PVT and Flow course - PVT Lab Tests

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PVT Lab Tests (20160216 Part 1)

There are three fundamental things that are done in the laboratory:

• quality control or a check on the samples that were collected (the main concern if there's leakage from the samples that have been sent from the field to the laboratory).
• compositional analysis - to get an idea what was in the samples.
• PVT experiments (CCE, SEP, DLE, CVD).

Sample QC

If you have separator samples a 20 litre gas container and a short of one liter oil separator well container and we'll probably have the duplicate of each, two of each. If it's a three day production test, you might take two sets after 24 hours another set after 36 hours and then a final set at the end. You might actually have like four or six of these bottles times two. So you might have a suite of these separators samples.

Separator samples.

Gas samples.
The pressure and probably the temperature the container are going to be the same as the separator conditions.
The lab should bring the container to the separator temperature (they probably won't but they they should). And then measure the pressure of the container. That pressure should be approximately equal to separated pressure. How close it should be? It depends whether or not the lab actually brings it to the separator temperature or maybe, if they don't, then they'll do some kind of ideal gas correction. Let's say that the temperature in the lab was not equal to separator temperature, then we could probably from the ideal gas law, as an approximation: I think that if the pressure over the temperature at the separator is approximately equal to the pressure over the temperature in the lab then you can assume that the number of moles is the same. If the temperature was cold enough you might be condensing some liquids out, so some of the moles are actually as a liquid and it won't have the same pressure because you'll lose some pressure as those compounds become liquid. So you have fewer moles as a gas phase so the pressure could be a little bit low if the temperatures low for that reason really why you should try to have them bring the temperature to the separator temperature.
I can't tell you that if the pressure they measure is like one bar less than the separator pressure it's ok or fit. But i'll say if it's within five percent it's probably okay.

Oil samples.
It's probably collected at separate pressure and separator temperature. We know that this oil should be in equilibrium with the gas. So if you have a saturated oil, the oil by definition is at its bubble point, it's fully saturated with gas. Anytime you've got two phases together in a system in there in equilibrium the gas is at dew point and oils that it's bubble point, it's fundamental.
So we know that the bubble point of that oil at the separator temperature should be equal to separator pressure. So what the lab does in a very simple test is they'll take a little bit of this oil and they'll actually measure pressure versus volume at the separator temperature and they'll take it to real high pressures and they measure the oil volume and as they lower the pressure it just barely increases but then they expect to see that jumping up and why is the volume getting so big - it's because gas is coming out of solution and it's much more compressible. So what they're going to be looking for is the discontinuity and that should be right around the separator pressure. This is the this is the QC for the lab on the separator oil sample and again within five percent.

That's those are really the the only to QC is that are that are done.
You got two of these samples, so the fact that both sample containers would leak it's it's probably unlikely.

Bottom well oil samples.
They do the same test at ambient temperature of the field (not reservoir), measure field bubble point. And then the lab would do the same test and it would be at the ambient temperature of the field.
And that was the QC, so if the bubble point in the lab is much lower or enough lower that would say there was leakage in the transportation. Then again, probably you have multiple bottomhole samples, so that maybe one of them didn't leak and you'll end up with at least one valid bottom well sample.
This does not work if you have an oil with a high gas or ratio. Probably we're talking greater than about 1500 standard cubic feet per stock tank barrel, which would be about 300 standard cubic meters per standard cubic meter. What you would see is that it's kind of curving above the bubble point and below the bubble point and that's because even as a single phase liquid above the bubble point it's quite compressible, so the volume is expanding not not linearly but it's kind of curved, the whole way, so you can't see any break point any discontinuity.

Measure compositions

What that means is the amounts usually in mass (and they'll probably convert them also to moles) of each component.
You get the pure compounds and a bunch of hydrocarbon isomers, until we get out to about C11 and then they quit trying to identify which isom is. And that will probably go out all the way C_N+ to some heavy amount. What they're measuring in the laboratory it is mass amounts and then convert them to moles by knowing the molecular weight.
Historically, compositional analysis stopped at C₇₊ and gradually increased in detail up to C₃₅₊.

In addition to the amounts that they would give you, they would also give you a molecular weight (estimate or measured, but probably estimate) and a molecular weight of the heaviest material and maybe even a specific gravity of that.

Lab Compositional Analysis (20160216 Part 2)

Gas chromatography

We've got these high pressure samples that we collect either bottomhole sample, which is like really high pressure, or separate oil samples, which could be high pressure anywhere from 10 bar 200 bar and separator gas samples, which also would be a high pressure. So we have pressurized samples that are typically from 10 to 500 bar pressure range.
Gas chromatograph is a device that helps analyze how much methane and how much ethane and how much propane, etc is in a mixture - it won't take such highly pressurized samples. This a low lower pressure device.
So, what we have to do is we have to have a kind of a three-step procedure in order to get a composition for these pressurized samples:

1. Flash the sample down to one atmosphere. It splits into two phases - a surface gas and its surface oil.
2. Gas chromatography of surface flashed samples. Send a surface gas into a gas chromatograph; flash oil we send into a gas chromatograph and in addition we also take that oil and measure its molecular weight.
3. Recombine this flash gas and the flash oil to find out how much methane total was in the sample that we started with. So we have to add the amount of the different components in the flash gas together with the amount of those same components in the flash oil.

We use this for all bottom oil samples that would be MDT type and the cased hole and this is the same method that's used for separator oils. Separator gases in general can be sent directly to the gas chromatography.

What is the gas chromatograph doing?


Watch the full video for more discussion.

Other lectures from the PVT and Flow course

• Blog:PVT and Flow course - Gas or Oil Reservoir?
• Blog:PVT and Flow course - Single Component Vapor Pressure.
• Blog:PVT and Flow course - Two-Component Phase Behavior
• Blog:PVT and Flow course - Multi-Component Phase Diagrams
• Blog:PVT and Flow course - K Values
• Blog:PVT and Flow course - Flash Calculations
• Blog:PVT and Flow_course - Surface Separation Processing
• Blog:PVT and Flow course - Sampling
• Blog:PVT and Flow course - PVT Lab Tests
• Blog:PVT and Flow course - OBM Decontamination
• Blog:PVT and Flow course - Lab PVT Tests CCE
• Blog:PVT and Flow course - LAB PVT Tests Multistage SEP
• Blog:PVT and Flow course - Lab PVT Tests DLE
• Blog:PVT and Flow course - Lab PVT Tests CVD
• Blog:PVT and Flow course - Black-Oil PVT
• Blog:PVT and Flow course - Rate Equation (Darcy) Intro

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/

See also

PVT tests
Curtis Whitson Petroleum Engineering Videos

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