The Behavior of Microemulsion in Unconventional Reservoirs: Insights from Molecular Dynamics Simulation

Abstract The use of microemulsions to enhance well productivity is becoming increasingly popular, especially in unconventional shale wells during fracturing. However, the subsequent behavior and stability of microemulsions remain unclear, particularly in clay-hosted nanopores where clay minerals have unbalanced surface charges. This study uses molecular dynamics (MD) simulations to analyze the behavior and stability of microemulsion droplets in clay-hosted nanopores with varying surface chemistries and salinities in shales, under typical reservoir pressures and temperatures. Our results show that in bulk conditions, microemulsion droplets are stable across all salinities. However, their stability in clay-hosted nanopores depends on the surface chemistry (charge distribution) of the clays and the salinity. When clay surfaces are charge-balanced or have a moderate imbalance, the droplets remain stable. In contrast, a strong charge imbalance creates local electric fields in the slit pore that disrupt the microemulsion droplets. Increasing salinity forms an electrical double layer that reduces the strength of the electric field, thus maintaining the stability of the droplets. This modeling-based study improves our understanding of how salinity variations affect microemulsions in charged clay-hosted nanopores. Significantly, this is the first study to combine and assess the sensitivity of microemulsions to both salinity and pore surface chemistry, opening new possibilities for the effective delivery of EOR agents to shales.