Novel Slightly Crosslinked Hydrophobic Zwitterionic Polymer for Modifying Rheology and Filtration Loss of Clay-Free Water-Based Drilling Fluids

Summary In this study, a novel, slightly crosslinked hydrophobic zwitterionic polymer (RMJ-1) was synthesized to modify the rheology and filtration loss (FL) of clay-free water-based drilling fluids (CFWBDFs) for use under high-temperature, high-salinity conditions. The temperature resistance of RMJ-1 was up to 180°C, and the salt (Na+, Ca2+) tolerance was up to saturated. RMJ-1 exhibited a contracted linear shape in fresh water, and it was capable of forming a “pea-pod”-shaped network structure through covalent bonding, electrostatic force, and hydrophobic association. Due to the combined effects of antipolyelectrolyte and hydrophobic association, RMJ-1 was able to stretch in a saturated-salt environment and formed a denser “honeycomb”-shaped network. The network structure formed by RMJ-1 imparted desirable shear-thinning, viscoelastic, and thixotropic properties to CFWBDFs, effectively reducing FL. Especially at higher salt concentrations, RMJ-1 salt solutions exhibited higher viscosity, higher yield point (YP), higher modulus, stronger thixotropy, greater shear resistance, and lower FL. The CFWBDFs exhibited appropriate rheological properties and FL within the density range of 1.20–1.60 g/cm³, both before and after aging at 180°C. RMJ-1 surpassed the reference-grade polymer (RGP) at 180°C and under saturated-salt conditions, and it is expected to be widely applied in oil fields in the future. This research introduces novel concepts for developing high-performance additives in CFWBDFs.