Preparation of the Tetrameric Poly(VS-St-BMA-BA) Nano-Plugging Agent and Its Plugging Mechanism in Water-Based Drilling Fluids

[Image: see text] 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.