ASPHALTENE PRECIPITATION AND SOLVENT PROPERTIES OF CRUDE OILS

Improved prediction of the onset of asphaltene precipitation may be ac hieved using refractive index (RI) to characterize crude oils and thei r mixtures with precipitants and solvents. Experimental measurements o f RI for mixtures of several crude oils with the precipitant n-heptane , are reported at ambient conditions. Theoretical developments are des cribed that will permit extension of these observations to reservoir c onditions. Measurements of RI at the onset of precipitation have shown that the onset occurs at a characteristic RT for each oil/precipitant combination, supporting the premise that precipitation is dominated b y London dispersion interactions and thus, that RI can be used to pred ict the onset of precipitation. Reports in the literature showing that the onset of precipitation occurs at constant solvent-to-precipitant ratios provide additional confirmation. The theory is developed on the assumption that London dispersion forces dominate aggregation and pre cipitation of asphaltenes. The interaction energy of asphaltene molecu les or aggregates in a medium of oil can be expressed as a function of the difference between the RI of asphaltene and oil. The RI of live c rude oil during pressure depletion can be calculated from the RI of th e stock tomb oil, the molar refraction of the separator gas, the forma tion volume factor, Bo, and the solution gas/oil ratio, R-s.