Using an Advanced Geomechanics-Based Workflow to Prevent Wellbore Shear Failure When Drilling Across Depleted Formations

Abstract

Abstract Drilling laterally and in the direction of minimum stress assists in optimizing hydraulic fracture stimulation designs. However, it poses a huge risk of hole instability due to stress concentration while drilling across the minimum stress direction. Generally, when breakouts are encountered, the strategy is to increase the mud weight, but in scenarios where the mud weight window is very narrow, trying to avoid fracture initiation will potentially lead to shear failure or breakouts. This makes drilling highly challenging. One would need to have a deep understanding of the weak formations and plan a wellbore strengthening strategy to mitigate instability issues when drilling across these formations. This research used a semi-analytical model based on the Gauss-Chebyshev polynomial to quantify wellbore strengthening. The model provided a comprehensive analysis of the formation strengthening in different scenarios and predicted fracture width distribution, coupled with the fracture initiation pressure. Combined with the mud loss volume, this algorithm helped quantify the loss and provided a strategy for corrective action. We used the learnings from the model to design a wellbore strengthening material to prevent shear failure of the weak formations or the formation with high porosity. This effectively prevented the wellbore from collapsing. In this paper, we present the results from formations in the Middle East and Asia where this approach was used to predict the instability in the formation at the planned mud weight. The effect of the mud weight was quantified to predict the breakout so that the appropriate strategy was utilized for wellbore strengthening. This integrated approach provides a workflow for drilling through depleted zones, weak formations, and challenging wells by considering stress behavior. Additionally, the case studies presented in this paper provide key information on mitigation strategies for unstable or weak formations.