Researching the Borehole Instability of Upper Variegated Mudstone Strata and Optimizing Drilling Fluid in Xihu Sag

The drilling complication of trip block or backreaming difficulty is occurred frequently when the variegated mudstone stratas of Longjing Formation & Huagang Formation are drilled at Φ311.15mm interval in Xihu Sag of the East China Sea Basin. The lithological fabric characteristics, mechanical properties, physical chemistry are researched by X-ray diffraction, scan with electric mirror, Mercury intrusion porosimetry, compressive strength test. It is indicated that the variegated mudstone is an extremely instability characteristic. Experimental study about sealing wellbore capacity, wellbore stability capability, wettability and lubricity of oil-based drilling fluid on site, the main controlling factors are discovered that the drilling fluid is insufficient performances of glueing and sealing and wettability. Targeting formation characteristics and insufficient properties of drilling fluid, optimization countermeasures determined for the drilling fluid are strengthening the sealing and wall fixing properties to improve the bearing capacity of the formation, and optimizing emulsifiers to improve the high-temperature stability and wettability. Optimized oil-based drilling fluid system can effectively seal micro and nano pore cracks in mudstone, improve wellbore stability and the lubricity of mud cakes, and reduce the risk of trip block. The good application results have been achieved on stability of wellbore, smooth tripping and casing running, improvement 96%～353% tripping efficiency compared to offset wells. As a result, the problem of borehole wall instability in the upper variegated mudstone formation is solved effectively.