An Experimental Approach to Prediction of Asphaltene Flocculation

Many of the existing models that have been developed to predict the onset of asphaltene flocculation rely on approximations to a full thermodynamic treatment of asphaltene phase behavior and on parameters that are not available except by fitting HTHP data to the model. A much simpler approach has been developed based on experimental measurements of refractive index (RI) at the onset of flocculation for precipitants representing a range of molecular sizes. The onset of asphaltene flocculation is often reported in mixtures of oil, solvent, and asphaltene precipitants. In this work, direct measurements of RI replace estimates of solubility parameters. RI is readily measured and is a good indicator of the ability of the oil to maintain a stable dispersion of asphaltenes. Tests with stock tank oil samples have shown that there is a common trend relating RI at the onset of instability to precipitant molecular size. This observation facilitates predictions of asphaltene instability for a wide range of conditions from as few as one or two onset determinations with precipitants of known molecular size. Results obtained at ambient conditions for stock tank oil can, in principle, be extended to reservoir conditions using standard PVT and compositional data, as demonstrated in this paper.