Drilling problems associated with wellbore instability are aggravated when shale beddings are drilled, which constitute a high percentage of drilled rocks in the world. Shale in contact with water base mud (WBM), when submitted to the in situ stresses, could have a semipermeable membrane system behavior. This behavior and the osmotic pressure developed around the wellbore have a fundamental role in the force equilibrium that makes the formation unstable. On this document, a computer model based in the poroelastoplasticity with numerical formulation based on finite element code is applied to estimate the influence of rock-drilling fluid interaction on the stability of borehole drilled through shale. Modeling conditions explains how the model couples and deals with driving forces involved (hydraulic diffusion, chemical osmosis, chemical diffusion, advection). Unfavorable consequences about the pore-pressure and rock strength are showed with the time rock exposure. Wellbore instability is discussed according to pore-pressure, effective stress, plastic zone and radial displacement results obtained in this document.