The Role of Water on Asphaltenes Aggregation Onset from Petroleum Fluids: Molecular Dynamics Simulations

To practicing engineers, the importance of knowledge about the behavior of asphaltenes in petroleum is similar to the knowledge needed by cardiologists regarding cholesterol in the arteries of patients. Asphaltenes, if present in a petroleum fluid, tend to aggregate and deposit in flow paths causing plugging in the reservoir porous media, wellbores, and transmission pipelines. Molecular dynamics simulations are performed on oil/water systems at reservoir conditions to investigate the influence of aqueous solutions on asphaltenes aggregation onset. In addition to pure-water, brine and acid are used to study the effect of salinity and pH. The oil phase consists of asphaltenes and ortho-xylene, in which asphaltene molecules are completely soluble. Molecular dynamics results illustrate that asphaltenes become less soluble in oil when water is partially miscible in the oil phase. The driving force behind asphaltenes aggregation is shown to be an asphaltenes-water hydrogen bonding. Aggregation onset of asphaltenes follows three sequential steps including asphaltenes-water interaction; water bridging; and face-to-face stacking. Molecular dynamics results also reveal that the “salt-in effect” of high-salinity brine (25 wt.% NaCl) causes a significant reduction of the onset of asphaltenes aggregation as compared with pure-water. Furthermore, molecular dynamics results show that the accumulation of the ionized asphaltenes at the oil/aqueous interface is responsible for the asphaltenes aggregation onset when HCl acid (low pH) is used as an aqueous phase.