Novel Use of a Superhydrophobic Nanosilica Performing Wettability Alteration and Plugging in Water-Based Drilling Fluids for Wellbore Strengthening

: Severe shale wellbore instability accidents in water-based drilling fluids encourage research and development of new inhibitors and nanoplugging agents. In this work, we synthesized a multifunctional superhydrophobic nanosilica (SA), which could form a hydrophobic film on the shale surface, imparting inhibiting and plugging properties. The composition and surface morphology of SA were characterized, and the results showed that SA was spherical at around 30 nm. The inhibiting performances were evaluated by hotrolling recovery, linear swelling, and water spontaneous imbibition tests. The plugging performance of SA was appraised by the filtration test with nanofilter paper and pressure transmission tests. The strengthening of shale core mechanical stability properties of SA was also appraised by the uniaxial compressive tests. The results showed that SA performed excellently at inhibiting shale hydration. The recovery rate of shale cuttings at 180 degrees C reached 64.8%. In addition, due to the low surface energy, SA particles could disperse stably at the nanoscale and perform well at plugging the nanopores and fractures, effectively reducing the filtration loss by 75% compared with the base fluids and limiting the pressure transmission through the shale. The compressive tests showed that after the shale cores were immersed in deionized (DI) water, 2% SA, and 3% aqueous suspensions at 150 degrees C for 48 h, the compressive strength of the shale cores decreased by 26.69, 3.88, and -2.22%, respectively, proving that SA could effectively alleviate the decrease of shale strength and even increase it at higher concentrations. The mechanism research showed that SA could decrease the surface energy of the shale from 81.86 to 1.15 mN/m and change its wettability to superhydrophobic, significantly inhibiting surface hydration. By changing the wettability, SA reversed the direction of the capillary force to offset the osmotic pressure, thereby decreasing the amount of water entering the shale through the nanopores or fractures and effectively inhibiting osmotic hydration. In addition, SA could accumulate on the surface of the shale and form a dense superhydrophobic membrane, providing proper plugging properties.