Abstract:Nano-scale pores and fractures are well developed in shale. Because of the large particle size, conventional plugging agent is difficult to form effective plugging for shale. First, nano SiO2 particles were modified by silane coupling agent KH570 under ultrasound conditions. Furthermore, nano plugging agent with core-shell structure was synthesized through the radical graft copolymerization on the surface of the modified nano SiO2 particles by emulsion polymerization method. In the reaction, we choose styrene and methyl methacrylate as monomers and potassium persulfate as initiator. Nano plugging agent was characterized by infrared spectroscopy, transmission electron microscopy and thermogravimetry experiments. The plugging performance of nano plugging agent was studied by the pressure transmission tests on shale and the mechanism was discussed. The results showed that this nano plugging agent can effectively plug nano-scale pores and fractures of shale, reduce pressure transmission, filtrate invasion and shale permeability, thus enhancing wellbore stability of fissured shale. A high performance water-based drilling fluid system with good key performaces such as polugging, inhibition, lubrication an so on for shale horizontal wells was developed with the nano-plugging agent as the key component. During the application of this system in the third section of X Well in weiyuan block, the wellbore remains stable, the friction is low, and there is no drag pressure display in the directional section.