Analysis of Pore Pressure and Stresses around a Borehole: A Local Thermal non-Equilibrium Approach

The conductive-convective thermo-poroelastic theory has been successfully applied to the problems of geotechnical engineering, in particular wellbore stability in the area of drilling industry. The assumption of instantaneous local thermal equilibrium has been employed to derive the governing equations for such problems. This assumption, however, falls short in describing the heat transfer in high permeable rocks. The aim of this study is to replace the assumption of instantaneous local thermal equilibrium with the real assumption of local thermal non-equilibrium to solve the conductive-convective thermo-poroelasticity. To do this, a finite element based model of conductive-convective thermo-poroelasticity is developed which takes into account the later assumption to capture the real change in rock’s temperature and resulted stresses. From the results of this study it was revealed that there is a significant difference in heat transfer between the classical theory (instantaneous local thermal equilibrium) and LTNE theory (local thermal non-equilibrium). The effective stresses are remarkably affected by this difference in heat transfer. Consequently there is an increase in effective tangential stress around the wellbore whereas a decrease in effective radial stress when LTNE is considered. The results also showed that the change in effective tangential stress is more pronounced than effective radial stress.

Keywords:

Local thermal non-equilibrium , Borehole stability , Instantaneous local thermal equilibrium , Finite element method , Thermo-poroelasticity