Polyampholyte-grafted silica nanoparticles for shale plugging under high-temperature and extreme-salinity conditions

Nano-/submicron-scale plugging is critical for wellbore stability during shale gas exploitation but typically fails under high-temperature and extreme-salinity conditions due to colloidal instability. Herein, polyampholyte polymers (ZP) were grafted from the surface of silicon dioxide (SiO2) nanoparticles through surface-triggered free radical polymerization to generate nanoplugging additives (SiO2-g-ZP). SiO2-g-ZP exhibited excellent colloidal dispersion stability with a nano/submicron size distribution in 35 w/v% sodium chloride (NaCl) and 11 w/v% calcium chloride (CaCl2) brines at 160 °C for a long time and over a wide pH range, and efficiently plugged 220 nm and ∼20 μm pores and 5 μm fractures at 160 °C and 3.5–5.5 MPa. Three critical aspects of observation and analysis obtained through microstructure analysis and molecular dynamics (MD) simulations helped elucidate the mechanisms that control the SiO2-g-ZP's performances: an increase in SiO2-g-ZP's hydration capability with salinity due to the antipolyelectrolyte behaviours of ZP coatings with zwitterionic nature, an efficient mitigation of Na-bent agglomeration even at harsh conditons due to the preferential adsorption of SiO2-g-ZP rather than Na+, Ca2+ and Cl− on Na-bent, and the presence of a nanosilica core and rigid groups and massive hydratable groups. The first two aspects caused a perfect matching of particles in muds to pores and fractures in formation and the third aspect helped enhance the SiO2-g-ZP's thermal resistance and residence in the pores and fractures. This work provides a novel approach that can be broadly generalized to address the thermal- and salt-tolerance challenges of colloidal systems used in the exploration and development of deep and complex shale formations.