Wellbore instability problems are induced by changes in near wellbore stresses and pore pressure. Thermo-mechanical stresses coupled with the osmotic contributions are used to compute conditions under which the wellbore becomes unstable. Changes in pore pressure due to osmotic effects are a function of the water activity in the mud and the membrane efficiency of the shale. Shale strength alterations with hydration time are considered while determining the critical mud weights. The model presented in this paper is the first complete attempt at including mechanical, chemical and thermal contributions into a general three-dimensional wellbore stability model.  the results presented from teh model clearly indicate the conditions under which all three contributions play an important role in wellbore stability.