The Yen−Mullins model, also known as the modified Yen model, specifies the predominant molecular and colloidal structure of asphaltenes in crude oils and laboratory solvents and consists of the following: The most probable asphaltene molecular weight is ∼750 g/mol, with the island molecular architecture dominant. At sufficient concentration, asphaltene molecules form nanoaggregates with an aggregation number less than 10. At higher concentrations, nanoaggregates form clusters again with small aggregation numbers. The Yen−Mullins model is consistent with numerous molecular and colloidal studies employing a broad array of methodologies. Moreover, the Yen−Mullins model provides a foundation for the development of the first asphaltene equation of state for predicting asphaltene gradients in oil reservoirs, the Flory−Huggins− Zuo equation of state (FHZ EoS). In turn, the FHZ EoS has proven applicability in oil reservoirs containing condensates, black oils, and heavy oils. While the development of the Yen−Mullins model was founded on a very large number of studies, it nevertheless remains essential to validate consistency of this model with important new data streams in asphaltene science. In this paper, we review recent advances in asphaltene science that address all critical aspects of the Yen−Mullins model, especially molecular architecture and characteristics of asphaltene nanoaggregates and clusters. Important new studies are shown to be consistent with the Yen−Mullins model. Wide ranging studies with direct interrogation of the Yen−Mullins model include detailed molecular decomposition analyses, optical measurements coupled with molecular orbital calculations, nuclear magnetic resonance (NMR) spectroscopy, centrifugation, direct-current (DC) conductivity, interfacial studies, small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS), as well as oilfield studies. In all cases, the Yen−Mullins model is proven to be at least consistent if not valid. In addition, several studies previously viewed as potentially inconsistent with the Yen−Mullins model are now largely resolved. Moreover, oilfield studies using the Yen−Mullins model in the FHZ EoS are greatly improving the understanding of many reservoir concerns, such as reservoir connectivity, heavy oil gradients, tar mat formation, and disequilibrium. The simple yet powerful advances codified in the Yen−Mullins model especially with the FHZ EoS provide a framework for future studies in asphaltene science, petroleum science, and reservoir studies.

The molecular architecture of asphaltenes is still a matter of debate. Some literature reports provide evidence that the contrast of petroleum asphaltenes versus coal-derived asphaltenes is useful for understanding the governing principles of asphaltene identity. Coal-derived asphaltenes provide an excellent test for understanding the relationship of asphaltene molecular architecture with asphaltene properties. Diffusion measurements have shown that coal-derived asphaltenes are half the size of many crude oil asphaltenes, but there are relatively few studies comparing coal-derived and petroleum asphaltenes using liquid state 13C NMR. 13C NMR confirms that the molecular sizes of these coal-derived asphaltenes are smaller than virgin petroleum asphaltenes. DEPT-45 experiments were performed in order to determine the relative amount of nonprotonated and protonated carbon in the aromatic region of the spectrum. In contrast to previous NMR work on asphaltenes that ignored interior bridgehead carbon, we show this is an important component of asphaltenes and that correctly accounting for this carbon enables proper determination of the number of fused rings. XRS data supports interpreting the NMR data with a model that weighs circularly condensed structures more heavily than linearly condensed structures. Significantly more carbon exists in chains at least 9 carbons long in petroleum asphaltenes (≥7%) compared to coal-derived asphaltenes (≥1%).

Asphaltenes are an important class of compounds in crude oil whose surface activity is important for establishing reservoir rock wettability which impacts reservoir drainage. While many phenomenological interfacial studies with crude oils and asphaltenes have been reported, there is very little known about the molecular level interactions between asphaltenes and mineral surfaces. In this study, we analyze Langmuir-Blodgett films of asphaltenes and related model compounds with sum frequency generation (SFG) vibrational spectroscopy. In SFG, the polarization of the input (vis, IR) and output (SFG) beams can be varied, which allows the orientation of different functional groups at the interface to be determined. SFG clearly indicates that asphaltene polycyclic aromatic hydrocarbons (PAHs) are highly oriented in the plane of the interface and that the peripheral alkanes are transverse to the interface. In contrast, model compounds with oxygen functionality have PAHs oriented transverse to the interface. Computational quantum chemistry is used to support corresponding band assignments, enabling robust determination of functional group orientations.

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.

Using fluorescence correlation spectroscopy (FCS) we measure the translational diffusion coefficient of asphaltene molecules in toluene at extremely low concentrations (0.03-3.0 mg/L): where aggregation does not occur. We find that the translational diffusion coefficient of asphaltene molecules in toluene is about 0.35 x 10(-5) cm(2)/s at room temperature. This diffusion coefficient corresponds to a hydrodynamic radius of approximately 1 nm. These data confirm previously estimated size from rotational diffusion studied using fluorescence depolarization. The implication of this concurrence is that asphaltene molecular structures are monomeric, not polymeric.