Wellbore stability in shales is very much influenced by the type of drilling fluid. The pore pressure generated during drilling and its diffusion into the shale are major contributors to both short and long term wellbore stability. The membrane effect and the subsequent osmotic pressure developed around the well, play an important role in the force balance that may destabilize the rock. The present paper focuses on assessing shale behavior when exposed to some real water- based drilling fluids. The physical-chemical aspects of the shale-fluid interaction are evaluated, in particular, in relation to osmotic pressure and membrane effects developments. Rock-fluid interaction tests, using shale samples collected from Campos Basin, Brazil, and the Norwegian North Sea, confirmed the efficiency of the designed equipment in carrying out experiments with highly viscous, waterbased fluids. The experiments showed that the fluid pressure transmission behavior, for the shales and fluids used, does not differentiate between the viscous, saline water-based fluid and pure saline water solution. The development of osmotic pressure suggests that the shale reflection coefficient in more influenced by its pore size distribution than by its mineralogy.