Core-shell structured polystyrene microspheres for improving plugging performance of oil-based drilling fluids

To avoid damage to wellbore stability in shale formations from drilling fluid intrusion and pressure transmission, it is important to develop a plugging material that can effectively plug micropores and fractures in shale formations. In this paper, polystyrene microspheres (CPM) with core-shell structure were synthesized by seed emulsion polymerization, and characterized by infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), particle size analysis, TEM observation and contact angle test. The effects of CPM on emulsion stability and emulsion rheological performance were evaluated by electrical stability tests and rheological performance tests, respectively. The plugging performance was evaluated using filter materials with different pore sizes as filter media. The experimental results showed that CPM had good thermal stability with a thermal decomposition temperature up to 374.3 °C. CPM had a core-shell structure with a median value (D50) of 229 nm and a three-phase contact angle of 116.2°. The synergistic effect of CPM and organoclay can significantly improve the stability of emulsions and viscosity at low shear rates. For different pore sizes of filter media, CPM achieved effective plugging of micropores, reducing fluid loss volume to less than 6 mL for different pore sizes of micropore filter membranes and 29.6% for ceramic sand discs. In this paper, a preparation method of plugging material with good plugging performance was given, which provided technical support for the development of plugging agents for Oil-based drilling fluids (OBDFs).