A model that combines chemical effects with mechanical effects and provides a quantitative tool for evaluating wellbore stability is presented. In the past, wellbore stability models have introduced chemical effects by adding an osmotic potential modified by a membrane efficiency to the pressure acting at the wellbore wall [Fonseca, C.F., 2000. Chemical–mechanical modeling of wellbore instability in shales. Proceeding of ETCE 2000 and OMAE 2000 Joint Conference: Energy for the New Millenium, Feb. 14–17, 2000, New Orleans, LA.]. In this paper, an entirely different approach is adopted. The fluxes of water and ions into and out of the shale are accounted for. The pressure profiles obtained using our model differ significantly from the error function decline in pressure that is predicted by earlier models. As a consequence of this near wellbore pore pressure profile, wellbore failure can now occur inside the shale not just at the wellbore wall (as predicted by earlier models). The onset of instability now depends not only on the activity of the water but also on the properties of the solutes.