ABSTRACT
Also, there are different opinions about the solubility properties of already precipitated asphaltenes. Years back, it was the general opinion that already precipitated asphaltenes would never go back into solution again. Supporters of this idea saw asphaltenes dissolved in an oil mixture as aggregates, only staying in solution because of an outer protective layer consisting of resins. Removal of this protective layer would make the asphaltenes form even larger aggregates that would be insoluble, because it would be impossible to regenerate the protective resin layer. Resins form another solubility class. As may be seen from Figure 12.1, resins are soluble in n-heptane. They can be adsorbed on silica or alumina from an n-heptane solution, from which state they can be extracted using a methanol-benzene solution. The understanding of asphaltene precipitation as a nonreversible process was essentially based on experimental observations of asphaltenes precipitated from stabilized oils by the addition of large quantities of either n-pentane or n-heptane. This precipitation technique gives asphaltenes in almost pure form, and the cohesion between the individual asphaltene molecules may be so high that it becomes almost impossible to dissolve the asphaltenes again. As opposed to these types of experiments, experimental studies of oils precipitating asphaltenes at reservoir conditions (e.g., Angulo et al. 1995; Jamaluddin et al. 2000 and 2002; Hustad et al. 2014) suggest that asphaltenes may precipitate and dissolve again as is the case with an “ordinary” equilibrium phase. The rest of this chapter is based on the understanding of gas-oil-asphaltene phase separation being determined by classical phase equilibrium criteria and sees the asphaltene phase as a heavy noncrystalline liquid phase.
