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Salt-Responsive Zwitterionic Polymer Brush Based on Modified Silica Nanoparticles as a Fluid-Loss Additive in Water-Based Drilling Fluids

Jinsheng Sun,Xiaofeng Chang,Fan Zhang,Yingrui Bai,Kaihe Lv,Jintang Wang,Xinyu Zhou,Bo Wang

Wellbore instability and formation collapse are crucial issues in the process of well excavation in the oil industry under extreme salinity and high-temperature conditions. This study demonstrates that a salt-responsive zwitterionic polymer brush (NS-DAD) based on modified nanosilica as a fluid-loss additive utilizing the anti-polyelectrolyte effect in water-based drilling fluids (WDFs) to overcome the wellbore instability caused by the failure of polyelectrolytes at extreme salinity and high temperature. Additionally, a nonionic polymer brush (NS-D), an anionic polymer brush (NS-DA), and a cationic brush (NS-DD) were also prepared for comparison. Compared with NS-D, NS-DA, and NS-DD, NS-DAD exhibited the anti-polyelectrolyte phenomenon, in which the sodium chloride electrolyte shields the electrostatic interaction in the molecular chain of the polyzwitterion and the molecular structure changes from a collapsed sphere to a more open helix. Macroscopically, NS-DAD exhibited a higher viscosity than NS-D, NS-DA, and NS-DD in saturated salt-based mud (SSBM). A typical “star-net” structure was observed between NS-DAD and the bentonite layer. Energy-dispersive spectroscopy (EDS) analysis of filter cakes showed that NS-DAD could significantly reduce the content of chloride and sodium ions in the bentonite layer. Therefore, compared with NS-D/SSBM, NS-DA/SSBM, and NS-DD/SSBM, NS-DAD/SSBM had excellent rheological properties, thermal stability, less fluid-loss volume, and thinner filter cake under extreme salinity and high-temperature conditions. The fluid-loss additive can be used to reduce the fluid-loss volume of WDFs in harsh reservoir conditions of high temperature and high salinity.