Over the years wellbore stability in shales has been achieved using oil base muds, however water base muds still plague the industry. Up until now it was not obvious what mechanisms were in operation during such shale/water base mud interactions. The research reported herein discusses the development of swelling pressures when water base muds are in contact with a troublesome North Sea shale. A mechanistic theory is presented that focuses on increases in total pore pressures that are reflected in swelling pressures. The results obtained showed that different mud filtrates produce swelling or shrinking pressures that are related to the type of salt present and its ionic concentration. Fluids tested included de-ionized water and solutions of CaCl2, Potassium Formate, Glycerol, Methyl Glucoside and NaCl/Methyl Glucoside. The range of swelling pressures measured were from +2800 psi (swelling) to -1400 psi (shrinkage). In addition to swelling pressures, the compressive strength of the shales was also measured under downhole insitu stress and conditions after the chemical interactions were complete. Strength reductions as high as 35% were measured for certain fluid systems.