Experiments were conducted to evaluate the membrane character of native sha
le samples by measuring the electrochemical potential across the shale memb
rane. Results suggest that the composition of the interstitial pore fluid i
n the shale plays a determining role on the establishment of the electroche
mical potential and that, in some cases, the behavior of the shales is clos
e to the expected behavior of a perfect cation-selective membrane. Shales w
ith intermediate electrochemical potentials appear to be more sensitive to
water-based fluids than shales that are closer to perfect membranes thigh e
lectrochemical potential). A model to simulate the transport of water and i
ons through shales was developed. Hydraulic pressure, concentration, and el
ectric potential gradients are the driving forces for the flow of water and
solute between two solutions separated by the shale. The reflection coeffi
cient and the modified diffusion potential are calculated from the model an
d are used to characterize the membrane behavior of shales. A sensitivity s
tudy on the various model parameters was conducted, Results show that the m
embrane efficiency of the shale is strongly affected by the concentration o
f the interstitial fluid, the separation distance between the platelets and
the type of ions in the membrane. The higher the ion concentration and the
interplatelet spacing, the lower the efficiency. Results obtained using th
e model produce reflection coefficients that are consistent with experiment
al data, The model provides excellent insight into the physical mechanisms
responsible for the membrane behavior of shales and a way to conduct sensit
ivity studies on the important model parameters. The membrane efficiencies
obtained from the model may be used in wellbore stability simulators to acc
ount for chemical interactions between shales and water-based drilling flui
ds. (C) 2000 Elsevier Science B.V. All rights reserved.