Preparation and Properties of Low‐Viscosifying Polymer Microsphere/Linear Polymer Composite Fluid Loss Additive

To address the trade‐off between filtration control and viscosity increase in linear polymer‐based fluid loss additives, a composite additive (MLS) combining polymer microspheres (PMS) and linear P(AM‐SAS) was prepared. At MLS dosages below 0.6 wt%, higher P(AM‐SAS) content improved filtration control, whereas at 0.6 wt% or above, the blending ratio effect diminished. When the P(AM‐SAS) fraction did not exceed that of PMS, MLS showed a weaker viscosifying effect than P(AM‐SAS) alone; when it exceeded that of PMS, viscosity increased but remained lower. At a 1:1 mass ratio and 0.8 wt% dosage, MLS performed best, with a filtration loss of 4.5 mL and an apparent viscosity of 13.0 mPa s. MLS also showed better salt resistance than P(AM‐SAS) alone, reducing filtration loss by 86.22% under 20 wt% NaCl and 8 wt% bentonite; however, under such high salinity, its viscosifying effect became pronounced, indicating that the low‐viscosity advantage of MLS is salinity‐dependent. After aging at 180°C, filtration loss increased from 4.5 to 7.5 mL and apparent viscosity from 13.0 to 15.0 mPa s, showing lower temperature sensitivity than P(AM‐SAS). Mechanistically, PMS inhibits excessive growth of the P(AM‐SAS) network to reduce viscosity while improving the filter cake through physical plugging.