The phase stability of crude oil is dependent on a multitude of factors, including temperature, pressure, and component fractions, especially long chain paraffin and polar asphaltene fractions. Paraffins precipitate out of the crude oil during pipeline transportation due to solubility limits, and form paraffin-oil gel deposits on the pipe walls. The presence of asphaltenes in crude oil is postulated to affect the formation of these paraffin gels. To quantify this effect, a controlled stress rheometer was used to study the gelation temperature and the yield stress of a model paraffinoil system. It was observed that the addition of asphaltenes in small proportions (∼0.1 wt %) resulted in a significant decrease both in the gelation temperature and the yield stress of the model system, indicating that the presence of asphaltenes hinders the gelation mechanism. Addition of higher amounts of asphaltenes resulted in macroscopic phase separation: a deposit consisting of asphaltenes and paraffins separated out of the liquid. The effects of operating conditions and the asphaltene polarity on the gelation process were also studied. Polarized light microscopy and nuclear magnetic resonance spectroscopy were used to obtain insights into the rheometric results.

Asphaltenes separated from two different North Sea oils were studied upon addition of two kinds of naphthenic acids by employing PFG-SE NMR (pulsed field gradient-spin echo nuclear magnetic resonance) and NIR (near-infrared) spectroscopy. The results indicated that there were interactions between asphaltenes and naphthenic acids. It also appeared as if the tendency of the acids to interact with asphaltenes depended on the asphaltene type. Furthermore, a concentration series of one of the asphaltenes was prepared, and a dramatic decrease in diffusion coefficients upon increased concentration was observed. This suggests that the asphaltenes studied were prone to self-associate in toluene-d 8.