Determination of rock properties by low-frequency AC electrokinetics

In brine-saturated rock the existence of a mobile space charge at the fluid /solid interface leads to the electrokinetic phenomena of electroosmotic pr essure and streaming potential. The coupling coefficients of these electrok inetic effects, when combined with the conductivity of the brine-saturated rock, determine the brine permeability of rock exactly. A sensitive low-fre quency AC technique has been used to measure electrokinetic response of a c ollection of eight rock and four glass bead samples saturated with NaCl bri ne as a function of salt concentration (fluid conductivity of 0.5 to 6.38 S /m); the response of four of the original 12 samples has also been measured as a function of temperature from 0 degrees to 50 degrees C. All data veri fy the predicted permeability relationship. Additionally, the frequency res ponse of the electroosmotic pressure signal alone can also be used to deter mine the permeability, given knowledge of experimental parameters. The conc entration and temperature dependence of electroosmosis and streaming potent ial is found to mostly conform to the predictions of a simple model based o n the Helmholtz-Smoluchowski equation, the Stern model of the electrochemic al double layer, and an elementary theory of ionic conduction.