Micro-nano polymer microspheres as a plugging agent in oil-based drilling fluid

The wellbore instability of shale formations with micro fractures was mainly ascribed to drilling fluid invasion and pressure transmission during drilling using oil-based drilling fluids (OBDF). Nano-plugging agents or micro-plugging agents at present only match a narrow size range of nanopores or microcracks. Plugging agents with good suitability to multidimensional fractures used in OBDF are lacking. This paper synthesized micro-nano polymer microsphere dispersion (PMD) with a solid content of 30 % through inverse emulsion polymerization. PMs had good wettability for water in the oil environment. The filtration of the emulsion with 4 % PMD dropped the filtration through 0.1–3 µm filters to within 14 mL while the emulsion with 1 % CaCO3 only plugged the filter with a pore size of 3 µm. Hydrophilic PMs with a wide size distribution have excellent suitability with multi-scale pores or cracks. PMs could reduce the filtration of OBDF after aging at 120 and 150 ℃ without significantly influencing their rheology and electrical stability. Based on Darcy's law, the HTHP mud cake permeability calculated decreased by half with the addition of PMD at 120 and 150 ℃. Moreover, the addition of PMD could decline the disc HTHP filtration by 100–50 % when the disc diameter was 5–120 µm respectively. Compared with CaCO3, the PMD showed superior plugging performance and self-suitability when plugging 5–120 µm discs. Micro-nano PMs could interact with organic clays through hydrogen bonds to form thinner and denser mud cakes. They could also form hydrogen bonds with the formation surface. PMs could enter, aggregate and arrange tightly at multi-scale pores or cracks owing to their wide size distribution. In addition, they formed a dense plugging layer and prevent the filtration from invading the formation. The PMD with good compatibility, suitability and plugging performance has a promising application prospect in shale oil and gas exploration.