A transient flux model for fluid loss from the wellbore into shales during drilling has been developed. The model takes into account the coupled flow of water and ions due to hydraulic, osmotic, and electrical potential gradients imposed on the formation. Solute concentration profiles and hydraulic pressure propagation are calculated as a function of time and the flux of water and ions are obtained for various drilling conditions. The model provides a method to quantify interactions between drilling fluid and shales and constitutes a useful tool for formulating drilling fluids. It assists in the establishment of proper bottomhole hydraulic and osmotic drilling fluid pressures so as to minimize flow into or out of shales, which could result in wellbore instability problems.