Aggregation of silica nanoparticles and its impact on particle mobility under high-salinity conditions

Abstract

Abstract CO 2 foams that are stabilized with nanoparticles are being considered to improve volumetric sweep efficiency for CO 2 enhanced oil recovery (EOR) processes. To better understand the nanoparticle retention/transport under high salinity conditions often associated with typical hydrocarbon reservoirs, the aggregation of the surface-treated silica nanoparticles was studied as a function of salinity and nanoparticle concentration. At pH 8.5, an enhancement in aggregation kinetics occurred with increasing calcium and particle concentration. However, when the pH was reduced to ca. 3, close to the CO 2 saturated condition, the aggregation was insignificant regardless of the salinity and the particle concentration over 10 days. In the transport test of silica nanoparticles in the sandpack column, a negligible amount of nanoparticles was retained with non-aggregated nanoparticles and for aggregates less than 235 nm regardless of flow rate. However, the retention of larger aggregates (>ca. 1000 nm) was enhanced as Darcy velocity decreased. Modeling based on a modified version of clean-bed filtration theory was able to explain experimental breakthrough curves only for the non-aggregated and slightly aggregated particles, suggesting that further considerations are essential to understand the physical effect of aggregation on the transport of larger aggregates. These results suggest a possible breakdown of the aggregates due to the hydrodynamic forces, hence retrieving the mobility of silica nanoparticles in the porous media. These observations demonstrate that silica nanoparticle aggregation can be prevented by increasing flow rate and lowering pH. The findings of this study would provide an insight on the behavior of nanoparticle in the reservoirs and a guideline for the application of the nanoparticles in enhanced oil recovery operations.