Nanoscale Insights into Carbonated Water Flooding for Enhanced Oil Recovery Using Molecular Dynamics Simulation

Carbonated low/smart salinity flooding is a promising technique for enhanced oil recovery (EOR) that optimizes the salinity and ionic composition of injected carbonated water. However, the nanoscale mechanisms driving its effectiveness remain poorly understood. This dissertation investigates the rock/brine/oil interfacial behavior in such systems using molecular dynamics simulations, focusing on both carbonated water/polar oil and calcite/polar oil-carbonated water interactions.
Simulations explored polar hydrocarbons with oxygen-bearing functional groups (e.g., benzoic acid, decanoic acid, phenol, decanol) at carbonated water interfaces. Despite their structural differences, the functional groups exhibited similar interfacial behaviors. CO₂ accumulated at the decane/water interface, reducing interfacial tension (IFT) and enhancing oil fluidity. Polar molecules, acting as surface-active agents, and their competition with CO₂ altered the charge distribution of water molecules, disrupting the structured water layer. Meanwhile, CO₂ diffused into the oil, enhancing the decane’s movement, a viscosity indicator.
The calcite/carbonated brine/polar oil simulations revealed that changes in salinity and ionic composition formed an electrical double layer (EDL) at interfaces, with Na+ ions forming a Stern-like positive layer and Cl– or SO₄²⁻ ions forming a negative layer above them. In NaCl-carbonated brines, sodium ions concentrated at brine–calcite and brine–oil interfaces at lower salinities, reducing their presence in the bulk brine. High-salinity brine systems enhanced the adsorption of polar molecules at the brine film interfaces. NaCl-carbonated brine (60,000 ppm) facilitated the uniform alignment of benzoic acid at the brine/oil interface, acting as a slippery surface and improving polar oil model viscosity.
The presence of ions in carbonated brine, particularly divalent ions (Mg²⁺, SO₄²⁻), significantly reduced the apparent viscosity of polar oils. Adding benzoic acid (BA) and ions highlights CO₂'s impact on the oleic phase's apparent viscosity. Magnesium ions effectively lowered IFT but showed limited affinity for the calcite surface due to their high charge density and strong hydration shell. They tend to follow negatively charged ions like sulfate, forming ion pairs and aggregates in the brine phase. Higher sulfate concentration promotes the formation of ionic aggregates in the bulk brine-oil interface, which further facilitates the detachment and reorganization of polar molecules. The co-presence of Mg²⁺ and SO₄²⁻ enhances the water-wetting tendency of calcite by reducing the positive charge on the surface and promoting the restructuring of polar molecules, shifting the wettability towards a more water-wet state. Comparing the capillary force driven by IFT and contact angle, the IFT plays a more significant role than contact angle in promoting capillary-driven oil recovery, with polar oil components serving as key surface-active agents relative to the impact of ionic composition.