Modified Nanopolystyrene as a Plugging Agent for Oil-Based Drilling Fluids Applied in Shale Formation

Most nanoplugging agents have high surface free energy and tend to aggregate in oil-based drilling fluids (OBMs), failing to plug the nanopores in the formation. This paper introduces the synthesis and application of a surface-modified nanoscale polystyrene (NS) as a plugging agent for OBMs. With oleic acid acting as the surface modifier, the surface free energy of the NS can be effectively reduced, which is a crucial step in enhancing its dispersion stability in mineral oil. Besides, due to the nanoscale size and low surface free energy, NS adsorbing onto the rock surface could improve the contact angle of the mineral oil on the rock surface, reduce the imbibition amount of oil into the rock, and maintain the mechanical stability of the rock. The structure of NS was characterized by infrared spectroscopy and transmission electron microscopy. Thermal gravimetric analysis demonstrated that NS could resist up to 252 °C. The particle size distribution test showed that NS could maintain the nanoscale dispersion in mineral oil. NS could also increase the contact angle of mineral oil on the rock surface from 12 to 61° and reduce the oil spontaneous imbibition amount by 52.7%. After the addition of 2% NS to the base mud, the permeability plugging apparatus filtration loss was reduced by 67% compared with that of the base mud without NS, and in the pressure transmission test, the downstream pressure was still kept constant with no noticeable change after 60 h, showing an excellent plugging effect. Besides, after the rocks were immersed in the base mud and base mud containing 2% NS, the uniaxial compressive strength of the rock declined by 25.41 and 10.76%, respectively, indicating that NS could effectively alleviate the mechanical stability decrease of the rock.