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# PVT and Flow course - Surface Separation Processing

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## Surface Separation Processing (Part 1 of 2)

A lecture on surface separation. Information from chapter 6 of "Phase Behavior" SPE Monograph (Whitson and Brule).
PDF of notes available here: http://www.ipt.ntnu.no/~curtis/courses/YouTube/Video-Notes/Whitson-PVT-Flow-20130920-Surface-Separation.pdf

The processing, the stream being produced be it a well stream or multiple well streams coming through a manifold or an entire pipeline of streams coming in, and we want to process it into at least two products: a sales product of oil stock tank oil and a sales product of gas.

Surface separation processing calculation is not usually a petroleum engineers' job. That's usually the chemical engineers.
But generally we want to maximize the resulting what we call stock tank oil volume from a produced stream. That's because we get the most value out. And in reality what we're really trying to do here is that we're trying to to maximize getting as liquid as many of the components typically starting at propane, butane, pentane, hexane and heptane (C3-C7) - these are the components that can end up in part in the gas and in part in the oil. Most of the stuff heavier like C8 and C9 and so forth no matter how bad you do it will end up in the stock tank oil. Methane for the most part is going to to end up pretty much all in the gas, ethane as well. There's not a lot you can do without spending a lot of money to get the ethane to stay much in the oil.
So we want to basically operate on trying to to get as much of the C3 through C7 components in the stock tank oil.
Reason: any component mole found in the stock tank oil has considerably more value than that component mole being sold as part of the gas.

One stage process is what you do if you don't care or if you have a low gas oil ratio. So a single stage process basically goes more or less straight to standard conditions of one atmosphere and 60 degrees fahrenheit. This process will always give you the lowest amount of stock tank oil with minimum API degree and typically give us a maximum amount of surface gas.

### Oil or Gas - calculations of the value difference

We're going to take some arbitrary amount of isobutane. Let's say that we have entering the separator 1234 kg-moles/day. What we want to know is that if we put it through some kind of a process a one stage or a five stage a complicated process and we can bring it into the stock tank oil or if we can bring it into thesales gas. What would the value difference be?
And we are going to make the simplifying assumption that it either goes all to the gas or all to the oil.

So what we need to do is we need to have our prices. Basically we have in the worst situation now in the States where there's a gas glut (year of 2015) as low as in the worst case 2 dollars per thousand standard cubic feet to as much as 10. On average in the world i'm just going to put pick \$3 per thousand standard cubic feet. And the other price is the price of the stock tank oil which varies and it's around 100 per barrel (year of 2015).
To make this calculation we need to convert moles to volume. And we need to have volume as a gas but we also need to know how to convert those moles if it becomes a liquid.

The results of the calculations: \$3610 if we sell these isobutane moles as gas and \$80300 if we sell these isobutane moles as oil.
Here is the calculations:

## Surface Separation Processing (Part 2 of 2)

Multi-stage separator process is the kind that's typically used for engineering calculations. It reflects either quite accurately, or approximately, the actual process of even very complicated offshore large field processes.

Stock tank oil recovery factor (or recovery factor of stock tank oil component) is the percentage of moles that ends up as oil to moles of that component that come in the feed into our process.

The other thing is that this final oil has to be what they call stable at the end of the day and it shouldn't be too volatile. So you can put it in a tank or a tanker and it doesn't evaporate a lot in transportation. There's some criteria the more methane that gets in there the less stable it becomes, so there's actually a it is kind of an advantage to keeping the methane and probably the ethane as low as possible but getting to help get as much as the C3 and heavier.

## Other lectures from the PVT and Flow course

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/