If oil in a reservoir experiences a gas charge, the GOR of that oil can increase, causing reduction of asphaltene solubility and the onset of asphaltene flocculation, which can result in deposition of asphaltenes. The location of the asphaltene deposit depends on how the reservoir trap fills and how the GOR increases. To describe this and other processes involving asphaltenes in reservoir crude oils, scientists use an equation of state. However, until recently, there was little understanding of the molecular and aggregate sizes of asphaltenes in crude oils or laboratory solvents. Without knowing the size of the aggregate, scientists cannot solve Newton's second law of gravity, F = m × g, where F is force, m is asphaltene mass and g is Earth's gravitational acceleration; therefore, modeling is fruitless. The inability to understand and model asphaltenes in crude oils led to an inordinate focus on asphaltenes as only a problem of flow assurance-the analysis of reservoir fluids to characterize phase behaviors and anticipate associated flow problems within production systems that may interrupt hydrocarbon flow from the reservoir to the refinery. In fact asphaltenes have a much broader impact on the reservoir and can affect evaluation of reservoir connectivity and fault-block migration, as well as cause formation of heavy oil gradients, tar mats and disequilibrium in fluid gradients.
Asphaltene Structures and Sizes
The Yen-Mullins model for asphaltene structures and sizes codifies the nanostructures of asphaltenes into three distinct and separate forms: asphaltene molecules, nanoaggregates of individual asphaltene molecules and clusters of nanoaggregates (Figure 2). The model enables the prediction of asphaltene mass, which is required for modeling the thermodynamic behavior of asphaltenes. Asphaltene molecules are relatively small, and the dominant molecular structure has a single core of aromatic hydrocarbons that have peripheral substituents (side chains or pendant groups) composed of alkanes hanging off them. The typical asphaltene molecule has a mean molecular weight of 750 g/mol.
Asphaltene molecules readily combine or aggregate. This process originally hindered conventional techniques for determining the molecular weight of asphaltenes. At low concentrations, such as in light oils, asphaltenes are dispersed as molecules. As the asphaltene concentration increases, such as in black oils, nanoaggregates form comprising about six asphaltene molecules each. At high concentrations, such as in heavy oils, clusters form consisting of about eight nanoaggregates each.
The Yen-Mullins model delivers a basis for an equation of state for predicting asphaltene concentration gradients in oil reservoirs. From this model, the Flory-Huggins-Zuo equation of state (FHZ EOS) was developed, a simple equation that has few parameters. This EOS model enables analysts to interpret field data using downhole fluid analysis (DFA) from a wireline formation tester.
The Yen-Mullins model has been confirmed by ultrahigh-resolution molecular imaging (Figures 3 and 4). The atoms and bonds of the molecule have been imaged using atomic force microscopy (AFM). Individual electron orbitals have been imaged by scanning tunneling microscopy (STM) and they match theoretical molecular orbital calculations.