In response to the current problem that micron-scale
plugging agents
cannot effectively plug shale nanopores and fractures, tetrameric
poly(VS-St-BMA-BA) nanoparticles were synthesized by the Michael addition
reaction using sodium vinyl sulfonate, styrene, butyl methacrylate,
and butyl acrylate as raw materials. The nanoparticles poly(VS-St-BMA-BA)
were characterized by infrared spectroscopy, particle size analysis,
and thermogravimetric analysis. The particle size distribution of
poly(VS-St-BMA-BA) at room temperature ranged from 62.17 to 96.44
nm, with a median particle size of 75.8 nm, and could withstand high
temperature of 359.5 °C. The effects of poly(VS-St-BMA-BA) on
the rheological parameters of drilling fluid and the effects of different
temperatures on the median particle size were investigated by the
drilling fluid performance testing methods and high-temperature stability
testing methods. The results showed that the apparent viscosity, plastic
viscosity, yield point, and high temperature and high pressure water
loss of drilling fluid gradually decreased with the increase in poly(VS-St-BMA-BA)
dosage; when the addition of poly(VS-St-BMA-BA) was 2.0%, the overall
performance of drilling fluid was better, the filtration loss was
4.4 mL, and the drilling fluid had good water loss wall building performance.
The median particle size of poly(VS-St-BMA-BA) was 132.60 nm (the
particle size at room temperature was 75.8 nm) after standing for
16 h at 180 °C, indicating that poly(VS-St-BMA-BA) has good high-temperature
stability and dispersion stability. The plugging performance and plugging
mechanism of poly(VS-St-BMA-BA) under extreme conditions (high temperature)
were investigated by the plugging performance test method and pressure
transfer method. The results showed that the plugging rate of artificial
mud cake and artificial core reached 48.18 and 88.75%, respectively,
when the amount of poly(VS-St-BMA-BA) was added at 2.0%. In the pressure-transfer
experiments, poly(VS-St-BMA)-BA) could invade the 2 mm position of
the nanopore fracture on the core surface and form a sealing barrier
layer to prevent the further invasion of liquid. Combined with the
pressure-transfer experiment, it shows that poly(VS-St-BMA-BA) can
enter the nanopore and fracture at a certain distance under the action
of formation pressure and keep accumulating to form a tight blockage,
which can effectively prevent the filtrate from entering the nanopore
fracture of the shale formation. Poly(VS-St-BMA-BA) is expected to
be used as a promising nano-plugging agent in water-based drilling
fluids.