Electrostatic Effect on Core–Shell Micro-spheres with Mixed Charges as Adaptive Plugging Agents for Enhanced Oil Recovery

Core-shell micro-spheres (MS) with both negative and positive charges in the core and only negative charges in the shell were developed as adaptive plugging agents for in-depth conformance control for enhanced oil recovery. The MS were designed to propagate deeply into the reservoir due to the small particle size and electrostatic repulsion between the MS and the sandstone at the initial stage of injection and form aggregates by electrostatic attraction between the cores with mixed charges when the shells degraded at a given time during transportation, leading to an effective plugging of the highly permeable layers with low residual oil saturation. The self-assembling and plugging behaviors of the MS have been studied by Monte Carlo simulation. The results show that charge density (Dcharge), fraction of positive charge (Fp), MS concentration, temperature, and salinity are the key factors influencing the self-assembling behaviors. The electrostatic interaction would become stronger with the increase in Dcharge when it is larger than 0.5. The MS are more likely to form aggregates when Fp approaches 0.5. The higher the concentration of the MS, the stronger the electrostatic interaction between the MS. In addition, electrostatic interactions between the MS become stronger with the increase in temperature and decrease in salinity. Simulation results prove that the MS with mixed charges can effectively and adaptively plug highly permeable layers with low residual oil saturation through self-assembling by combination of electrostatic interactions along with physical bridging, leading to the improvement of oil recovery. Furthermore, block charge distribution will be helpful for the MS with mixed charges to form larger aggregates than that of the random mode to effectively plug the highly permeable layers.