The role of calcite and silica interfaces on the aggregation and transport of asphaltenes in confinement

Abstract

The structure and transport of the heavy asphaltene molecules in unconfined bulk fluids and fluids confined in a 4 nm slit-shaped nanopores of calcite and silica were studied using classical molecular dynamics simulations. The pores were filled with 7 wt% of asphaltene dissolved in an equimolar solvent of heptane and toluene. The mixture was considered since asphaltenes are insoluble and soluble in heptane and toluene, respectively. Our studies showed that asphaltene molecules tended to aggregate in unconfined bulk fluids to form a large cluster composed of all the asphaltene molecules. However, the size of nanoaggregates is smaller in the confined fluidic environments due to the adsorption of asphaltenes on silica or calcite surfaces. The aggregation and adsorption of asphaltenes initially located in the bulk fluid occurred at the pore entrances of calcite and silica. However, the asphaltene molecules located within the pore were adsorbed onto the pore surface. Further, confinement influenced the structure of the solvent. Toluene showed preferential adsorption for the calcite and silica surfaces over heptane due to differences in the polarities of these molecules. The adsorption behavior of the asphaltenes and solvents prove that the chemistry of the surfaces and edges have a crucial role in determining the structures of confined hydrocarbons. Further, confined fluids had a significant influence on the diffusivities of the asphaltenes and the solvents. Asphaltene and toluene diffusivities in confined fluids are lower compared to in bulk fluids. Further, the diffusivities of asphaltene with a higher molecular weight in confined and bulk fluids were lower compared to asphaltenes with lower molecular weights. The aggregation of asphaltenes proceeded via π-π stacking arising from the van der Waals affinity between the polyaromatics cores. These findings have important implications for predicting the influence of earth-abundant solid interfaces on the aggregation and transport behavior of complex heavy hydrocarbon molecules such as asphaltenes which are known to cause challenges in flow assurance for the recovery of hydrocarbons at the field-scale.