Chuncheng Li

Date of Award

January 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

First Advisor

Hui Pu

Abstract

The Middle Bakken Formation of the Williston Basin is a typical tight formation with the predicted primary oil recovery of less than 10%, which results in large amounts of oil remaining in the reservoir. Therefore, an effective enhanced oil recovery (EOR) method for recovering the residual oil is crucially important. To obtain the microscopic EOR mechanisms, molecular simulation methods including Monte Carlo (MC) simulations and molecular dynamics (MD) simulations were applied to study the various EOR methods, such as CO2 injection, hydrocarbon gas injection, and nanofluid flooding. A series of molecular models, including bulk systems, interfacial systems, and nanoconfined systems, were built to evaluate the potentials of the injected fluids to improve oil recovery.CO2 injection is a successful EOR technology that is being widely applied in North American oil fields. Studies have suggested CO2-based EOR is technically possible in the Middle Bakken Formation. The swelling of the crude oil/CO2 system plays a crucial role in the CO2 flooding process. Therefore, a better understanding of the effect of CO2 on crude oil swelling and viscosity reduction is critical for a successful CO2 EOR project. In this dissertation, a series of n-alkane/CO2 systems were studied by performing configurational-bias Monte Carlo (CBMC) simulations and MD simulations. The effects of alkyl chain length, pressure, and temperature on the CO2 solubility and the swelling factor were investigated. The solubility of CO2 and the swelling factor of CO2 saturated n-alkane are positively correlated to the pressure, while negatively correlated to the alkyl chain length and temperature. With more CO2 dissolved, the interaction energy between n-alkane molecules becomes less negative, which indicates the swelling of the n-alkane/CO2 system. N-alkanes with longer alkyl chain have more negative intermolecular interaction energy, and thus have a smaller swelling factor after saturating with CO2. With the increase of the CO2 mole fraction, the viscosity of the n-alkane/CO2 system is reduced. N-alkanes with longer alkyl chains have a larger viscosity reduction with increasing amounts of dissolved CO2. Besides CO2, hydrocarbon gases, like methane and ethane, can also mobilize the residual oil and enhance oil recovery. The gas solubility, volume swelling factor, oil diffusion coefficient, minimum miscibility pressure (MMP), and the oil extraction from nanoslits were then studied to compare the efficiency of different gases in the EOR process. Based on the Bakken oil composition, a molecular model of the crude oil containing different types of alkanes was built. MD simulations were carried out to study the interfacial interactions between the Bakken crude oil and the injected gases and the oil extraction from the calcite nanoslits. At various pressures and reservoir temperature, density profiles were plotted to show the distributions of different components, and the solubility of gases in crude oil was calculated. The simulation results show that all three gases hold great potential in further improving oil recovery. At constant temperature and pressure, ethane holds the highest solubility in crude oil and can induce the most pronounced oil swelling. Meanwhile, ethane can achieve the lowest MMP and the most significant oil diffusion coefficient. Without the effect of nano-confinement, ethane is most effective in mobilizing crude oil. However, CO2 is more effective in extracting oil from the nanoslits. Recent studies have also reported various types of nanoparticles (NPs) for improving oil recovery either alone or in combination with surfactants. The mechanisms of surface-modified silica (SiO2) NPs in improving oil recovery were investigated. Interfacial tensions (IFTs) of octane (C8H18)/water systems in the presence of different NPs were calculated. Quartz nanochannels were constructed to study the effect of NPs on oil flow through nanopores in rocks. Both water-wet and oil-wet surfaces were considered. Simulation results indicate that IFT reduction depends strongly on the distribution and the interfacial concentration of NPs. Surface-modified NPs with both hydrophilic and hydrophobic functional groups can reduce the IFT between oil and water. However, the IFT reduction is not significant in terms of EOR application. The alkanes/water/NPs transportation in confined nanochannels shows that the initial rock wettability affects the water flooding performance and the final oil recovery. The surface-modified NPs hold a higher capacity in detaching oil droplets from the oil-wet mineral surface regardless of their abilities to change interfacial tension. Surface modification is crucial to improve the surface properties of SiO2 NPs. The strong interactions between NPs and oil/rock lead to oil detachment and incremental oil recovery. The chemical composition of the functional groups and the surface coverage of the hydrophilic/hydrophobic functional groups should be carefully tuned to achieve the highest oil recovery rate. Molecular simulation study provides better insight into the interactions between oil components and injected fluids or mineral surfaces at the molecular level. The effect of injected fluids on the properties of the oil can be clearly explained. The application of molecular simulation methods could play an important role in interpreting experimental results and providing guidance for practical oil recovery processes in the Bakken Formation.

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