Wellbore stability in a depleted reservoir by finite element analysis of coupled thermo-poro-elastic units in an oilfield, SW Iran

Sustaining wellbore stability in depleted reservoirs is a crucial concern. With production from hydrocarbon reservoirs, the reservoir's pore pressure is reduced over time, and the reservoir is depleted since field development is one of the main purposes for oil companies. Heavy mud weight in depleted reservoir caused fracture due to reduced fracture gradient, and low mud weight caused blow out in high-pressure zone or well collapse due to shale beds that required high mud weight to prevent collapse. Considering geomechanics and coupled equilibrium equation, continuity equation, Hook’s law, compatibility equation, Darcy’s law, and thermal relation, the Thermo-poro-elastic equation was derived in this research. A finite element method has been designed to execute the fully coupled thermo-poro-elastic non-linear models. The finite element model was validated by analogizing it to the available analytical solutions for the thermo-poro-elastic wellbore troubles in shale. The non-linear thermal-poro-elasticity finite element model was used to analyze wellbore stability in a depleted limestone reservoir during drilling. The numerical results showed that a decrease drilling fluid’s temperature (cooling) causes to increase in the potential for tensile failure and reduces the potential of shear failure. Due to the depletion reservoir, the potential of tensile failure increased than shear failure, so heating the drilling fluid could cause wellbore stability in the depleted reservoir. Furthermore, based on the numerical results, it may be supposed that the drilling fluid’s temperature is one of the essential elements in the wellbore stability analysis in depleted reservoirs.