THERMODYNAMICS
Niels Lindeloff
1
, Abul K. M. Jamaluddin
2
and Karen S. Pedersen
1
1
Calsep A/S, Lyngby, Denmark,
2
Schlumberger, Jebel, UAE
Historically, most research on asphaltene precipitation has focused on downstream applications especially issues related to refinery operations. This is reflected in definitions and terminology, as well as in the type of data that are predominantly available. Over the last ten years an increasing amount of both modeling work and experimental techniques are focusing on upstream applications where typical concerns are asphaltene precipitation resulting from reduction of system pressure, and to a lesser extent, system temperature. The classical definitions of asphaltenes that are based on solubility classes do not appear to provide a good framework for describing this process.
Asphaltene molecular weights are of the order of 1000 g/mol, i.e. no heavier than the heavy end fractions normally considered in EoS modeling. The challenge of asphaltene modeling is therefore to explain why these heavy aromatic compounds under some circumstances find it thermodynamically favorable to form a separate phase, while under other circumstances the same components stay in solution with the remaining oil constituents. For single phase oils at reservoir conditions asphaltene precipitation may take place as a result of pressure reduction. A liquid-liquid split caused by a pressure change will usually have to do with the system trying to minimize its volume. Aromatic components will not necessarily take up the same volume in a primarily paraffinic solution as in a primarily aromatic solution. The phase split will then occur when separating out an asphaltene rich phase decreases the volume of the total system. This behavior will in the actual work be modeled using an EoS based model. The volumetric effect is accounted for by a proper assignment of EoS parameters to the various fluid constituents.
Onset asphaltene precipitation pressures from single-phase reservoir fluid samples taken from the Gulf of Mexico and various parts of the Middle East have been published in the recent literature. These onset pressures are experimentally obtained by subjecting single-phase reservoir fluids to isothermal depressurization tests. These experimentally measured data will be modeled using the EoS model.