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[daryon]

Hi all,

(sorry this is a long post, please bear with me)

I have question i've been trying to figure out the answer to, and hoped someone might be able shed some light on it. Its regarding crude oil characterisation. I've been working in the oil and gas industry for a few years now and have a bit of experience with crude oil compositions that are typically provided by the upstream reservior guys to the process engineers for detailed design of processing facilities.

A typical crude oil contains say 40% alkanes, 40% Cycloalkanes, 15% Aromatics, and 5% asphaltenes, the numbers aren't important here, but these four classes of hydrocarbons make up the bulk of all crude oils. A paraffin based crude will be rich in higher molecular weight alkanes and lean in asphaltenes.

What i don't understand is why most crude oil compositions (that i've seen) don't list any components apart from alkanes. Or list a few cycloalkanes and aromatics components with small mol fractions, but no where the factions that make up a typical crude. Take the below composition for example; this was provided by a client for a medium crude oil (32 API) to be produced from the gulf of mexico.



COMPONENTS Mole %
CO2 0.45
H2S 0
Nitrogen 0.47
Methane (C1) 20.03
Ethane (C2) 3.73
Propane (C3) 5.1
i-Butane (C4) 0.89
n-Butane (C4) 2.75
i-Pentane (C5) 1.32
n-Pentane (C5) 1.85
n-Hexane (C6) 3.13
Mcyclopentane (C5) 0.51
Benzene (C6) 0.11
Cyclohexane (C6) 0.22
n-Heptane (C7) 2.87
Mcyclohexane (C7) 0.42
Toluene (C7) 0.29
n-Octane (C8) 2.94
E-Benzene (C8) 0.24
m&p-Xylene (C8) 0.11
o-Xylene (C8) 0.17
n-Nonane (C9) 2.7
n-Decane (C10) 2.91
n-C11 2.64
n-C12 2.17
n-C13 2.14
n-C14 1.94
n-C15 2.02
n-C16 2.8
n-C17 1.76
n-C18 2.07
n-C19 1.38
n-C20 1.14
n-C21 1.60
n-C22 0.95
n-C23 1.05
n-C24 0.82
n-C25 0.76
n-C26 0.72
n-C27 0.67
n-C28 0.74
n-C29 0.59
C30+* 18.84
Total 100.01


---Hypothetical---
C30+ Properties
MW 750
Density @ 60F 1.01 g/cm3

Here we have a couple of cycloalkanes and aromatics for C6, C7,C8 but the total fraction of cycloalkanes and aromatics is less than 0.02. I can't believe this is an accurate representation of the actual crude composition.

The FPSO project have recently finished the process simulation for had a crude oil composition which just contained alkanes. THis I find even harder to believe. The component mol fractions we given up to C20 as alkanes and two hypothetical groups (C20+ and C30+) were provided.

Why are crude oil compositions often given with just the alkane components? Is this to do with accuracy of the composition and it being pointless to indentify the compostion of crude oil in the detail of determing what fraction of C8 hydrocarbons are alkanes, cycloalkanes and aromatics. Or is this labs being lazy. Cycloheptane has significantly different properties to heptane and toluene.


C7 components:


Cyclo-heptane
MW =98.18
SG = 0.810
BP (oC) =118 -120

n-heptane
MW = 100.2
SG = 0.684
BP (oC) = 98.4

toluene
MW = 92.13
SG = 0.866
BP (oC) = 110.8


I guess if the component fractions are identified by TBP distillation then the lab guys don't have a clue whats actually in the fraction they just know the boiling point and then say well thats C7, and it goes in the composition list as heptane?

I could understand what was going on if the composition specified C7 cut and gave boiling point, density and molecular weight, but it is given just as C7 which we take for pure heptane. If there is a significant amount of toluene (which is likely to boil in the same fraction as octane won't boil till 126 degC) then the density of this fraction could potentially be way off that of pure heptane.

Any light you can shed on this would be great. My understanding of crude characterisation is certainly lacking here

Thanks in advance

Daryon