Abstract
This study presents a new interface for wellbore stability analysis, which visualizes and quantifies the stress condition around a wellbore at shear and tensile failure. In the first part of this study, the Mohr–Coulomb, Mogi–Coulomb, modified Lade and Drucker–Prager shear failure criteria, and a tensile failure criterion, are applied to compare the differences in the critical wellbore pressure for three basin types with Andersonian stress states. Using traditional wellbore stability window plots, the Mohr–Coulomb criterion consistently gives the narrowest safe mud weight window, while the Drucker–Prager criterion yields the widest window. In the second part of this study, a new type of plot is introduced where the safe drilling window specifies the local magnitude and trajectories of the principal deviatoric stresses for the shear and tensile wellbore failure bounds, as determined by dimensionless variables, the Frac number (F) and the Bi-axial Stress scalar (χ), in combination with failure criteria. The influence of both stress and fracture cages increases with the magnitude of the F values, but reduces with depth. The extensional basin case is more prone to potential wellbore instability induced by circumferential fracture propagation, because fracture cages persists at greater depths than for the compressional and strike-slip basin cases.
Highlights
Reliable analysis of wellbore stability is one of the keys for a successful and safe drilling process, because if instability occurs it may result in pipe stuck by wellbore collapse or drilling fluid loss through induced tensile fractures, which require considerable time and effort to be solved
At the upper-bound of the safe wellbore pressure window, tensile failure is more likely to occur than shear failure for well depths shallower than 1868 and 2840 m, according to the Mohr–Coulomb and Mogi–Coulomb criteria, respectively
For the other shear failure criteria, tensile failure will be more likely to occur than shear failure for the entire 3048 m deep well
Summary
Reliable analysis of wellbore stability is one of the keys for a successful and safe drilling process, because if instability occurs it may result in pipe stuck by wellbore collapse or drilling fluid loss through induced tensile fractures, which require considerable time and effort to be solved. Knowing the stress trajectory pattern at the moment of failure is extremely useful as it provides additional conceptual insight and quantitative information relevant for real-time stability analysis during drilling and completion processes. The safe drilling window and the critical wellbore pressures at lower-bound and upper-bound shear failure are calculated for each Andersonian case using four different shear failure criteria: The Mohr–Coulomb, Mogi–Coulomb, modified Lade, and Drucker–Prager criterion. According to our knowledge such a systematic comparative study of shear and tensile failure criteria and the resulting critical wellbore pressures, with an analysis of the shifts in the safe drilling window under the three principal Andersonian stress regimes, has not been done before. The present study visualizes the principal deviatoric stress magnitudes and trajectories at the margins of the safe drilling envelope for a vertical wellbore under three different Andersonian tectonic stress regimes
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