EXPERIMENTAL STUDY ON THE INFLUENCE OF DRILLING FLUID PLUGGING PROPERTY ON THE SHALE WELLBORE STABILITY

China's shale is widely distributed, in which 70% of the strata are shale formations, and hard and brittle shale is more than 65%. Due to the structural properties of the hard brittle shale, the downhole complexes such as falling blocks, stuck drilling and wellbore instability are easily encountered during the drilling process, which seriously affects the progress of the oil and gas development process. Based on the mineral composition and microstructure of the shale, the shale hydration and the expansion of the shale under hydration are studied. It is concluded that the micro-cracks in the shale are extensively developed and expanded under hydration. It is the internal cause of rock mass destruction. Based on this, an evaluation method for the plugging effect of drilling fluid blocking agent on micro-cracks of hard and brittle mud shale is designed. Investigate the effect of plugging agent, addition amount and pressure difference on the cracking effect of micro-cracks, and propose the plugging mechanism of plugging agent in micro-cracks. The experimental results show that the combination of 4% CaCO3+2% EFD-2+4% NFA-25 in the drilling fluid system can greatly improve the plugging capacity of the system, effectively block the micro-cracks developed in the formation, and at the same time make the system A certain pressure bearing capacity. This has obvious effects on the improvement of the stability of the borehole wall, and provides new ideas and guidance for the design of the drilling fluid system.