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A novel terpolymer of acrylamide (AM), 4-vinylpyridine (VP), and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) was synthesized through free radical polymerization and characterized by proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, elemental analysis, and static light scattering measurement. The monomer ratio was shown to be the predominant factor to the fluid-loss control performance of this polymer in drilling fluids. The terpolymer under optimal polymerization conditions (PAAV) was prepared, and the dipolymer of AM and AMPS (PAA) was synthesized as a contrast sample. In an American Petroleum Institute (API) filtration test of bentonite-based mud with 10% CaCl2contamination after a 16 h aging at 150 °C, mud with 1% PAAV maintained an API filtrate volume (FLAPI) of 4.8 mL, whereas mud with 1% PAA reached a FLAPIof 96.0 mL. The fluid-loss control mechanism of PAAV was investigated through adsorption experiments, ζ potential measurements, and particle size distribution analysis. The results illustrate that the introduction of VP units into a polymer molecule greatly improves the temperature resistance performance of the polymer and enhances the interaction between the polymer and bentonite, which improves colloidal properties of bentonite particles, and these make PAAV a pronounced fluid-loss control agent in deep gypsum drilling operations.

With the increase of drilling depth, the density of drilling fluid required also increases gradually. Increasing the density of drilling fluid by weighting material usually worsens the performance of drilling fluid. Based on the fractal principle and mathematical fractal, a fractal grading model suitable for the grading calculation of drilling fluid weighting materials was established, and the theoretical proportion of barite with different particle sizes was given. Through drilling fluid performance evaluation, filter cake analysis, oscillatory rheology measurement and circulation simulation, the influence of barite gradation on the rheology, filtration and sag stability of high-density drilling fluid was studied, and the influence of barite gradation on drilling fluid performance was discussed. The results showed that the fractal grading model is suitable for the optimization of weighting materials of high-density drilling fluids, and the graded barite can improve the comprehensive performance of weighting drilling fluids.