Mechanism of hydrophobic nano-SiO_2 material in inhibiting clay swelling

Drag reduction technology of nano-materials is an effective new technique to solve the problem of under-injection in high pressure in oilfields,however,its drag reduction mechanism is not well understood.Hydrophobic nano-particles inhibiting clay swelling is one of the drag reduction mechanisms of nano-materials.A new testing method of the anti-swelling effect,suitable for strongly hydrophobic nano-powder,was applied to test the anti-swelling rate of bentonite with various hydrophobic nano-SiO2 materials.Adsorption features of nano-SiO2 particles on core surface were studied by means of adsorption experiments and scanning electron microscope(SEM).The result of anti-swelling experiments shows that most of the nano-SiO2 materials have certain anti-swelling properties,which however show a big difference in the anti-swelling effect.The anti-swelling effect is greatly influenced both by the surface modifier of nano-SiO2 particles and by the proportion of nano-SiO2 particles to bentonite,therefore,an appropriate modifier or an optimum proportion can achieve a good anti-swelling effect.The anti-swelling rate of nano-HNP2 is about 46.9% when the proportion is 1∶25,while the anti-swelling rate of nano-HNP4 becomes about-25.8% when the proportion is 1∶25.The SEM photos show that hydrophobic nano-SiO2 particles can form nano-particle layers on cores through adsorption and make core surface highly hydrophobic,here the water contact angle is 138.3°.The investigation indicates that through adsorption on clay mineral surface,hydrophobic nano-SiO2 can form isolation layers with high hydrophobicity,which separate clay minerals from water to produce an anti-swelling effect.This anti-swelling mechanism is somewhat different from that of commonly used clay stabilizers.