Theoretical Analysis of the Vertical Stability of a Floating and Sinking Drilled Wellbore Using Vertical Elastic Supports

This study addresses the calculation of vertical stability for shaft walls during floating and sinking processes in deep vertical shaft drilling in Western China. A mechanical model for the elastic support of the drilling shaft wall was developed by analyzing the forces during the transition from floating to sinking, and incorporating the cement filling behind the wall. This model was validated against empirical data. The analysis examined how shaft wall stability is impacted by parameters such as the elastic modulus of vertical support, borehole diameter, and water column height. Key findings include (1) the proposed elastic support model, which incorporates the viscoelastic properties of the cement slurry post setting, accurately reflecting the interaction between the wellbore and the surrounding rock mass; (2) the critical depth of the borehole wall initially increases and then decreases, correlating with cement slurry setting time, peaking about 18 h post initial setting, and stabilizing after 24 h as the support becomes a fixed support; and (3) a significant positive correlation exists between borehole diameter and critical depth, which increases and then decreases as the height of the ballast water rises. These results provide insights essential for assessing the stability of the floating sinking technique in drilling operations.