A significant portion of non productive time can be attributed to wellbore instability during drilling, in particular for high pressure and high temperature (HPHT) wells. Drilling introduces changes to stresses, formation pore pressure and temperature in the vicinity of a wellbore. These changes are fully coupled and interact with one another. Such coupling effects are often ignored in the conventional approach to wellbore stability analysis. A thermo-poroelastic 2D model recently developed at CSIRO considers the full coupling effect on wellbore stability.
This paper presents a study that uses this newly developed model to analyze the wellbore stability for a HPHT well drilled in the South China Sea. The study well encountered HPHT condition in the 8.5" hole section with a formation pore pressure gradient up to 1.95 SG and temperature up to 155 °C. The objective of the study was to gain a better understanding of the transient pore pressure and temperature effects on wellbore stability shortly after drilling, and to evaluate if conventional uncoupled wellbore stability models are adequate for HPHT wells. The main conclusions from this study are a) coupling effect caused by changes in stresses, formation pore pressure and temperature cannot be ignored for HPHT wells in low permeability formations, such as shales; and b) conventional uncoupled wellbore stability models are inadequate in deriving a reliable mud weight window for HPHT wells in early times for low permeability formations drilled underbalanced, but are adequate for formations with permeability in the orders of microdarcies or higher.