 0.5 mmSCFD and inject 0.5 mmscfd you got a balance with a total gas flow of 1 mmscfd - but you could also inject 9.5 mmscfd - and bleed of .5 - your compressor would then have to be able to compress 10 mmscfd. initially when you opened the well this gas would not be there and you would recycle 100% - but sooner or later your would reach your target LG flow and you would start to bleed of gas. So your colleague is right. The type of compressor does not matter. Well kick and such is a complicated matter the observations above is basically about the "steady state" case.

Best regards, Morten

 

[zdas04]

And there is no such thing as steady state multi-phase flow. If the reservoir is giving you 500 MSCF/day and you are injecting 500 MSCF/day then the gas in the tubing is 1 MMSCF/day (two sources of gas into the tubing must be additive). Problem is that 1 MMSCF/day plus your reservoir liquids is probably 4 times as much fluid friction as 500 MSCF/day and the same amount of reservoir liquid. That increased friction equates to a higher flowing bottom hole pressure, which often equates to a lower reservoir flow. At the margin you just cycle your 500 MSCF/day and reservoir flow stops.

Balancing a gas lift system is really difficult and requires both flexible software and competent operators. I see it done poorly far more often than I see it done 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

 

[pyMorty]

Thanks for all your quick repplies.

I´m talking about a self-fed system, how could the flow increase if the source is not able to produce such amount of gas?

In the attached sketch please see the stream (1) as the well production, # BPD, 0.5 mmscfd, the compressor takes all of that, compress it and then reinject it to the well, stream (2). That´s the first cycle, then, in the second cycle the gas flow in the stream (1) would be 1.0 mmscfd? this way the third time it would be 2.0 mmscfd, the forth 4.0, and so on.

I´m aware of there being an instant when the 1.0 mmscfd is there, but as soon as the "extra" 0.5 mmscfd go into the well it will start again the process of separating the current production and we will obtain again the 0.5 mmscfd, until we don´t reach the required conditions to increase the oil production and therefore the separated gas.

 

[zdas04]

What you drew has zero reservoir gas. Ever. Not a very likely event. There is always some gas (even if it is only the hydrocarbon products that are liquid at reservoir conditions and gas at the compressor suction). So let's say that it is 10 MSCF/day. You start day 1 injecting at a 500 MSCF/day rate and end the day injecting at a 510 MSCF/day rate. Since you do not have a connection to offload any of the gas in this closed loop, on day 50 you are injecting 1 MMSCF/day.

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]

Reinjection gas does not go into the oil producing layer of the oil bearing zone - it goes into the gas cap of the reservoir. Hence the term pressure maintenance is used for reinjection gas.

Hence cap gas does not flow into the oil production tubing (not for a long time)

Once the well has been drawn down, accumulated cap gas may break through into production fluids.

Reinjection water, in a similar way, goes down below the oil bearing zone to move the oil layer up and maintain pressure - it may be some time ( months to years ) before reinjection water breaks through into production fluids.

 

[georgeverghese]

Oops, just saw the title of your post, and there is a discrepancyin your posting - are we talking aobut lift gas or reinjection gas here ?

Obviously, lift gas is a recycle stream and becomes part of production fluids. Not all gas at the compressor exit can be recycled as lift gas - net gas produced (total gas minus lift gas) is exported out of the lift gas recycle loop

 

[zdas04]

George,
Did you see the new "Delete Post" button?

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 zdas04 and georgeverghese I really appreciate your patience.


georgeverghese, it´s gas lift, I think I wrongly used the term reinjection to stress on the fact the gas used for gas lift is the same previously separated from the same well´s production.

Obviously, lift gas is a recycle stream and becomes part of production fluids. Not all gas at the compressor exit can be recycled as lift gas - net gas produced (total gas minus lift gas) is exported out of the lift gas recycle loop

What do you mean with exported out of the lift gas recycle loop? what would that part out of the recily loop be?

 

[georgeverghese]

Onle net gas leaves the lift gas recycle loop.
As mentioned by others, it is not possible to recycle all gas as lift gas - the limit being compressor capacity.
You could in reality try to increase lift gas rate to use a larger proportion of gas leaving the separator - this - after a short while, the compressor will be running at max speed ( with compressor recycle valves fully closed)and all net gas will be flared off at the separator exit on compressor suction excess pressure control operation.