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Geomechanics and Engineering
  Volume 24, Number 3, February10 2021 , pages 281-293

Quantitative risk assessment for wellbore stability analysis using different failure criteria
Alireza Noohnejad, Kaveh Ahangari and Kamran Goshtasbi

    Uncertainties in geomechanical input parameters which mainly related to inappropriate data acquisition and estimation due to lack of sufficient calibration information, have led wellbore instability not yet to be fully understood or addressed. This paper demonstrates a workflow of employing Quantitative Risk Assessment technique, considering these uncertainties in terms of rock properties, pore pressure and in-situ stresses to makes it possible to survey not just the likelihood of accomplishing a desired level of wellbore stability at a specific mud pressure, but also the influence of the uncertainty in each input parameter on the wellbore stability. This probabilistic methodology in conjunction with Monte Carlo numerical modeling techniques was applied to a case study of a well. The response surfaces analysis provides a measure of the effects of uncertainties in each input parameter on the predicted mud pressure from three widely used failure criteria, thereby provides a key measurement for data acquisition in the future wells to reduce the uncertainty. The results pointed out that the mud pressure is tremendously sensitive to UCS and SHmax which emphasize the significance of reliable determinations of these two parameters for safe drilling. On the other hand, the predicted safe mud window from Mogi-Coulomb is the widest while the Hoek–Brown is the narrowest and comparing the anticipated collapse failures from the failure criteria and breakouts observations from caliper data, indicates that Hoek-Brown overestimate the minimum mud weight to avoid breakouts while Mogi–Coulomb criterion give better forecast according to real observations.
Key Words
    geomechanical key parameters; wellbore instability; quantitative risk assessment; response surface; failure criteria; likelihood of success
Alireza Noohnejad and Kaveh Ahangari: Department of Mining Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

Kamran Goshtasbi: Department of Mining Engineering, Tarbiat Modares University, Tehran, Iran

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