Impact of surface roughness, surface charge, and temperature on sandstone wettability alteration by nanoparticles

Abstract

The wettability of rocks affects the balance between capillary and viscous forces during multiphase flow through porous media, which in turn determines the fluid displacement process governing the recovery of oil from subsurface formations. In this work, the mechanism of wettability reversal of aged synthetic sandstones by metal oxide nanoparticles (SiO2 and Al2O3) was investigated with particular focus on the impact of surface roughness, zeta potential, and temperature. The synthetic surfaces were prepared from powders of Berea sandstone with known grain size ranges and their average roughness and roughness ratio were obtained from the 3D surface reconstruction of their microscope images. Each surface was subsequently aged in Permian crude oil to alter its wettability. For surfaces with larger grain sizes and lower surface roughness ratios, the lower capillary pressure allowed stronger oil/surface interactions, leading to enhanced oil-wetness. The wettability alteration effects of nanoparticles were then examined through real-time top view imaging and dynamic front view contact angle experiments. The negatively charged SiO2 nanoparticles rapidly reversed the sandstone wettability, indicating their potential applicability as wettability alteration agents. By contrast, the positively charged Al2O3 counterpart caused no wettability reversal. The mechanism of wettability alteration was further studied by microscale interaction analyses and nanoscale transmission electron microscopy. Because nanoparticles were only a few nanometers large, the microscale roughness had a negligible effect on the wettability reversal. Instead, the combined effect of van der Waals dispersion forces and surface-charge-induced electrostatic forces were recognized as the two key factors affecting the wettability of sandstone particles. Such interactions may be curbed at elevated temperatures due to a decrease in the zeta potential and colloidal stability of the particles.