Nanoparticles screening criteria for wettability alteration in carbonate reservoirs

Wettability alteration has been recognized as a key mechanism for enhanced oil recovery in oil-wet carbonate reservoirs. Nanoparticles can change rock surfaces from oil-wet to water-wet. However, there are multiple kinds of nanoparticles and different reservoir conditions. It is necessary to establish a screen system to guide nanoparticles selection. Temperature and salinity are two important reservoir conditions that may have a big influence on the performance of nanoparticles. Besides, divalent ions and monovalent ions can have a different impact. In this paper, different kinds of commercial nanoparticles were used as wettability modifiers on carbonate rock. Metallic oxide nanoparticles, SiO₂, and titania nanoparticles were dissolved into brine solution under varying temperatures (50°C – 100°C) and salinity (2500 – 210,000ppm). Carbonate rock samples were aged by crude oil to make them oil-wet. After that, rock samples were immersed in brine and nanoparticle solutions to measure the contact angles under different temperature. Rock surfaces were then examined by scanning electron microscope (SEM) to see the adsorption and distribution of nanoparticles. Stability data shows that nanoparticles tend to be more stable under high temperature, low salinity conditions. Besides, monovalent ions have a smaller impact on nanoparticle stability than divalent ions. By monitoring contact angle with time, a quicker wettability alteration was observed with SiO₂ and CaCO₃ nanoparticles on carbonate rock under higher temperatures, indicating that temperature has a significant influence on the performance of nanoparticles. SEM images show that under low salinity conditions, nanoparticles tend to be more scattered. This paper showed that nanoparticles are very effective materials for carbonate wettability alteration under varying temperature and salinity. Besides, the relative performance of nanoparticles under different conditions and a screening guide according to carbonate reservoir conditions are provided.