Impact of anionic surfactant on stability, viscoelastic moduli, and oil recovery of silica nanofluid in saline environment

Controlling nanofluid stability in saline environment has been a long-standing challenge as salt ions screen-out the surface charge of the nanoparticles (NPs), rendering them unstable. This study thus reports the use of an anionic surfactant (sodium dodecylsulfate, SDS), as co-stabilizer, that not only controls the effect of salt, but also dramatically improves nanofluid stability by controlling the agglomeration rate. Silica NPs of 0.5 wt%, SDS with concentration >= critical micelles concentration (CMC), and polymer polyacrylamide (PAM) of 1000 ppm were used while the concentration of NaCl was explored up to 5 wt%. The dispersion stability of nanofluid without salt (NaCl) was found to be similar to 4 weeks which further increased to similar to 7 weeks with increasing SDS concentration. With increasing SDS (>CMC), a reduction in NP agglomeration was observed that delayed the NP settlement. The effect of salinity was also investigated for flow properties of nanofluids and, an unstable rheological response (transition from viscoelastic to viscous) was observed with increasing NaCl concentration. However, SDS (>CMC) made nanofluid rheologically stable up to saline environment of 3 wt%, an optimum salinity was identified. SDS role on nanofluid stabilization in saline environment is also generalized for enhanced oil recovery (EOR) applications in a porous media enriched with varying salinity. With increasing salinity, the cumulative oil recovery from sand-pack reduced and reached to minimum level of 48% original oil in place (OOIP) at 3 wt% NaCl which was 58% OOIP without salinity. Oil recovery re-established to 55% OOIP in presence of increasing SDS concentration, which is remarkable for nanofluid applicability in saline environment where conventional methods typically show challenges. Even in the presence of 4 wt% NaCl, the synergy between surfactant and nanofluid enabled an improvement in oil recovery and subdued surfactant adsorption in both unconsolidated and consolidated porous media, paving future way for field implementation.