Transient thermoporoelastic model under local thermal non-equilibrium

Abstract

Wellbore stability is a key challenge for oil and gas industry since it adds a great deal of additional cost to the industry. Traditional wellbore stability models such as elastic and poroelastic models may not produce reliable stress field around a wellbore since they consider isothermal wellbore condition. During the drilling phase, the drilling fluid temperature is different from the formation temperature due to geothermal gradient and circulation of the fluid inside the wellbore. Therefore, the assumption of isothermal condition will not predict the correct wellbore stability condition, especially for high pressure and high temperature wells. Hence, consideration of a thermoporoelastic model in stress estimation is more appropriate. Although there are several thermoporoelastic models in the literature, they employ the assumption of local thermal equilibrium (LTE) in their modeling. However, thermal properties of phases in a porous medium are rarely similar. This dissimilarity evokes the validation of LTE assumption. The local thermal non-equilibrium assumption ignores additional pore and thermal stresses in the porous medium caused by temperature variations of the fluid and solid phases. Under pure heat conduction, LTE is valid if thermal boundary conditions in a medium is constant or thermal conductivities of phases are similar according to Vadasz (2005) 1. Hence, using a local thermal non-equilibrium (LTNE) approach is required to establish each phase temperature, and update thermoporoelastic model accordingly. In this paper, the effects of induced thermal stresses on wellbore stress resolution and LTNE on thermoporoelastic model around a wellbore during drilling phase are investigated in low and high permeable formations. The LTNE thermoporoelastic model developed herein is a transient model considering conductive and convective heat transfers.