Membrane Behavior of Shales and Ionic Solutions

Abstract

The transport of water and ions through shales was studied both experimentally and theoretically. Experiments were conducted on the diffusion of ions and water through shale at elevated stress levels similar to those encountered under normal drilling conditions. An experimental apparatus was designed to operate under confining pressures to 6,000 psi and to measure the diffusion rates of ions and water through shales using radioactive tracers under various levels of confining stress. Results show that the diffusion of both water and ions through shales decreases with increasing confining pressure; however, in all cases, ions are still free to move as a result of the "leaky" membrane character of shales. The membrane efficiency of shales was modeled using irreversible thermodynamics. The membrane system consisted of two electrolyte solutions separated by two charged parallel surfaces containing an electrolyte solution of known concentration. Results show that the flux of ions and water is determined mainly by the bulk concentration of the interstitial fluid, the separation distance between the platelets, and the type of solute. The membrane model was used in conjunction with a transient model of the wellbore to determine the pressure transmission and the solute concentration profiles within the shale formation as a function of time and to quantify the fluxes of water and salt through shales under different operational conditions. Results indicate that osmotic effects play a major role in determining the pressure propagation and the flux of water and ions within the formation. Experiments were carried out to evaluate the membrane character of downhole shale cores by measuring the electric potential difference between two electrolyte solutions of different concentrations separated by the shale membrane. Results suggest that the composition of the interstitial pore fluid in the shale plays a determining role on the establishment of the electric potential difference and that, in some cases, the behavior of the shales is close to the expected behavior of a perfect cation-selective membrane.