Overview of asphaltene nanostructures and thermodynamic applications

Abstract

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 sp2-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 sp2-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 sp2-hybridized carbon networks, in which these different aromatic ring systems are connected by one or more sp3-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.