The ion-selective membrane behavior of native shales

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.