The role of thermal stress in near wellbore instabilities and its implications: a case study from the HTHP well, offshore East India

Abstract

Drilling through a narrow pore pressure-fracture pressure window, especially under HTHP (high temperature-high pressure) conditions, is one of the most challenging and costly operations in the oil and gas industry. In this case study, drilling results and thermal stress modelling emphasized the role of thermal stress in several borehole failures encountered at shallow depths and severe formation breakdown incidents at deeper depths. The thermal stress from cooling effects delayed the shear failure conditions by reducing the maximum tangential stress of the formation around the wellbore, but induced formation breakdown as the minimum tangential stress exceeded the tensile strength of the formation. The result from this study shows both the rock stiffness (Young’s Modulus, Poisson Ratio) and differential temperatures (ΔT) have a critical control on thermal stress-related instabilities. Thermal stress-based wellbore stability modelling shows that temperature fluctuations in the dynamic drilling environment can induce a stress change at the rate of 300-500 psi/10°C. Our study results show that the range of thermal stress magnitudes in stiff rocks can vary between 300-4000 psi for a ΔT range of 20-60°C.