Analysis and implementation of different methods for characterization of heavy fractions in reservoir fluids

CISMONDI DUARTE, M.; HEREDIA, N.; PEREIRA DO CARMO, R.; MATAR NDIAYE, P.; TAVARES, F.W.

Alicante

Conferencia; EQUIFASE 2015: X Conferencia Iberoamericana sobre Equilibrio entre Fases para el Diseño de Procesos; 2015

The composition of petroleum, crude oil or -moregenerically- reservoir fluids, is usually known from a combination oflaboratory procedures involving gas chromatography for the lighter fractions,and standard distillation methods for splitting the rest into cuts named C6,C7, C8 and so on, depending on temperature ranges which are defined in terms ofthe normal alkanes boiling points. At present, it is common practice toquantify cuts until C19, while the heavier residue is known as C20+. Inthe equation of state (EOS) modeling of phase behavior and PVT or thermodynamicproperties for these fluids, the word characterization is commonly used to expressthe procedures and/or strategies implemented for representing those heavy cutsthrough a set of pseudo-components and their EOS parameters, based on thequantitative information obtained in the lab and the densities and averagemolecular weights for the different cuts and the residue CN+. Thischaracterization may involve different steps, correlations and lumpingprocedures, but probably the more sensitive part, which can have importanteffects on the prediction of different fluid properties, wax appearancetemperatures, asphaltene onset pressures, etc., is the assumption of a specificdistribution for the CN+ residue, tipically C20+.Inthis work we analyze the possibilities and limitations of the two probably mostapplied methods for obtaining such distributions, both in academia andindustry, namely the Pedersen and the Whitson methods. Although they aresimilar in terms of mathematical functionality, and the first could be consideredas a particular case of the second, there are important differences regardingthe way they are usually applied and the restrictions imposed in each case. Wediscuss the importance and necessity of some specific restrictions, speciallyin relation to molecular weight distribution, proposing variants and presentingresults for different reservoir fluids with information available in theliterature.