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# Specific Gravity Question

gas upstream

9 replies to this topic
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### #1 jtriplej

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Posted 13 June 2017 - 11:33 AM

I was wondering if someone can answer this question for me.  I have the separator gas composition and hydrocarbon composition, each with their own SG(which was given).  I have calculated the recombined mole fraction.  Am I able to calculate a new specific gravity based on the recombined Mole Frac?

### #2 MrShorty

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Posted 13 June 2017 - 11:58 AM

I am cautiously optimistic that you can calculate a new SG, but it is not clear to me exactly what you have with the recombined mixture, or exactly how you want to compute SG. For clarification: Are you wanting to treat the recombined fluid as a single phase or as two phases (vapor + liquid)?

If you are treating the recombined fluid as a single phase, you can compute the density using your desired methodology (equation of state or other). Then take the ratio of the calculated density with the density of your chosen reference fluid to get SG.

If the recombined fluid is to be treated as two phases, then you will need some estimate of "quality" or what fraction is vapor and liquid and the compositions of each phase. Once you have composition for each phase, you can estimate the density of each phase (using your favorite EOS or other algorithm). Then combine the phase densities with the quality/phase fraction to get a bulk density. Then choose a suitable reference fluid (I have no idea what a suitable reference fluid would be for a two phase mixture) and take the ratio of the bulk density to the density of the reference fluid to get SG.

Does that help, or is it more confusing?

### #3 jtriplej

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Posted 13 June 2017 - 12:17 PM

It does help and confuse.  I think I am trying to treat it as a single phase.  If you see my attached spreadsheet this is as far as I have made it. (I am not an engineer, I am in geology)

What I am trying to do in a broad sense is take my gas analysis and hydrocarbon analysis, recombine and calculate a z factor based on the recombined results.  The lab gives a relative density based on these recombined  values of .6012 and that is what I was trying to recreate to use to determine my new z factor?

### #4 Bobby Strain

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Posted 13 June 2017 - 01:11 PM

What are you trying to accomplish? That might help us to help you. Gas specific gravity is related to Air MW, liquid is related to water, both water and liquid hydrocarbon at atmospheric pressure and 60 F. So, what properties of the mixed fluids are you looking for?

Bobby

### #5 jtriplej

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Posted 13 June 2017 - 02:11 PM

Well basically I get alot of gas analysis and hydrocarbon analysis, and occasionally I get a recombined hydrocarbon analysis.  That is what I am after, a way to calculate my own recombined analysis.  I have got so far as recombining the mole fraction, but I am trying to figure out the rest of the properties that are usually given to me already on the recombined.

For example the data shown on attached.  What did they use to calculate the API of 103.6?

These fields are as far as I have to take it basically, other than using the relative density of the total sample to determine a z factor-if that makes sense even.  Hopefully that explains what I am trying to do, and maybe that doesn't even actually work-this is not my area of expertise at all

#### Attached Files

Edited by jtriplej, 13 June 2017 - 02:13 PM.

### #6 Bobby Strain

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Posted 13 June 2017 - 02:35 PM

And what do you do with the results? I'll check my files to see if I have something that will help. I usually use a simulator to get answers like these. I notice lots of calculations for heating value. Is this any way related to your quest? There are not many reservoir engineers on this site. Maybe you should also use Google search for oil and gas reservoir analysis and sampling.

Bobby

### #7 serra

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Posted 13 June 2017 - 02:36 PM

in API TDB (and similar documents) you can find the relation between Specific Gravity and API gravity

API (gravity) = 141.5 / Specific Gravity  - 131.5

for liquids specific gravity SG = density of liquid / density of water (see Bobby's answer)

the document example.pdf gives the composition including C7+ as pseudocomponent,

with the composition you can calculate the different properties at specified conditions

### #8 jtriplej

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Posted 13 June 2017 - 03:25 PM

ok so in this case the given density is 600.7 kg/m3,and the temp is 15 degrees celcius

so .6007/.999127=.0612

Eureka! It worked.

So how do I figure out the density based on the data I have to start with if I didn't already have the recombined PDF?

### #9 Pilesar

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Posted 13 June 2017 - 03:34 PM

The specific gravity of your gas is calculated by dividing the MW of the gas by 28.9625 (the MW of air) and is not on the same basis as the hydrocarbon liquid specific gravity. This make for a flimsy theoretical basis for combining specific gravities of the two streams. For your well-defined chemical components, consider using a steady state process simulator to generate properties. Prode Properties is available from prode.com and may be all you need. I don't understand the value of what you are trying to do, but if you insist on not using a simulator, you might look up the API methods (procedure 2B2.1) which predict molecular weight as a function of boiling point and specific gravity.

### #10 jtriplej

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Posted 13 June 2017 - 03:57 PM

Ok I will give the simulator a shot!  I see what you are saying, I can't really combine the two specific gravities and that is why I was having such a hard time finding a way to do so.

So if I can figure out the molecular weight of my recombined composition then I can find my specific gas gravity.

Thanks for all the help everyone!