An Objective Method for Wellbore Stability Analysis

: This paper proposes a new method for determining the quality of wellbore stability through an objective consideration of the cumulative effects of wellbore failure. The method combines the weighted functions of hole enlargement from a caliper measurement and calculated shear failure. The method is called the Wellbore Quality Index (WQI). Unlike other methods, WQI assesses wellbore stability without being constrained by the failure mechanism (cause), regardless of whether it is mechanical, hydraulic, chemical, or operational. This is because using the hole-enlargement measurement is non-discriminative. Consequently, WQI is effective even when limited data is available (e.g., only a routine one-arm caliper log is available and the hole-enlargement mechanism is unknown). When high-quality data is available (e.g., oriented caliper measurements, image logs, pore pressure and fracture gradient measurements, and rock core testing results), WQI becomes a valuable and objective calibration tool for assessing wellbore instability. It demonstrates a significant improvement over conventional mechanism-biased wellbore stability analysis methods used to match field observations. Furthermore, when using dimensionless analysis with filtering, WQI can differentiate the effect of each failure mechanism and determine the underlying physics of the wellbore failure. This is because the scaling of the wellbore failure tends to be mechanism-dependent and the associated failure dimensions are often clustered in particular regions.