Polyethyleneimine/nano-silica core–shell materials with different degrees of branching for improving shale hydration inhibition and plugging in water-based drilling fluids

Abstract

Clay mineral hydration dispersion and pressure transfer are the main factors leading to well wall instability during shale development. In this study, the surface of nano-SiO₂ was modified with silane coupling agent KH-550, polyethyleneimine PEI_30 000 and PEI₃₀₀, and three kinds of end-amidated nano-SiO₂ core–shell structural materials with different degrees of branching were successfully prepared. The results show that –NH₂ is truly present on the surface of three modified SiO₂ nanostructures. Additionally, the modified nanosilica nanoparticles show better dispersion in water and enhanced thermal stability compared with the unmodified sample. Linear swelling and drum recycling were used to investigate the inhibitory effect of sodium bentonite on water sensitivity, with BPEI@SiO₂ exhibiting the strongest inhibition against clay hydration expansion. At a 2.0 wt% concentration, it reduced the 16-hour expansion rate of sodium bentonite by 30.77%. Pressure transmission and microporous filtration membrane blocking demonstrated that the amine-terminated nano-SiO₂ exhibits a smaller particle size than the other samples, enabling effective blocking of pore channels within the rock core. The zeta potential and the clay-layer spacing test were used to thoroughly analyze the mechanism of action of the end-amidated nano-SiO₂. The results showed that all three types of modified end-amidated nano-SiO₂ exhibited effective chemical inhibition and physical sealing, as evidenced by a pronounced increase in the zeta potential of bentonite suspensions from −32.6 mV to −18.95 mV and a significant reduction in clay interlayer spacing after treatment with BPEI@SiO₂. The interactions between modified silica nanoparticles and shale showed different mechanisms of action for the modified nano-SiO₂ with different chain-length shells. KH550@SiO₂ neutralises the negative charge on the clay surface by protonating –NH₃⁺. BPEI@SiO₂ inserts into the bentonite crystal layer and reduces its crystal layer spacing to inhibit its hydration and swelling. In contrast, HPEI@SiO₂ features a larger and more distinctive hyperbranched polymer shell, which extends outward from the nanoparticle surface and enables simultaneous electrostatic adsorption onto the external surfaces of multiple bentonite grains.