Improving the shale stability with nano-silica grafted with hyperbranched polyethyleneimine in water-based drilling fluid

Shale hydration and dispersion leading to wellbore instability is always a challenge in oil and gas drilling engineering. In this study, specifically designed nano-particles by grafting hyperbranched polyethyleneimine (Mw = 600) onto the surface of nano-silica particles were synthesized. The modified nano-particles were characterized with Fourier transform infrared spectrum (FT-IR), particles size distribution measurement, thermal gravimetric analysis (TGA), transmission electron microscope (TEM) and zeta potential measurement. The results indicated that after modification, the nano-particles became positively charged and showed better dispersion in water compared to pristine nano-silica particles. Shale cuttings hot-rolling dispersion test and linear swelling test as chemical inhibitive tests in combination with pressure transmission test and shale sample spontaneous imbibition test as physical plugging tests were used to evaluate the shale stabilizing performance of the modified nano-particles. The results showed that the modified nano-particles exhibited both effective chemical inhibition and physical plugging, whereas nano-silica only showed limited physical plugging. The interaction between modified nano-silica particles and shale demonstrated that the grafted hyperbranched polymer strongly adsorbed onto the clay surface via electrostatic interaction and hydrogen bonding, which resulted in effective inhibition of shale hydration and swelling. After adsorption, the modified nano-silica particles were trapped on the micropores of shale formation and prevented the fluid invasion. The dual functions of the modified nano-silica particles demonstrated a remarkable improvement of shale stability.