Keywords: 
• nano silicon dioxide /
• drilling fluid /
• filtration /
• shale /
• wellbore stability

Abstract: The main reason causing clay shale formation wellbore instability is that the expansion and sloughing of clay shale after water absorption. The thought was proposed that silicon dioxide nano-particles was used to plug the nano-size pores and throats of shale to reduce the permeability of shale, resulting in decreasing and delaying water invasion into shale. On the basis of preliminary studies, the paper evaluated the improvement effect of three types of drilling mud with two brands of silica nano-particles at different temperature, by means of transmission electron microscopy analysis, regular performance test of drilling fluid and scanning electron microscope analysis. Results show that silica nano-particles could effectively reduce the filtration of Fresh Water Mud （FWM） and Bentonite Mud （BM）with temperature increase. It could reduce the loss of water by 56.25% at room temperature and 78% at 140 ℃. It has little effect on Low Solid Mud （LSM）. Finally, to the drilling mud with the weight concentration of 10% had better plugging capacity than that with the weight concentration of 5% because of forming more continuous and tight mud cake. Based on the experiment results, FWM and BM’s filtration properties could be improved using nano-particles with temperature increase（from room temperature to 160 ℃）.