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 gradi
ents imposed on the formation. Solute concentration profiles and hydraulic
pressure propagation are calculated as a function of time and the flux of w
ater and ions are obtained for various drilling conditions. The model provi
des a method to quantify interactions between drilling fluid and shales and
constitutes a useful tool for formulating drilling fluids. It assists in t
he 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. (C) 2000 Elsevier Science B.V. All rights
reserved.