Understanding the interaction of drilling fluid with shale formation is critical to properly describe pore pressure propagation which directly affects the wellbore stability. In the case of reactive shale formations, it has been shown that clay minerals within shale matrix have a tendency to adsorb water from drilling fluid. Using a concentrated drilling fluid, available models which are based on the ideal solution assumption fail to properly address the true shale-drilling fluid interaction. It is shown that the moisture content of shale is correlated with water activity using the multilayer adsorption theory. It is found that the GAB model fairly well describes the adsorption process for the selected shale types. The adsorption parameter signifies the characteristic of specific shale to interact with aqueous fluids. The constitutive equations are generalized to consider the case of non-ideal solutions. The coupled transport equations are solved using a finite difference method and numerical computations are conducted to predict the stability of the wellbore. Having developed a transient model which predicts the instantaneous moisture content around the wellbore, it is possible to update the compressive strength of shale as a function of its moisture content using the empirical correlations already proposed in the literature. The proposed model predicts that due to the moisture transport, the range of safe mud weight reduces substantially. The results of this investigation assist in drilling fluid design and address wellbore stability issues in troublesome shale formations.