The behavior of surfactant microemulsion in clay-hosted nanopores

Abstract

The utilization of microemulsions to boost well productivity is gaining traction, particularly in unconventional shale wells during fracturing. However, the subsequent behavior and stability of microemulsions remain less clear, especially in clay-hosted nanopores where clay minerals exhibit unbalanced surface charges. This study employs the molecular dynamics (MD) approach to characterize the behavior and stability of microemulsion droplets through clay-hosted nanopores with different surface chemistries and salinities in shales, under typical reservoir pressures and temperatures. Our findings reveal that in the bulk, microemulsion droplets remain stable across all salinities. Yet, depending on the surface chemistry (charge distribution) in clays, the stability of microemulsion droplets becomes contingent on salinity. When clay surfaces are charge-balanced or have a moderate imbalance, microemulsion droplets remain stable. However, with a strong charge imbalance, induced local electric fields in the slit pore affect the fluids, leading to the essential role of both the electric field and clay surface in breaking the microemulsion droplet. Increasing salinity results in the formation of an electrical double layer, diminishing the strength of the electric field, and preserving the stability of microemulsion droplets. This modeling-based work enhances our comprehension of salinity variations' impact on microemulsions in charged clay hosted nanopores. Notably, this is the inaugural study combining and assessing microemulsion sensitivity to salinity and pore surface chemistry, thereby broadening possibilities for the effective delivery of EOR agents to shales.