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This study presents a shale/drilling fluid interaction Martin E. Chenevert Robert S. Schechter model developed to explain mechanisms. The model uses total pressure concepts and diffusivity equation to predict shale swelling pressures. The limitations of the model are the assumptions that the shale acts as a perfect membrane, and that confining pressures are carried by the fluid within the shale (no grain stresses exist). Suitable equipment and testing techniques were developed to evaluate the modeling of shale/drilling fluid interaction at elevated conditions of temperatures and pressures. The testing techniques emphasized the importance of shale handling procedure during coring, and preparation of shale test samples. The model and the laboratory techniques were used to determine the effects of wellbore pressure, confining stress, temperature, drilling fluid composition and shale types on shale swelling pressures. Results show that shale swelling rates decrease with increase in confining stress and cationic concentration of the drilling fluid, and increase with increase in wellbore pressure and temperature. In addition to the development of the model and the testing techniques, the adsorption isotherm phenomenon as related to shale/drilling fluid interaction was studied. The adsorption isotherms using the isopiestic method provide the basis for establishing the total aqueous chemical activity of a shale. The adsorption isotherm is the primary relationship used in this work to determine the aqueous chemical activity of the shale. THE CHIEF TECHNICAL CONTRIBUTIONS OF THIS STUDY ARE: 1. A model which takes into account the aqueous chemical activity of the shale and the drilling fluid has been developed to predict shale swelling pressures at elevated temperatures and pressures. 2. A simplified equipment and testing technique has been developed to study shale swelling rates at elevated temperatures and pressures on a timely basis. 3. The theory and testing techniques developed will provide ,, understanding and information on many factors which produce unstable wellbores drilled through shale formation. This information will allow for the optimization of drilling fluid design and help minimize total drilling costs.