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A thermo-poroelastic model which is fully coupled to conductive and convective transport processes is employed to analyze wellbore stability and borehole breakout. Using the two-dimensional finite element method, stress distribution around an open borehole subjected to non-isothermal, non-hydrostatic loading is investigated. Four strength criteria: Mohr–Coulomb, Drucker–Prager, modified Lade, and Mogi–Coulomb, are utilized to the stability of boreholes. Borehole breakout propagation is assumed to occur as successive spalling of thin layers of rock caused by stress concentration and redistribution around boreholes and breakouts. Stabilization of breakouts is examined by deploying Mogi–Coulomb failure criterion for different wellbore fluid pressures and temperatures. To evaluate the influence of various prominent factors on borehole breakouts, a parametric study is conducted. Results indicate that Mohr–Coulomb criterion offers the highest breakout depth, so it underestimates rock strength and requires the highest value of the least mud weight necessary for wellbore stability and Drucker–Prager provides the lowest estimation of the breakout depth. Borehole breakout stabilization is concluded to occur as consequences of reduction in effective tangential stress and increase in confinement effect of radial stress around breakout periphery. Moreover, confinement effect of borehole fluid appears to be more influential in borehole stability than pore pressure effect. Parametric analysis reveals that drilling a borehole with small radii, applying overbalanced fluid pressure condition, and maintaining drilling fluid temperature lower than rock temperature boost the creation of a favorable borehole stability condition. |
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