Drilling instability is always one of the major challenges to drilling engineers. The temperature differential and pressure differential affect the borehole instability by altering the stress concentration at near wellbore region through poro-elastic and thermal elastic stresses. The temperature gradient and pressure gradient between the drilling fluid and formation change not only from conduction and convection, but also from interacting to each other. In low mobility formation such as shale, the thermo-induced pore pressure is important while in high mobility formation, the pressure effect on the temperature is not negligible.
Furthermore, the imbalance in chemical potentials between the formation pore fluid and wellbore drilling fluid will cause the solvent and solutes to diffuse and transport. This will alter the fluid pressure due to chemical osmosis pressure. In this paper, a fully coupled borehole stability model is proposed including thermal, pore, chemical effect. In the model, not only pressure, heat conduction and convection are included, but also the interaction of pressure and heat are incorporated. The model is solved by superimposing method and finite difference method which otherwise is impossible to be obtained from analytical solution.