Temperature-sensitive polymer based nano-SiO2 composite multi-component synergistic improvement of shale stability in water-based drilling fluids

Shale instability caused by the development of nano-micron-scale fractures, pores, and bedding is always a challenge for drilling complex formations. In this work, two additives (Temperature-sensitive polymer based nano-SiO2 composite (SNAS) and poly (NVP-TAAC-AMPS) (NTA)) were synthesized. The lower critical solution temperature (LCST) value of SNAS could be controlled by adjusting the monomer ratio. As temperature-sensitive nanocomposites, SNAS has the rheological control of drilling fluid. The effects of SNAS and NTA on shale stability were tested based on analytical approaches, including shale wettability, microporous membrane fluid loss, pressure transmission rate, shale specific surface area, shale pore volume, shale strength, linear swelling percentage and shale cuttings recovery. When the temperature is higher than the LCST value, due to the transition from hydrophilicity to hydrophobicity of SNAS, it not only changes the wetting angle of shale, but also enhances the plugging effect of SNAS. The composite system could form tight plugging layer and hydrophobic region compared with single SNAS, which was more conducive to improve plugging performance of shale. Finally, the synergistic improvement mechanism of shale stability combined with SNAS and NTA were further proposed.