Compatibility and efficiency of hydrophilic/hydrophobic nano silica as rheological modifiers and fluid loss reducers in water-based drilling fluids

Nano-silica (nano-SiO2), as the most widely used nanometer-sized additive, exhibits various functions in water-based drilling fluids (WBDFs), such as rheological adjustment, filtration control, and shale inhibition. However, the exertion of these functions greatly depends on their type, concentration, and other characteristics. In this study, typical hydrophilic nano-SiO2 (phi-SiO2) and hydrophobic nano-SiO2 (pho-SiO2) have been utilized as the research samples. Their compatibility and efficiency in four different WBDFs containing sodium carboxymethyl cellulose (CMC), potassium humate (KHm), sulfonated phenolic resin (SMP), and white asphalt (NFA) were investigated by measuring their influence on the filtration and rheological properties. Results indicated that these nano-SiO2 displayed different effects in WBDFs and didn't always have a positive effect. At room temperature and 150 °C, both phi-SiO2 and pho-SiO2 had a thickening and shear force-increasing effect. The viscosity and yield point of four WBDFs improved with increasing the concentration of these two nano-SiO2, whereas the enhanced degree of pho-SiO2 was stronger than that of phi-SiO2. Meanwhile, phi-SiO2 hasn't decreased the fluid loss of WBDFs while pho-SiO2 could be applied as a fluid loss reducer and performed good compatibility with CMC and KHm, even at 150 °C. However, the filtration control performance of pho-SiO2 was also dependent on its concentration and the additive type in WBDFs. The analysis based on microstructure demonstrated that phi-SiO2 were easy to agglomerate and much sediment was formed after the filtration measurements, leading to more permeation channels and an increase in fluid loss volume. The improved viscosity, enhanced shear force, stable colloidal structure as well and the formation of a hydrophobic barrier after adding pho-SiO2 contributed to reducing the fluid loss volume of WBDFs. It was suggested that the use of nano-SiO2 should be determined according to the characteristics of nanoparticles, types of used additives, and other formation conditions.