This paper highlights and prioritizes various rock mechanical parameters, in-situ stresses and pore pressure conditions that affect wellbore instability with failure criteria frequently used in such analysis such as the Mohr-Coulomb, Drucker-Prager, modified Lade, and tensile failure. In addition, different far-field stress regimes including the normal fault, thrust fault, and strike-slip fault were considered in this study to evaluate the influence of such geological settings on parameter sensitivity for each of the failure criterion. The results showed that the magnitude of impact of the various parameters depends on both the failure criterion in use and the regional far-field stress state. In addition, for all shear failure criteria and stress regimes, the maximum horizontal principal stress, SH, and formation pore pressure, Ppore , were shown to have greater effects on collapse mud weight (CMW) compared to the minimum horizontal principal stress Sh. Moreover, while SH is the most sensitive parameter for Mohr-Coulomb criterion in all far-field stress states, Ppore were shown to have the largest effects on the modified Lade criterion regardless of regional stress regimes. The results also showed that among the various rock mechanical properties, the cohesion and friction angle yield greatest influence on CMW analysis. Lastly, the role of formation tensile strength on tensile failure analysis was shown to be subtle compared to in-situ stresses and formation pore pressure for all stress regimes.
The most frequently used failure criteria by petroleum engineers for borehole instability analysis and design of ultra-deep, high angle drilling are the Mohr-Coulomb (M-C), Drucker-Prager (D-P), modified Lade (ML), and tensile failure criteria [1-6]. Though we may argue the applicability of each criterion in terms of rock types and/or in-situ conditions [4, 5], we do not have consistent studies on the priority and importance of the various in-situ factors such as the far-field stress regimes, pore pressure and rock mechanical properties that affect the analysis outcomes for those failure criteria. Existing studies on the sensitivity of various parameters involved in wellbore stability analysis [7-9] have some limitations when compared with this study. In this paper, various rock mechanical parameters and in-situ conditions that directly affect wellbore stability analysis with failure criteria frequently used in our technical community were prioritized using Monte Carlo simulation method. In addition, different states of regional far-field stress that are the normal fault, thrust fault, and strike slip fault as classified by Anderson [10] were considered to evaluate the impacts of geological settings on parameter sensitivity. This study will enhance future risk reduction for borehole stability analysis by identifying parameters most critical for a particular failure criterion and geological stress regime.
In addition to the commonly used Monte Carlo simulation, several error estimation methods have been employed for sensitivity and uncertainty analysis. For example, the fuzzy set theory, the total differential method, the first order second moment (FOSM), and the first order reliability method (FORM) have been used in existing sensitivity studies [7-9].
