Study and Application on Risk Stratification of Wellbore Collapse For the Longmaxi Formation Shale in Zhaotong City

The national shale gas demonstration block was in Zhaotong city, where the distribution of limestone was uneven. These complex downhole accidents, such as collapse failure and cavernous leakage, sticking of bit, overflow, gas invasion, and leakage, were frequent when high-density oil-based drilling fluid was adopted. The correlation between geological stratification and underground working conditions was found to be poor according to the statistics of complex downhole conditions of 26 wells. This paper first analyzes the non-correlation between geological stratification and downhole complex accidents. And it summarizes the factors such as anisotropy of reservoir geomechanical parameters, a difference of longitudinal horizontal stress, borehole trajectory and formation dip, lithology difference and weak surface structure. Taking safe drilling as the main objective, the risk stratification of wellbore collapse is proposed from the engineering perspective. Compared with traditional geological stratification, it considers the lithological difference and integrates the rock's weak surface structure, in-situ stress state, drilling parameters and other comprehensive indicators. Finally, the collapse risk assessment is carried out for the complex small layers of a typical well, the safe mud density window is optimized, and the well deviation angle and azimuth angle are optimized. It is suggested to drill according to the lower limit of the safe mud density window. In the high collapse risk formation, improving the plugging property of drilling fluid and reducing the wall permeability can largely avoid complex accidents. Exploring risk stratification of wellbore collapse can effectively guide drilling engineering operations, optimize engineering design schemes, and provide a scientific basis for safe drilling in long horizontal stages of deep shale reservoirs to prevent complex downhole accidents.