LAUSR

To tackle the viscosity surge in oil‐based drilling fluids (OBDFs) due to heavy oil invasion, a novel tetrapolymer viscosity reducer (PVR) was synthesized from diisobutylene, isobutylene, isoprene, and maleic anhydride. FT‐IR, NMR, and TGA confirmed its structure and high thermal stability (decomposition at 398°C). Tests showed PVR reduced plastic viscosity of contaminated fluids by 21 mPa s and yield point by over 12.5 Pa, achieved by disrupting asphaltene π‐π stacking and hydrogen bonds, reducing aggregate size (D50 from 31.345 to 6.265 μm). At 2% dosage, it maintained emulsion stability (breakdown voltage 486 V, above 400 V threshold). After 72‐h 180°C aging, PVR performed better: plastic viscosity showed little change (72–71 mPa s), yield point dropped sharply (16–7 Pa), and density difference was 0.01 g/cm3 (vs. 0.04 g/cm3 for blank), indicating superior high‐temperature long‐term stability. This offers a reliable solution for optimizing deep‐well high‐temperature drilling fluids.