ABSTRACT
Wellbore collapse is a classic rock mechanics problem, it is usually determined by the stress distribution and the failure criterion. The most stable well path is always the inclined or horizontal borehole for the traditional method, whatever the in-situ stress is. However, it’s unconformable with the real drilling practices, the drilling practices show that the stability of vertical borehole is better than the others. Therefore, a risk-controlled model was involved to overcome the above problem and improve the predicted veracity of collapse pressure. The analytical solutions of collapse pressure were derived for vertical and deviated borehole using the stress distribution, the Mohr-Coulomb criterion and the risk-controlled model. The Equivalent Mud Weight of Collapse Pressure (EMWCP) was predicted for three types of in-situ stress state (NF, SS and RF), and the most stable well path was also investigated and contrasted for these three types of in-situ stress state. Finally, the risk-controlled model had been applied to several field cases, from the Sichuan Basin, Tarim Basin and Ordos Basin. The results show that the EMWCP predicted by the risk-controlled model is always lower than the traditional model. It also illustrated that allowing a presupposed failure to lower the required mud weight could prevent the happening of collapse accidents. The optimal well path, which has the low EMWCP, usually involves the vertical borehole under the risk-controlled conditions. The risk-controlled model in each case is consistent with the field experience. We conclude that this risk-controlled model could predict the EMWCP more accurately, and provide more reasonable and beneficial well path for drilling engineers.
