Probabilistic-Analysis of Deviated Wellbore Stability in Carbonate Formations of SE Iraq


Probability theory has been increasingly applied to address the uncertainty of geomechanical parameters for prediction of allowable drilling mud weight. When conducting wellbore stability analysis, input parameters are estimated from well log data and empirical correlations. In depleted reservoirs, and deviated infill drilling, the uncertainty of these parameters is magnified with stress contrasts. A field case in SE Iraq, which highlights this issue, was studied post drilling operations, to assess the impact of this uncertainty in wellbore stability analysis. Sensitivity analysis showed that the maximum horizontal stress magnitude had the greatest effect on estimated conditions at failure, followed in decreasing order by internal friction angle, rock cohesion, vertical stress, minimum horizontal stress and pore pressure. Applying P90 as a threshold on the estimated minimum mud weight proved to be conservative. Field data showed wells drilled without observed instability issues with mud weights even below the P50 value. For fracture mud weight, on the other hand, the field data showed the P50 threshold for maximum mud weight did not prevent fluid losses. This illustrates the issue of applying threshold of P50 for selection the mud weight window to ensure wellbore stability analysis.

Meeting Name

51st US Rock Mechanics / Geomechanics Symposium 2017 (2017: Jun. 25-28, San Francisco, CA)


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Deflected boreholes; Infill drilling; Oil field equipment; Rock mechanics; Sensitivity analysis; Stability; Well logging, Carbonate formations; Depleted reservoirs; Drilling operation; Empirical correlations; Geomechanical parameters; Internal friction angle; Probabilistic analysis; Wellbore stability analysis, Uncertainty analysis

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Document Type

Article - Conference proceedings

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© 2017 American Rock Mechanics Association (ARMA), All rights reserved.

Publication Date

01 Jun 2017

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