Drilling fluid solutions to well instability in deep layers of Linhe block of the Bayan Hetao Basin

Deep drilling in the Linhe block of the Bayan Hetao Basin suffers from severe well instability, which restrains the high-efficient development of the oilfield. The clay mineral composition and micro structure of the core samples of the troublesome layers were investigated via the X-ray diffraction analysis and scanning electron microscopy, respectively, which reveals that the well instability is mainly attributed to the high water sensitivity and well-developed micro fractures of the formations. Micro fractures are expanded by the water wedge of the invading mud filtrate and formation hydration raises the collapse pressure. These jointly lead to borehole wall caving. The key to solving well instability is to improve the inhibition and plugging performance of the drilling fluid system. Given this, the micro-nano plugging agents were introduced to improve the plugging performance of the drilling fluid system and the polyamine was added for synergy with KCl to enhance the inhibition performance. The laboratory experiments demonstrate that the optimized drilling fluid system presents a reduction of the shale swelling by 62% and rolling recovery of cuttings by over 90%. The field applications in three wells result in the average borehole enlargement ratio below 20% for the Palaeogene formations. The optimized drilling fluid system effectively improves deep well stability in the Linhe block of the Bayan Hetao Basin and technically supports the high-efficient development of the oilfield.