P(AM-co-AA)/PSt nano-microspheres with releasing polymer chains function for filtration loss reduction in oil-based drilling fluids

High filtration loss of the oil-based drilling fluids restricts their application in reservoirs production at high temperature and high pressure. In this work, modified nano-microsphere P(AM-co-AA)/PSt filtration reducers are designed to reduce filtration loss by excellent dispersion in oil medium, excellent water-absorption swelling and auxiliary emulsification and prepared by inverse phase emulsion polymerization using acrylamide, acrylic acid and styrene, followed by modification with stearyl trimethyl ammonium chloride. The modified nano-microspheres absorb water to reach an equilibrium swelling rate of 950%. Additionally, the water contact angle increases to 70.3° and the temperature resistance reaches up to 240   o C. The permeability of the filter cake can be decreased through the utilization of swelling microspheres to plug tiny cracks, as well as by intertwining organo-montmorillonite sheets with polymer chains detaching from weakly crosslinked microspheres. Especially, due to their excellent dispersion in the oil medium, the microspheres gather at the oil-water interface, thereby stabilizing the emulsification state of oil-based drilling fluids. These two points are conducive to reducing filtration loss. After thermal aging for 16   h at 180   o C, oil-based drilling fluid by adding 4% microspheres exhibits a low filtration loss volume of 7.8   mL at 180   o C and 3.5   MPa. These hydrophilic nano-microspheres with a partially alkylated structure have a considerable practical application prospect as filtration loss reducers of oil-based drilling fluids.