Asphaltene Nanoscience and Reservoir Fluid Gradients, Tar Mat Formation, and the Oil-Water Interface
Authors: Mullins Oliver C. , Pomerantz Andrew E. , Zuo Julian Y. , Andrews A. Ballard , Hammond Paul , Dong Chengli , Elshahawi Hani , Seifert Douglas J. , Rane Jayant P. , Banerjee Sanjoy , Pauchard Vincent 
Published: June 2014

Abstract
Recent advances in understanding asphaltene nanoscience have led to important developments in related but distinct disciplines of reservoir fluid gradients, fault block migration, tar mat formation and oil-water interfacial properties. Here, we provide an integrated overview of recent advances in asphaltenes nanoscience and corresponding implications in recent oilfield studies. Specifically, the Yen-Mullins model codifies the dominant molecular structure and two hierarchical colloidal species of asphaltenes for condensates through mobile heavy oils. Recent mass spectrometry studies confirm the asphaltene molecular weight and architecture as well as the aggregation number of the nanoaggregate. With the size known, the gravitational effect is resolved enabling development of the industry's first equation of state (EoS) for asphaltene gradients, the Flory-Huggins-Zuo (FHZ) EoS. Many case studies prove its validity. The formation of different types of tar mats are understood within these asphaltene science developments. Specifically, some tar mats are formed by solution gas increase throughout the column via late gas charge yielding discontinuous increases of asphaltene content at the oil-tar contact. Other tar mats are formed by asphaltene gravitational accumulation at the base of the oil column which can yield heavy oil and tar with a much more continuous increase of asphaltene content. This asphaltene gravitational accumulation is associated with redistribution and equilibration of the asphaltene colloidal species. Sulfur x-ray spectroscopy corroborates mechanisms proposed for creation of these heavy oil gradients and tar mats addressed herein. A recent breakthrough in understanding oil-water interfaces shows that simplifying universal curves are obtained for the reduction of oil-water interfacial tension merely as a function of asphaltene molecular coverage and independent of many potential complexities. In particular, the first and only direct measurement of asphaltene molecular orientation at the interface shows that the asphaltene aromatic ring system is in plane at the oil-water interface while the asphaltene alkanes are perpendicular, which agrees quite closely with interfacial tension measurements. In addition, the asphaltene nanoaggregates are shown not to contribute to the surface in accord with known basic chemistry principles. These new interfacial results prove applicability of the Yen-Mullins model to the interface as well as to bulk oil. These important, new results will impact understanding of emulsions and provide a foundation for investigating oil-mineral interfacial science and enhanced oil recovery concepts. The fact that diverse crude oil and asphaltene properties are understood with simple models and universal curves confirms validity of this approach and portends rapidly expanding field application of these basic science precepts.