Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop-drop coalescence. As a result, asphaltenes unfavorably impact water-oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water-oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barrier-controlled regime rather than to a conformational relaxation regime.

In an earlier study, oil-water interfacial tension was measured by the pendant drop technique for a range of oil-phase asphaltene concentrations and viscosities. The interfacial tension was found to be related to the relative surface coverage during droplet expansion. The relationship was independent of aging time and bulk asphaltenes concentration, suggesting that cross-linking did not occur at the interface and that only asphaltene monomers were adsorbed. The present study extends this work to measurements of interfacial rheology with the same fluids. Dilatation moduli have been measured using the pulsating droplet technique at different frequencies, different concentrations (below and above CNAC), and different aging times. Care was taken to apply the technique in conditions where viscous and inertial effects are small. The elastic modulus increases with frequency and then plateaus to an asymptotic value. The asymptotic or instantaneous elasticity has been plotted against the interfacial tension, indicating the existence of a unique relationship, between them, independent of adsorption conditions. The relationship between interfacial tension and surface coverage is analyzed with a Langmuir equation of state. The equation of state also enabled the prediction of the observed relationship between the instantaneous elasticity and interfacial tension. The fit by a simple Langmuir equation of state (EOS) suggests minimal effects of aging and of nanoaggregates or gel formation at the interface. Only one parameter is involved in the fit, which is the surface excess coverage Γ∞ = 3.2 molecules/nm(2) (31.25 Å(2)/molecule). This value appears to agree with flat-on adsorption of monomeric asphaltene structures consisting of aromatic cores composed of an average of six fused rings and supports the hypothesis that nanoaggregates do not adsorb on the interface. The observed interfacial effects of the adsorbed asphaltenes, correlated by the Langmuir EOS, are consistent with the asphaltene aggregation behavior in the bulk fluid expected from the Yen-Mullins model.

Previously, asphaltene science had been hindered by many significant uncertainties regarding molecular weight, molecular structure, and nanocolloidal characteristics in laboratory solvents and crude oils. These debates were of sufficient magnitude to forestall development and utility of asphaltene modeling for various applications. In the last 2 decades, advances in asphaltene science using many sophisticated techniques have greatly reduced corresponding uncertainties, enabling development of simple asphaltene models for a variety of applications. Here, we provide an overview of key findings in asphaltene science; dominant molecular and nanocolloidal structures are described. These structures with simple thermodynamic formalisms are shown to work well in oilfield reservoirs, specifically including light oils, black oils, and heavy oils. This novel thermodynamic approach using asphaltene nanostructures has enabled characterization of different processes that impact or preclude equilibration of reservoir fluids in geologic time. These processes combine to form the new technical discipline, "reservoir fluid geodynamics", that has proven value in many reservoir studies. In addition, these asphaltene nanostructures are shown to apply to interfacial tension of asphaltene solutions, using simple thermodynamics for surfaces. In addition, we contrast past and current debates in asphaltene science, especially regarding asphaltene molecular architecture, which has been largely resolved. Molecular structural characterization of asphaltenes reviewed herein shows that asphaltenes are dominated by island structures, but some asphaltenes also have a secondary content of structures with an "aryl-linked core", which we propose as a third class of molecular architecture along with island and archipelago designations. The aryl-linked core structure is defined as having a single, contiguous sp 2 -hybridized carbon network containing one or more aryl linkages, in which adjacent aromatic rings are directly bonded together but do not share a common bond in a ring. In contrast, a traditional island structure also has a single, contiguous sp 2 -hybridized carbon network but has adjacent aromatic rings exclusively fused (sharing a common bond in a ring) with no aryl linkages. The definition of archipelago remains unchanged and consists of multiple, discontinuous sp 2 -hybridized carbon networks, in which these different aromatic ring systems are connected by one or more sp 3 -hybridized carbons. This new classification, "aryl-linked core", has been assigned to both island and archipelago structures in different publications; thus, this new designation should reduce confusion and help resolve structure− function relations, especially regarding aggregation and reactivity. More broadly, the advances in asphaltene science have ushered in powerful, new applications that are continuing to expand.

Previous studies indicated that asphaltenes adsorbed as monomers on oil-water interfaces and the early stage kinetics of the process was controlled by diffusion and hence dependent on oil viscosity. By measuring interfacial tension (IFT) as a function of surface coverage during droplet expansions in pendant drop experiments, it was also concluded that the IFT data could be interpreted with a Langmuir equation of state (EoS), which was independent of oil viscosity, time of adsorption, and bulk asphaltenes concentration. The surface excess coverage was calculated to be ∼0.3 nm(2)/molecule, which suggested adsorption in face-on configuration of asphaltenes monomers at the interface and average PAH core per molecule of about 6 for the asphaltenes investigated, consistent with the Yen-Mullins model. The current study focuses on the kinetics of asphaltenes adsorption at longer times and higher interfacial coverage. Long-term IFT data have been measured by the pendant drop method for different asphaltenes concentrations and for different bulk viscosities of the oil phase (0.5-28 cP). The data indicate that when coverage reaches 35-40%, the adsorption rates slow down considerably compared to the diffusion-controlled rates at the very early stages. The surface pressure increase rate (or IFT decrease rate) at these higher coverages is now independent of oil viscosity but dependent upon both surface pressure itself and asphaltene monomer concentration. The long-term asymptotic behavior of surface coverage is found to be consistent with the predictions from surface diffusion-mediated random sequential adsorption (RSA) theory which indicates a linear dependency of surface coverage on 1/√t and an asymptotic limit very close to 2D random close packing of polydispersed disks (85%). From these observations RSA theory parameters were extracted that enabled description of adsorption kinetics for the range of conditions above surface coverage of 35%.