Gas Lift - Is the compressor capacity the limitant for well self feeding?

[pyMorty]

For an oil well which produced gas be used as gas lift for itself, is it possible to reach, for example 1 MMSCFD if the well only produces 1.0 MMSCFD? assuming the gas is being recycled in a compressor of 2.0 MMSCFD capacity

For example, if we separate the well´s production (x number of BPD and 1.0 MMSCFD), take the gas into the compressor at 10 kg/cm2, which capacity is theoritically 1.0 MMSCFD (at the given pressure due to compressor "fixed" capacity) will it be possible to feed the compressor at its required capacity (2.0 MMSCFD) over time?

I know it won´t obviously do it the first day, the compressor should be capacity-controlled to operate at half its capacity, but since it is a recycle in a "closed loop" (vast mayority of separated gas goes into the compressor) it would do it on the second or third day once the production has been added up to the recycle process. Is this correct?

Bottomline, the compressor´s capacity will be the limit for the flow of gas that will be recycling (remember, the gas comes from the same well is being gas lifted).


Thanks

 

[georgeverghese]

There are 2 recycles here, one of which is the overall recycle of lift gas you have described: well - separator - compressor - well; while the other smaller recycle is the capacity recycle within the compressor. This smaller recycle occurs whenever net feed gas rate is lower than the compressor capacity at the given compressor dp. For this case you've described, the smaller recycle would be 1mmscfd within the compressor, enabled by the discharge to suction recycle valve.
The well must produce some net solution gas to sustain lift gas, what ever that lift gas rate might be, to account for losses out of the lift gas loop, such as gas carryunder into the oil and water phases at the separator, leakage through shaft seals on the compressor, emissions through safety and blowoff control valves, vents etc. In your case, gross gas produced may notionally have to be say 1.05mmscfd at least to sustain lift gas at 1.0mmscfd.

 

[pyMorty]

I found a report from our client monitoring the closed-loop gas lift system, reading the following:

Formation gas: 0.5 MMSCFD
"Injected" gas (gas lift flow): 1.5 MMSCFD
Measured gas at separator: 2.0 MMSCFD

This is a closed system and no external source of gas is available, so above information means produced gas will recycle until compressor capacity is reached over time, then wath cannot be handled by compressor is sent as flare or excess gas.

 

[georgeverghese]

Agreed, if you dont have a means of exporting the solution gas of 0.5mmscfd, then most of this may have to be flared off on compressor excess suction pressure control. This solution gas would usually be more than enough to sustain the lift gas recycle loop, after losses as suggested previously.

 

[pyMorty]

Thanks georgeverghese,

Your second reply seems to contradict your first one, or maybe I´m not getting you. In the information I showed it was reported by client monitoring it states the well´s gas production of 0.5 MMSCFD is enough to reach, over time, a gas lift flow (self-fed) of 1.5 MMSCFD, which would be the maximum compression capacity of given unit at given conditions, so there will be a time where the excess of produced gas will be flared or sent to another available pipeline.

I mean, first day compressor would be underutilized, just compressing roughly 0.5 MMSCFD (for practical puporses I´m not considering losses thru surface system), then second day could handle 1.0 MMSCFD, an then 1.5 MMSCFD, fourth day would be excess gas.

Am I right or totally lost?

 

[georgeverghese]

Yes, the transient startup case will start from a low gas lift recycle flow of 0.5mmscfd, and gradually increase to the required flow over some time, but the time taken to reach the required gas lift flow is dependent on the system volume. The smaller the system volume, the quicker the required gas lift recycle rate will be reached.