Synthesis of the Oil-Based Nanoblocker Poly(MMA–BMA–BA–St) and the Study of the Blocking Mechanism

[Image: see text] Well wall instability is one of the problems that seriously affect the efficiency of oil and gas drilling and extraction, and the economic losses caused by accidents due to well wall instability amount to billions of dollars every year. Aiming at the fact that well wall stabilization is the current technical difficulty of drilling shale gas horizontal wells with oil-based drilling fluids, the oil-based nanoplugging agent poly(MMA–BMA–BA–St) was synthesized by the Michael addition reaction with compounds such as styrene, methyl methacrylate, and butyl methacrylate as raw materials. The structure and characteristics of the oil-based nanoblocker poly(MMA–BMA–BA–St) were characterized by infrared spectroscopy, particle size analysis, and thermal weight loss analysis. The particle size distribution of poly(MMA–BMA–BA–St) is 80.56–206.61 nm, with an average particle size of 137.10 nm, and it can resist the high temperature of 372 °C. The effects of poly(MMA–BMA–BA–St) on the performance parameters of oil-based drilling fluids were investigated by rheological experiments, electrical stability tests, and HTHP filtration loss experiments. The results show that when poly(MMA–BMA–BA–St) is added at 0.5 wt %, it has less influence on the rheological parameters of drilling fluids, the breaking emulsion pressure remains basically unchanged, the stability of the drilling fluid is better, the dynamic-plastic ratio of the drilling fluid is higher than 0.27, the filtration loss is the lowest, and it shows good rock-carrying properties. The results of mud cake experiments and artificial lithology experiments show that poly(MMA–BMA–BA–St) has the best sealing effect, with a mud cake permeability of 1.12 × 10(–4) mD and a sealing rate of 30.00% when added at 0.5 wt %; the artificial core permeability was 4.0 × 10(–4) mD, and the sealing rate was 91.23%. Poly(MMA–BMA–BA–St) showed good sealing performance. The oil-based nanoplugging agent poly(MMA–BMA–BA–St) has good dispersion in oil-based drilling fluids and can enter the nanopore joints to form a dense plugging layer under the action of formation pressure to prevent the intrusion of drilling fluids, thus reducing the impact of drilling fluids on the formation, maintaining the stability of the well wall and reducing downhole complications.