Fractionation of Hydrocarbons Between Oil and Gas Phases

Abstract

LÕŽtude du fractionnement des hydrocarbures lŽgers entre les phases gazeuses et liquides intŽresse plusieurs domaines dans le cadre de lÕexploitation des gisements. En gŽochimie de rŽservoir lÕŽvolution de la composition de la fraction lŽg•re peut expliquer certains phŽnom•nes de migration. Lors de lÕinjection de gaz, la dissolution prŽfŽrentielle de certains constituants dans le gaz peut modifier la composition et le comportement le lÕhuile en place. Le stockage souterrain de gaz dans des rŽservoirs contenant encore de lÕhuile peut Žgalement •tre concernŽ par ce probl•me.Des Žtudes ont ŽtŽ menŽes ˆ lÕIFP, dans le cadre de diffŽrents projets, de fa•on ˆ mettre en Žvidence ce phŽnom•ne et ˆ le quantifier. Deux dÕentre elles, qui concernent la gŽochimie de rŽservoir, ont ŽtŽ menŽes sur des fluides rŽels. La troisi•me Žtude, qui est un test de gonflement, concerne lÕinjection de gaz, le fluide considŽrŽ est un mŽlange synthŽtique comprenant des hydrocarbures jusquÕĈ 30 . Les Žquipements utilisŽs sont, dÕune part, une cellule saphir ayant une pression maximale dÕutilisation de 400 bar, et dÕautre part un Žquipement appelŽ Hercule dont la pression maximale dÕutilisa-tion est de 1500 bar. Pour chacun des fluides ŽtudiŽs, la pression de saturation a ŽtŽ mesurŽe. Pour plusieurs paliers, infŽrieurs ˆ la pression de saturation, les phases liquides et gazeuses ˆ lÕŽquilibre ont ŽtŽ prŽlevŽes ˆ pression constante et analysŽes. Dans le cas de lÕinjection de gaz, les compositions ˆ lÕŽquilibre ont ŽtŽ dŽtermi-nŽes pour des mŽlanges contenant des quantitŽs variables de gaz.Par rapport aux n-alcanes de m•me nombre dÕatomes de carbone, les hydrocarbures aromatiques restent prŽfŽrentiellement dans la phase liquide, de m•me que les cycloalcanes pour lesquels ce phŽnom•ne est moins net toutefois. Par contre, la phase gazeuse sÕenrichit lŽg•rement en isoalcanes. Ces phŽnom•nes de fractionnement sont moins marquŽs au voisinage de la rŽgion critique. Ils ont ŽtŽ modŽlisŽs au moyen dÕune Žquation dÕŽtat cubique assortie dÕune r•gle de mŽlange par contribution de groupes. FRACTIONATION OF HYDROCARBONS BETWEEN OIL AND GAS PHASESThe investigation of hydrocarbon fractionation between oil and gas phases is of interest for several purposes in reservoir exploitation. In reservoir geochemistry, the evolution of light hydrocarbon fractions of oils may explain some migration phenomena. In gas injection projects, the preferred dissolution of some components in gas may alter the composition as well as the properties of the oil. Underground gas storage in depleted oil reservoirs may also be concerned by these problems.Results of several IFP studies are described here to illustrate and to quantify the phenomenon. Two of them, using real reservoir fluids, Copyright © 1998, Institut français du pétrole concern reservoir geochemistry, while the third, which is a swelling test, aimed to study gas injection, investigated a synthetic reservoir fluid with hydrocarbon components up to C 30 . Two pieces of equipment were used: a sapphire cell with a maximum...

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