Abstract

In this study, fully coupled poro-thermo-elastic geomechanics modeling of wellbore stability was conducted on high pressure and high temperature (HPHT) wells in Hai Thach field offshore Vietnam. The drilling environment is extremely challenging with formation pore pressure gradient of more than 17.0 ppg and formation temperature above 170°C near TD. Post-mortem analyses of drilled wells were carried out using an analytical poro-thermo-elastic wellbore stability simulator, taking into account the coupled time-dependent evolution of pore pressure, temperature, and stresses around the wellbore. The analyses revealed very narrow time-dependent safe mud weight windows that became narrower as time progressed. The HPHT environment created very challenging drilling situations. During drilling and hole cleaning, the flowing mud cools off rock formations, inducing tensile thermal stresses, and therefore reducing effective compressive stresses around the wellbore. As a consequence, the fracturing pressure is reduced when the ECD is higher due to circulation, increasing the chances of mud losses. On the other hand, during static mud condition the formations retain their very high temperature and compressive stresses, which require higher mud weight to prevent wellbore collapse while the mud pressure only equals the static mud weight, increasing the chances of shear failure. This study highlights the importance of accurate prediction of safe mud weight window before drilling as well as timely updates to the mud weight window during drilling. Furthermore, guidelines of good mud weight management practices to drill the HPHT zones with very narrow mud weight windows are also given. For example, when the safe mud weight window is too narrow and shrinks with time, a higher static mud weight might be required during openhole logging to combat borehole collapse. On the other hand, during hole cleaning with increased mud circulation rate and increased ECD, a lower static mud weight might be required to mitigate mud losses. Moreover, with the time-dependent safe mud window available, the drilling and logging program can be optimized to minimize exposure time and mitigate the damaging thermal effects on the mechanical stability of the drilled borehole.

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