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 hot-rolling 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 °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 °C for 48 h, the compressive strength of the shale cores
decreased by 26.69, 3.88, and â2.22%, 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.