Study on Collapse Instability Mechanism of Shale Formations in Bohai Oilfield

There are lots of complex problems due to borehole collapse instability in Bohai oilfield, which include mainly mud ball, fall-blocks returning, stuck pipe, pump suffocation and hole enlargement and so on. The complex problems affect drilling speed and increase drilling spending. Therefore, it is necessary to research wellbore stability from rock mechanics and drilling fluid chemistry, and clarify the instability mechanism of borehole collapse. The dominate mechanism was made clear through analysis of drilling features, clay mineral content and the physical-chemical properties of shales and the relationship between collapse pressure and drill-ing time in some oilfields of Bohai Sea region. In the upper member of Minghuazhen Fm, the main mechanism is that mudstone with high content of montmorillonite is easy to collapse due to hydration. From the lower member of Dongying formation to Shahejie formations, the hard brittle shales with cracks are easy to fall blocks due to weak plane and drilling fluid seepage. For the different instability mechanism, the corresponding countermeasures of drilling technology were put forward.