Molecular Simulation Study on the Volume Swelling and the Viscosity Reduction of n-Alkane/CO₂ Systems

Abstract

Carbon dioxide (CO 2 ) injection is a successful enhanced oil recovery (EOR) technology that is being widely applied in North American oil fields. Studies have suggested CO 2 -based EOR is technically possible in the Middle Bakken Formation of the Williston Basin in North Dakota. The swelling of a crude oil/CO 2 mixture plays a crucial role in the CO 2 flooding process. Therefore, a better understanding of the effect of CO 2 on crude oil swelling and viscosity reduction is critical for a successful CO 2 EOR project in unconventional reservoirs such as the Bakken Formation. In this work, a series of n-alkane/CO 2 systems were studied by performing configurational-bias Monte Carlo (CBMC) simulations and molecular dynamics (MD) simulations. The effects of carbon chain length, pressure, and temperature on the CO 2 solubility and the swelling factor were investigated. The solubility of CO 2 and the swelling factor of the CO 2 -saturated n-alkane are positively correlated to the pressure, while negatively correlated to the carbon chain length and temperature. With more CO 2 dissolved, the interaction energy between n-alkane molecules becomes less negative, which indicates the swelling of the n-alkane/CO 2 system. n-Alkanes with a longer carbon chain have a more negative intermolecular interaction energy and thus have a smaller swelling factor after saturation with CO 2 . With the increase of the CO 2 mole fraction, the viscosity of the n-alkane/CO 2 system is reduced. n-Alkanes with longer carbon chains have a larger viscosity reduction with increasing amounts of dissolved CO 2 . This study provides a reliable assessment of the volume swelling and viscosity reduction of n-alkane/CO 2 systems and can further assist the CO 2 EOR application in the Bakken Formation.

Bulk and Interfacial Properties of the Decane + Brine System in the Presence of Carbon Dioxide, Methane, and Their Mixture

“…The knowledge of the species mole fractions and swelling effects plays an important role in the use of carbon dioxide for the recovery of hydrocarbon fluids from underground reservoirs 30,32,40,53,56 . Figure 4 shows the bulk mole fractions of methane and carbon dioxide in the decane-rich phase as obtained from the MD simulations (open symbols) and the corresponding theoretical data (solid lines) for all studied systems.…”

“…5 shows the swelling factor for the decane-rich phase as obtained from the MD simulations (open symbols) and the corresponding theoretical data (solid lines) for all studied systems. The swelling factor is defined as ratio of the volume of the saturated decane-rich system to the volume of the decane alone 53,56 . Though not shown, both theoretical and simulation results for densities of the pure n-decane system at the studied conditions are in good quantitative agreement with the NIST data 72 .…”

“…Also, similar to the solubility vs temperature behavior, the swelling of the CH 4 + n-decane system is not significantly affected by temperature. It is known that the interaction between CO 2 and alkane molecules plays a key role in the solubility and swelling properties of the alkane-CO 2 system 53,56 . For example, Fig.…”

“…1). These values were chosen to ensure reasonable bulk phases with enough molecules and also to reduce the truncation and system size effects in the calculations of the bulk and interfacial properties 39,46,[52][53][54][55][56] . These systems are energy-minimized using the steepest descent algorithm and subjected to equilibration MD runs of 10 ns in the NPT ensemble.…”

“…The bulk and interfacial properties of binary mixtures of methane or carbon dioxide with alkanes such as n-decane (model oil in this work) under reservoir conditions receive continuous attention and many experimental [30][31][32][33][34][35][36][37][38][39][40][41][42] , theoretical [38][39][40][41][43][44][45][46][47][48][49][50][51] , and simulation 39,46,[52][53][54][55][56] studies have been reported. These studies, for example, showed that the mole fractions of methane and carbon dioxide in the decane-rich phase increase with pressure.…”