Multi-scale simulation of asphaltene aggregationand deposition in capillary flow

摘要

Asphaltenes are known as the 'cholesterol' of crude oil. They form nano-aggregates, precipitate, adhere to surfaces, block rock pores and may alter thewetting characteristics of mineral surfaces within the reservoir, hindering oilrecovery efficiency. Despite a significant research effort, the structure,aggregation and deposition of asphaltenes under flowing conditions remainpoorly understood. For this reason, we have investigated asphaltenes, theiraggregation and their deposition in capillary flow using multi-scale simulationsand experiments. At the colloid scale, we use a hybrid simulation approach: forthe solvent, we used the stochastic rotation dynamics (also known as multiparticle collision dynamics) simulation method, which provides bothhydrodynamics and Brownian motion. This is coupled to a coarse-grained MDapproach for the asphaltene colloids. The colloids interact through a screenedCoulomb potential with varying well depth E. We tune the flow rate to obtainPe_(flow) 1 (hydrodynamic interactions dominate) andRe 1 (Stokes flow).Imposing a constant pressure drop over the capillary length, we observe that thetransient solvent flow rate decreases with increasing well depth e. The interactionsbetween the mesoscopic asphaltene colloids can be related to atomistic MDsimulations. Molecular structures for the atomistic calculations were obtainedusing the quantitative molecular representation approach. Using these structures,we calculate the potential of mean force (PMF) between pairs of asphaltenemolecules in an explicit solvent. We obtain a reasonable fit using a —1/r2attraction for the attractive tail of the PMF at intermediate distances. Wespeculate that this is due to the two-dimensional nature of the asphaltenemolecules. Finally, we discuss how we can relate this interaction to the mesoscopiccolloid aggregate interaction. We assume that the colloidal aggregates consist ofnano-aggregates. Taking into account observed solvent entrainment effects, wededuct the presence of lubrication layers between the nano-aggregates, whichleads to a significant screening of the direct asphaltene—asphaltene interactions.