Understanding the Role of Amphiphilic Chemical Additives in Enhanced Oil Recovery from Molecular Perspectives
Creators: Nan, Yiling
Publisher: name: University of Alberta Library
Year: 2022

<p>Carefully designed chemical formulas are often applied in the enhanced oil recovery (EOR) process. Understanding the molecular distribution and working mechanism of each component can provide theoretical guidance in designing chemical formulas with desired functionalities. In this dissertation, we employ the molecular dynamics (MD) simulation to study the atomic distribution as well as the working mechanism of the amphiphilic chemical additives (surfactant, co-surfactant) applied in the petroleum industry, especially during the chemical flooding and gas injection process. <br>
In chemical flooding, surfactant formulas are injected into the reservoir to reduce the interfacial tension (IFT) between oil and brine. The injected surfactant should be stable and effective under reservoir conditions, generally associated with high-pressure high-temperature, and formation water (omnipresent in the reservoir, contains various salt ions with its salinity can be up to 35 wt.%). We first explore the effect of ion valency and concentration on sodium dodecyl sulfate (SDS) surfactant arrangement and efficiency. Two different cations (Na+ and Ca2+) with a wide range of ion concentrations (up to 3.96 M) are employed to simulate reservoir conditions. We demonstrate that ion valency has a significant effect on molecular configurations. Ca2+ ions can form unique pentagon-like SDS-Ca2+ complexes through SDS-Ca2+-SDS cation bridging. Monovalent Na+ can also generate SDS-Na+-SDS cation bridgings, while their concentration is much lower than that of SDS-Ca2+-SDS. The non-ionic (propanol) and cationic [cetrimonium bromide (CTAB)] surfactants with a wide range of concentrations are introduced to the primary SDS formula, to study the effect of chemical additives. We find that CTAB can disaggregate the cation bridging when their concentration is above a certain threshold. The cation bridging density is maintained at a low level when the sum of surfactants and cosurfactant interface charges is neutral or positive. On the other hand, propanol barely disaggregates the cation bridging. Both propanol and CTAB can further decrease the oil-brine IFT while having different efficacies. More rapid IFT decrement is observed when cation bridging is disaggregated. Propanol, as a cosurfactant, can be transported through oil and brine phases; such a dislocation of propanol in the system is a dynamic process. In the meantime, the introduction of propanol does not always increase the local fluidity of surfactants at the interface. A local maximum fluidity was observed when the SDSs are more perpendicular to the interface. These works should guide surfactant formula design in the chemical flooding process.<br>
Alcohol blending is often employed in the super critical CO2 (scCO2) gas injection. Ethanol can increase the sodium bis(2-Ethylhexyl) sulfosuccinate (AOT) solubility in scCO2 during the gas injection, while their working mechanism is not well established yet. Spontaneous aggregation processes in two systems (one consists of AOT and scCO2; the other consists of AOT, scCO2, and 10 wt.% ethanol) are conducted under a typical tight oil reservoir condition (333 K and 200 bar) to investigate the working mechanism of ethanol. After 600-ns runs, the AOT molecules aggregate together and form rod-like reversed micelles (RMs) in the System without ethanol, while forming several small sphere-like RMs by introducing the ethanol to the system. We propose that the ethanol molecules can better solvate and surround Na+ ions, preventing the further aggregation of AOT clusters. Other than increasing surfactant solubility, as an amphiphilic molecule, alcohols can also distribute at the interface region and further affect the water/scCO2 (foam interface) interfacial properties. Alcohols with varying tail lengths (C2OH-C16OH) under a wide range of concentrations are introduced to water/AOT/scCO2 interface systems to study their effects. We demonstrate that alcohol can distribute in water, interface region, and scCO2 phases, and their participation in phases is affected by the alcohol tail length. Alcohols&#39; tail length has a negligible effect on alcohol distribution at the interface when their concentration in the scCO2 phase is fixed. On the other hand, alcohol concentration in the water phase increase as tail length decrease. The ability in decreasing IFT is similar for different tail length alcohols when alcohol concentration is relatively low (before reaching the inflection point). However, the lowest available IFT (inflection point) increases as alcohol chain length increases. The mean-squared displacement (MSD) of AOT decreases as alcohol concentration increases, and such a decrement trend is more significant in systems with long-chain alcohols. These works should provide important insights into designing chemical formulas for gas injection.</p>