Abstract

Ions in brine significantly affect EOR. However, the mechanism of EOR with different brine is still controversial. By Combining Molecular Dynamics (MD) method and Quartz Crystal Microbalance with Dissipation (QCM-D) technology to analyze ions distribution and the mechanisms in detaching acidic components on the sandstone, an effective method to determine the detaching capacity was established. The results show that detaching capacity is related to ions distribution and hydration capacity. In the oil/brine/rock system, ions far from the rock are favorable for detaching, while ions near the rock are unfavorable for detaching due to ion bridging effect. The hydrogen bond between water and naphthenic acid is key to detaching. Cations strengthen the detaching by forming hydrated ions with water, and the detaching capacity is negatively correlated with hydrated ions radius and positively correlated with the water coordination number. The detaching determination coefficient was established by considering the ions distribution, ions types, and hydration strength, then verified by QCM-D. The brine detaching capacity with different Ca2+/Mg2+ ratios was predicted based on MD and detaching determination coefficient, and verified by QCM-D. The optimal Ca2+/Mg2+ ratio gave 7:3. This study provides theoretical guidance for targeted regulation of brine composition to improve the recovery of tight sandstone reservoir.