The paper presents experimental PVT data of reservoir fluid mixtures with API gravities ranging from 8 to 30. As can be seen from the presented data, heavy oils may under influence of gas injection split out an extra liquid phase, for which type of system a VLLE approach would be necessary to fully describe the phase equilibrium. Reservoir simulators are largely set up with 2-phase flash calculation algorithms that are capable of handling VLE, but not VLLE systems. The majority of today's reservoir simulators are therefore unable to fully represent the phase behavior of heavy oils in fields with gas injection. The paper outlines a heavy oil fluid characterization procedure that allows the user to get 2-phase flash results for VLLE systems that are approximately right with respect to gas/liquid ratio and saturation points. It is further shown how the characterization can be modified to give the full VLLE picture.
PVT simulations on heavy reservoir oil mixtures have traditionally been carried out using black oil correlations expressing the fluid properties in terms of easily measurable quantities like API oil gravity, gas gravity and gas/oil ratio. With the application of secondary recovery techniques like gas injection and thermal stimulation, it has become more interesting, also for heavy reservoir oils, to make compositional equation of state based simulations
A heavy oil has a high density at standard conditions. Crude oils are essentially mixtures of paraffinic (P), naphthenic (N) and aromatic (A) compounds. The densities of aromatics are higher than that of naphthenes and paraffins of the same molecular weight. This is consistent with chemical analyses showing that heavy oil mixtures are rich in aromatic compounds. The density of an oil at standard conditions is conventionally expressed as API gravity (Equation 1 is available in full paper)
SG is the 60 ° F/60 ° F specific gravity, which is defined as mass ratio of equal volumes of oil and water at the appropriate temperature. As the density of water at 60 ° F is close to 1 g/cm3, the specific gravity of an oil sample will take approximately the same value as the density of the oil sample in g/cm3. The term heavy oil may be used for oil mixtures of an API gravity below 30.
The C10+ aromatics present in a crude oil mixture will be components containing one or more aromatic ring structures with paraffinic side branches. The melting temperature of that type of compound is low as compared with that of normal and slightly branched paraffins of approximately the same molecular weight. For this reason wax precipitation is unlikely to take place from a heavy oil mixture. The fact that high molecular weight compounds may be kept in solution in the oil at low temperatures has the side effect that the viscosity of heavy oil mixtures can be very high indeed at production conditions and even at reservoir conditions. Lindeloff and Pedersen (2004) have reported viscosities as high 8,500 cP for a heavy reservoir fluid.
