PVT and Flow course - LAB PVT Tests Multistage SEP

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by Petro Engineer

created 12:30, 3 December 2023 , last edited 12:50, 13 December 2023

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A multi-stage separator test. Typically it's two to four stages, most common maybe in the old days was two stages.

Reasons:

They are really trying to find out what pressure and particularly temperature of the first stage gave the most stock tank oil. Basically you're taking a reservoir fluid mixture in single phase conditions to the surface and you're trying to figure out how can you get the most stock tank barrels out as sellable oil. That's the purpose of the test. Stock tank oil volume is the dependency of the separator temperature but probably even more so the separator pressure of the first stage.
The second thing would be particularly the primary gas if there's the gas oil ratio is large enough let's say more than 500 750 cubic feet per barrel, be like 100 cubic meters per cubic meter, if there's enough gas particularly at the first stage, the composition of that first stage separator gas can be important, because you can extract liquids out of that propane butanes and gas liquids that can be very valuable particularly as the GOR increases over time. So another reason would be to get the composition of the separator gases and that's again primarily for estimating potential - if you add enough money to the processing center processing of the gases - potential natural gas liquids that can be extracted. That would mainly be C3 C4 and C5+.
One more reason - it helps build the equation of state Pvt model.
And of course getting the general gas total gas volume. Which is just the sum of the gas volumes from each stage.

Instead of composition the old days they would just measure the gas gravity, that could also be be used for for each stage.

Couple of practical comments: it's only used today if the GOR is approximately greater than or equal to somewhere between five and ten thousand cubic feet per barrel. The labs will basically tell you we can't do this experiment very accurately if you're higher than ~ five thousand cubic feet per barrel. If it's more than that the lab can't measure the resulting surface oil accurately, it's just too inaccurate.

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Class notes developed during lectures are available as PDF files, named with the format yyyymmdd.pdf located on: http://www.ipt.ntnu.no/~curtis/courses/PVT-Flow/2016-TPG4145/ClassNotes/