Analysis of the stress state around wellbores in saturated porothermoelastic rock

Abstract

In the oil and gas exploitation or geo-thermal energy exploitation industries, wellbores can be drilled at great depths where the formation would be hot and saturated. In such case, a large temperature difference between the rock mass and drilling fluid can occur and cannot be ignored. During drilling the wellbores, thermic, hydraulic and mechanical phenomena appear simultaneously and interact with each other within the rock. This study presents the analysis of stress state around the wellbore located in saturated hot rock based on the fully thermo-hydro-mechanical behavior model of the rock mass by the finite element method. Two scenarios involving thermal conditions at the well wall are taken into account, i.e. the drilling fluid temperature is lower or higher than the formation temperature so-called the cases of “cooling” and “heating”, respectively. In this study, the influence of some thermic, hydraulic and initial stress field parameters of the rock mass on the stress state around the wellbore was also clarified. The obtained results showed that, in the cooling case, the well wall may be destabilized by fracture failure while in the heating case this would be collapse failure. The maximum points of tangential and axial stresses appear at the same locations for the two scenarios. In addition, the thermal expansion coefficient, the initial shear stress in the rock mass greatly affect the stress state around the wellbore whilst the permeability of the formation does not influence on the stresses on the well wall but only on the stresses inside the surrounding formation.