MICROSCOPIC INVESTIGATION OF THE ONSET OF ASPHALTENE PRECIPITATION

A microscopic study of the onset of asphaltene precipitation is report ed. The onset conditions can be quantified by measurement of mixture r efractive index, together with microscopic observations of particulate formation in mixtures of oil and precipitant, with or without added s olvents. For isooctane mixtures with a variety of hydrocarbon solvents and a crude oil from Alaska, the onset of precipitation occurs over a narrow range of solution refractive index. Addition of polar solvents or different precipitating agents can shift the refractive index at w hich precipitation begins. Refractive index decreases when a crude oil is diluted by precipitant, as in this study, or when changes in tempe rature and pressure alter the relative molar volumes of species in the oil. If it falls below some critical value, resin/asphaltene aggregat es that had been in stable dispersion become unstable and precipitate. These observations provide a method of screening solvents to differen tiate between those that prevent precipitation mainly by maintaining a higher mixture refractive index and others that may participate in or disrupt asphaltene/resin interactions.