Investigation of Asphaltene-Asphaltene Association and Aggregation for Compositional Reservoir Simulators By Quantitative Molecular Representations

Abstract

The objective of this research is to properly calculate the association energy with molecular models of asphaltenes using Molecular Dynamics (MD) simulations. Asphaltene precipitation clogs pore throats of reservoir rocks, and significantly reduces the production rate. To better understand the asphaltene precipitation, a process-modeling methodology has been developed. The cubic-plus association equation of state (CPA) (Li & Firoozabadi, 2010) is effective in the modeling, and contains only one adjustable parameter, that is, association energy of asphaltene, however, we were not able to correctly evaluate the association energy by the currently available methods. Quantitative Molecular Representations (QMR) method provides systematic structural representations of asphaltenes based on analytical data of a crude oil, from which molecular models of asphaltene can be obtained. These models will be employed in this study for calculation of the association energy. Two different asphaltene models were generated for two different crude oils (Oil 'A' and Oil 'M'). The potential of mean force (PMF) is calculated with two models of asphaltene molecules in liquid benzene for two kinds of asphaltenes using umbrella sampling method. The association energy is calculated by the difference between values at the lowest bottom of a trough and at infinity of the PMF curve. As the calculation conditions, temperature and pressure are 300 K and 100 bar, respectively. The PMF profiles for asphaltenes from Oil 'A' and Oil 'M' were obtained, and the values of association energy were estimated as -8.39 kJ/mol and -8.91 kJ/mol, respectively. Two aromatic planes of asphaltene molecules are stacked with each other at the minimum energy point. This π-π stacking is one of the main interactions to form asphaltene nano-aggregation. The results of asphaltenes from Oil 'A' and Oil 'M' have a similar tendency; for example, the distance between the centers of mass of two asphaltenes at the minimum energy points factually equal. The data obtained in this work should be used in the thermodynamic model of phase behavior of asphaltene dealing with asphaltene association and precipitation. It will improve the prediction ability of reservoir simulators with cubic-plus association equation of state.

Evaluation of Asphaltene Adsorption Free Energy at the Oil–Water Interface: Role of Heteroatoms

“…They are corresponding isomers of asphaltene models shown in Figure , because the QMR method was used to generate the asphaltene model, which can best reproduce all the analytical experiments. So far, the analytical experiments (used in QMR) include elemental composition, molecular weight, and 1 H and 13 C NMR spectroscopies. ,,, Since the element ratio, molecular weight, and hydrogen and carbon structural types of the modified asphaltene models (As01-s–As06-s) are the same as the original models (As01–As06), they are identical in terms of the QMR method. So, this effort is also of some help to check how our “interpretation” of the analytical data can affect the adsorption behavior.…”

“…The PMF is known as a tool to calculate the system’s energy changes as a function of the distance between two molecules. ,,,, The adsorption Gibbs free energy was obtained from PMF ( r ) by the following equation: where r is a reaction coordinate, k B is the Boltzmann constant, T is temperature, and P ( r ) is the distribution function of the system along a reaction coordinate. In this study, we calculated adsorption Gibbs free energy of the asphaltene molecule at the oil–water interface and the unit is represented by kJ/mol.…”

“…During the pulling and umbrella sampling simulations, a dummy atom was frozen to keep its position. D was controlled by the spring constant K of 1000 kJ/mol nm 2 determined from previous studies , on asphaltene molecules, while the asphaltene molecule was free to rotate in all simulations. During the pulling simulation, the asphaltene molecule was pulled from the oil–water interface to the oil phase.…”

“…Because of the development of computational technology in recent years, molecular dynamics (MD) simulations have been widely used in the research relevant to petroleum engineering. ,− ,− A range of different asphaltene models have been constructed and employed in various studies. ,− ,− ,− In this study, we used 13 types of asphaltene molecules. The original model of these asplaltene molecules was generated from an improved quantitative molecular representation (QMR) method, which provides a set of molecules consistent with experimental data, such as elemental composition, average molecular weight, and the structural types of hydrogen and carbon atoms, as revealed by 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopies. ,,, MD simulations were performed to investigate the asphaltene adsorption behavior and the stability of the asphaltene adsorption structures at the oil–water interface, focusing on the role of heteroatoms. First, we considered a set of six types of asphaltene models, which have essentially the same structures but with different heteroatoms (such as nitrogen, oxygen, and sulfur) on the aromatic ring (i.e., heteroaromatic ring).…”

“…The original model of these asplaltene molecules was generated from an improved quantitative molecular representation (QMR) method, which provides a set of molecules consistent with experimental data, such as elemental composition, average molecular weight, and the structural types of hydrogen and carbon atoms, as revealed by 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopies. 46,50,51,53 MD simulations were performed to investigate the asphaltene adsorption behavior and the stability of the asphaltene adsorption structures at the oil−water interface, focusing on the role of heteroatoms. First, we considered a set of six types of asphaltene models, which have essentially the same structures but with different heteroatoms (such as nitrogen, oxygen, and sulfur) on the aromatic ring (i.e., heteroaromatic ring).…”