Wellbore Stability Analysis in Oil and Gas Drilling by Mechanical, Chemical and Thermal Coupling (Case Study in the South of Iran)

Inadequate wellbore stability has always been the most important issue during drilling and can cause many drilling complications. The most important tools for studying this phenomenon are the use of geomechanical modeling and failure criteria. Drilling fluid temperature and chemical interactions between drilling fluid and wellbore are also significant factors in wellbore stability. Decreasing the fluid temperature changes the concentration of induced stresses around the well, influencing the wellbore's safe mud window. This study considered a vertical well drilled in a field in southwest of Iran to determine the mud window and find out the optimal path under different conditions. The upper and lower bound of the mud window were calculated separately and simultaneously with three criteria, namely Mohr–Coulomb Failure, Hoek–Brown, and modified Lade. In the modified Lade failure criterion that gives more realistic results, the lower bound under the initial conditions is 7.81–10.51 ppg; by applying the thermal effect, it is 7.68–10.48 ppg, and with a fluid chemistry effect, it is 8.47–10.69 ppg. Given these two factors simultaneously, the lower bound of the mud window changed to 8.34–10.66 ppg. The optimal path in all failure criteria is horizontal drilling of well and maximum in horizontal stress direction. Furthermore, the upper bound of the mud window under mechanical conditions was 21.36–31.13 ppg, and with the effect of temperature was reduced to 21.09–30.84 ppg. Moreover, after applying the chemical effect, it increased to 21.29–31.05 ppg and converted to 21.02–30.76 ppg simultaneously.