Abstract
Wellbore instability is always inevitable in shale formation due to hydration, swelling, and dispersion of clay, especially when using water-based drilling fluids (WBDFs). To mitigate the wellbore instability of shale formation and avoid correlative detriments such as formation damage, various nano-silica particles (nano-SiO(2)) were employed to plug the nano-sized pores, inhibit water invasion into shale, and prevent shale swelling. Firstly, the influence of various nano-SiO(2) on rheological and filtration properties of a set WBDF was evaluated. Then, the linear swelling test, shale recovery test, zeta potential test, imbibition test, contact angle measurement, scanning electron microscopy (SEM) observation, and computed tomography (CT) analysis were conducted to assess the characteristic of nano-SiO(2). Experimental results showed that solid phase nano-SiO(2) could dramatically increase the viscosity and yield point while liquid phase nano-SiO(2) only caused small fluctuations on these parameters. Besides, hydrophobic nano-SiO(2) displayed better filtration performance than hydrophilic nano-SiO(2). On the whole, the hydrophobic nano-SiO(2) dispersion, called as nano-2, showed the best performance in WBDFs. Furthermore, nano-2 exhibited better inhibition than hydrophilic nano-SiO(2), KCl, and polyether amine (PA). Mechanism analysis demonstrated that nano-2 could improve the hydrophobic degree of shale surface and prevent water from contacting with the shale. Meanwhile, nano-2 plugged the pores and throats in the shale. As a consequence, water in the drilling fluid was prevented from invading the shale, and the shale was inhibited. Nano-2 could form a thin barrier in the surface of shale, and mitigate the damage degree of shale cores after perforation operation. As a result, nano-2 displayed great potential to stabilize shale and protect formation in WBDFs.