Streaming potential in porous media 1. Theory of the zeta potential

Electrokinetic phenomena are responsible for several electrical properties of fluid-saturated porous materials. Geophysical applications of these phen omena could include the use of streaming potentials for mapping subsurface fluid flow, the study of hydrothermal activity of geothermal areas, and in the context of earthquake prediction and volcanic activity forecasting, for example. The key parameter of electrokinetic phenomena is the zeta potenti al, which represents roughly the electrical potential at the mineral/water interface. We consider silica-dominated porous materials filled with a bina ry symmetric 1:1 electrolyte such as NaCl. When in contact with this electr olyte, the silica/water interface gets an excess of charge through chemical reactions. Starting with these chemical reactions, we derive analytical eq uations for the zeta potential End the specific surface conductance. These equations can be used to predict the variations of these parameters with th e pore fluid salinity, temperature, and pH (within a pH range of 6-8). The input parameters to these equations fall into two categories: (1) mineral/f luid interaction geochemistry (including mineral surface site density and s urface equilibrium constants of mineral/fluid reactions), and (2) pore flui d pH, salinity, and temperature. The zeta potential is shown to increase wi th increasing temperature and pH and to decrease with increasing salinity. The proposed model is in agreement with available experimental data. The ap plication of this model to electric potentials generated in porous media by fluid flow is explored in the companion paper.