Matching the oil density from PVT analysis with HYSYS results

This is something I wrote to a customer complaining that HYSYS was producing totally wrong densities for his stock tank oil.

Quite often you may get an analysis of your reservoir fluid that provides you a composition that mentions what looks like "component names" like C4, C5, C6, C7 etc. These aren't really component names in the chemical sense. They probably come from a chromatographic analysis. The indication C12 for example really only means "material with a boiling point similar to n-C12" or something like that. The actual components present in that part of the oil are a very diverse combination of paraffins, olefins, naphtenics, aromatics etc... It is a bit tempting however in HYSYS to select the C1 to C16 components for example and to assume that now you only need to provide the composition and that will yield the correct result. However, you have in fact selected for most components the normal paraffin isomer to represent a whole series of components and those normal paraffins are usually the lightest components of the possible ones. So, this is definitely not the correct representation of your oil. Ideally you would like HYSYS to have a C12 component that DOES represent your oil, but, no oil is equal and so HYSYS cannot provide you that.

If you do not have info on the stock tank oil density, you could use what I call "Katz cuts". This comes from the following publication: Katz., Characterising Hydrocarbon Plus Fractions, SPE 12233 (1982) and SPEJ Aug 1983, p683 -- 694. M. Katz has determined the average properties of components ranging from C6 to C45 that you typically find in an oil.Note, this is still an average, so your results will not be accurate. BUT, if you use normal paraffins the results will be totally wrong!

If you have something like the chromatographic analysis, you can use the oil environment in HYSYS and that will alow you to provide the measured liquid density and the oil manager will take care of fine tuning the pseudo component densities to match your PVT analysis. A similar thing can be done for viscosities. As an alternative you could use the Katz cuts and manually adjust the liquid density of these components, but this is a bit more tedious to do.

If you need an easy way to add these Katz cuts to your model, I can provide a macro that does so, just drop me a line.