Asphaltene behavior at the interface oil-nanofluids: Implications to adsorption

Abstract

The mechanisms pertaining to the asphaltene adsorption at the interface oil/silica-based nanofluid and oil/wetted sandstone are examined. Asphaltene sample, extracted from a dead heavy oil using n-heptane, was dissolved into toluene to obtain a solution with a concentration of 3.23 mg/g. The solution was allowed to contact crushed Berea sandstone wetted with different silica-based nanofluids in the presence of carbon dioxide (CO2) bubbling. The silica-based nanofluid was prepared by dispersing silica oxide nanoparticle (0.1 wt%) into an aqueous solution of modified polyvinyl alcohol (1 wt%). Both batch and dynamic adsorptions were performed. A Solid-Liquid Equilibrium (SLE) model was employed not only to describe the adsorption mechanisms, but also to obtain the adsorptions parameters including the adsorption affinity and the asphaltene-self aggregation. The batch adsorption results reveal that asphaltenes develop a stronger affinity with the silica nanofluid film, which leads to an adsorption four-fold lower compared to sandstone wetted with the base fluid (polymer). It is also shown that the adsorption decreases with the decrease in the concentration of the polymer. In respect of the intrinsic properties of asphaltenes, it is found that the asphaltene molecules with a large aromatic core diffuse less, thus have a low adsorption. Irrespective of the core size, the affinity at the nanofluid film increases with the ratio in heteroatom-to-total carbon. Furthermore, the chemistry of the base fluid acts upon both the adsorption affinity and asphaltene self-association. Dynamic adsorption results on a nanofluid-wet sandstone were found three-fold larger compared to batch adsorption results. The primary reason is attributed to the alteration of the diffusion-barrier by the water, which promotes owing to the self-association of asphaltenes.