# Associated Gas Flashing Calculations From Gas / Oil Mixtures - Vasquez-Beggs Method

Flashing calculations from crude oil -associated gas are mostly done using a simulation software such as HYSYS or UNISIM or any other. By flashing it is meant that a crude oil - associated gas mixture is reduced from a higher pressure to a lower pressure. An example of flashing would be to transfer an oil-gas mixture from an inlet separator to a storage tank where the pressure of the oil-gas mixture is reduced from the operating pressure of the separator to the pressure of the storage tank which is most likely operating at atmospheric pressure or a few kPa above atmospheric pressure.

What if a process engineer does not have access to a simulation software and is still desirous to find out the quantity of flashed gas from a crude oil - associated gas mixture. Today's blog entry deals with the Vasquez-Beggs flashing calculation method from crude oil-associated gas mixture which can be configured on a simple excel spreadsheet. The Vasquez-Beggs correlation is fairly accurate and can be used by process engineers to estimate flashing losses from crude oil storage tanks. Let us get on to the equations:

Rs = C1*γg*PC2*EXP((C3/γo*T) - (C4/T))

where:
Rs = solution gas-to-oil ratio (m3/m3) at standard conditions of 101.325 kPa (abs) and 15°C
γg = specific gravity of solution gas with respect to air, dimensionless = MWg / 28.96
MWg = molecular weight of associated gas, kg / kgmole
T = Temperature at which the flashing needs to be evaluated, K
P = absolute pressure in the vessel whose flashing needs to be evaluated, kPa(abs)
γo = oil specific gravity with respect to water, dimensionless = 141.5 / (131.5 + deg API)
deg API = means of classifying petroleum oils and normally range from 45.4 deg API (light crude oils) to 10 deg API (heavy asphaltic crude oils)

Values of C1, C2, C3 and C4:

γo < 0.876

C1 = 3.204E-04
C2 = 1.1870
C3 = 1881.24
C4 = 1748.29

γo ≥ 0.876

C1 = 7.803E-04
C2 = 1.0937
C3 = 2022.19
C4 = 1879.28

The most likely application for the Vasquez-Beggs correlation would be to estimate flashing losses when transferring crude oil-gas mixtures from separators / treaters at higher pressure to atmospheric pressure oil storage tanks. The value of "Rs" as calculated from the above equation is the flashed gas from the crude oil-gas mixture.

Example Calculation:
An oil train consisting consisting of an inlet separator and oil storage tank receives oil @200 m3 per day. The oil is transferred from the oil storage tank and flashes in the storage tank . The conditions in the separator and the storage tank are as follows:
Separator
T = 40 deg C
P1 = 350 kPag
Storage Tank
P2 = 0 kPag

ΔP = P2- P1 = 350 - 0 = 350 kPa = P (for the example calculation)
Inlet Oil gravity = 45.4 deg API
Solution gas molecular weight = 22.46 kg / kgmole

Calculate the flashing loss Fvap from the oil storage tank

Solution:
Rs = 1.787 m3 / m3
Fvap = 357.5 m3 / day at standard conditions of 101.325 kPa (abs) and 15°C

Hope the readers of my blog like this blog entry, specially the engineers working in upstream oil and gas engineering. Looking forward to some comments on this blog entry.

Regards,
Ankur.

Hi Ankur,
My name is Harshad Gandhi. I would like to calculate nozzle erosion rate Vs tube erosion rate for flow of heavy oil combined with gas and vapour under certain downhole oil well temperature. Your help would be highly appreciate.Is there any formula to calculate it ?
Thanks,
Yassif
Thanks a lot for this precious entry Ankur,

What if the production banks consist of many separators. The gas analysis of the flashed gas from each separators are available so as the API gravity of the stock tank. The API gravity of the oil existing each separators can not be predicted in the laboratory because the oil under pressure. At this case how can the flashed gas can be calculated.
ankur2061

Thanks a lot for this precious entry Ankur,

What if the production banks consist of many separators. The gas analysis of the flashed gas from each separators are available so as the API gravity of the stock tank. The API gravity of the oil existing each separators can not be predicted in the laboratory because the oil under pressure. At this case how can the flashed gas can be calculated.

The API gravity is a necessary input for using the Vasquez-Beggs correlation. If you cannot obtain the necessary inputs for the method described in the blog entry then you need to use a simulation software and use the black-oil model for property prediction. Aspen HYSYS can do the necessary flashing calculations based on black-oil model.

Regards,
Ankur.
shailesh2706
Hi Ankur,
how to calculate Gas to oil ratio if system consist of 3 Separators.That is crude oil is being separated from associated gas in 3 stages with 4th stage separator(intermediate tank) at atmospheric pressure.Can we use Vasquez-Beggs correlation in that case.
regards
shailesh tripathi
ankur2061

Hi Ankur,
how to calculate Gas to oil ratio if system consist of 3 Separators.That is crude oil is being separated from associated gas in 3 stages with 4th stage separator(intermediate tank) at atmospheric pressure.Can we use Vasquez-Beggs correlation in that case.
regards
shailesh tripathi

Yes, I don't see any problem. The gas-oil ratio (GOR) is a function of the system pressure. If you are reducing the pressure in 3-stages, the gas-oil ratio will change according to the pressure in the separator.

Regards,
Ankur.
benabed

Good day

Can we use this correletion to estimate flashing losses of natural gas condensate. Because sometimes we are obliged to mix natural gas condensate with stabilised oil and we want to know how much natural gas condensate was added to oil.

thank you very much

ankur2061

Good day

Can we use this correletion to estimate flashing losses of natural gas condensate. Because sometimes we are obliged to mix natural gas condensate with stabilised oil and we want to know how much natural gas condensate was added to oil.

thank you very much

benabed,

Yes, we can use it for natural gas condensate as long as the API gravity of the NGC falls between 45.4 and 10.

Regards,

Ankur

benabed

Good day

Can we use this correletion to estimate flashing losses of natural gas condensate. Because sometimes we are obliged to mix natural gas condensate with stabilised oil and we want to know how much natural gas condensate was added to oil.

thank you very much

benabed,

Yes, we can use it for natural gas condensate as long as the API gravity of the NGC falls between 45.4 and 10.

Regards,

Ankur

Thank you for the reply, but as you know the API gravity of the NCG doen't fall between  45.4 and 10.

Regards

Benabed

mofiyinfolu

hello Ankur, I try using the approach but am getting different solution.

Ankur,

Please, i dont understand how you determine thë F vap and what does it mean?

chestudent123

Hi Ankur,

I want to calculate flashing of gas from a stream of water. How can I do that?

The water comes from HP separator operating at 20 bar and stored in an accumulator at 18 bar. How much flashing of gas shall occur at delta P of 2 bar?

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