We have studied the electrokinetic signals generated by acoustic sources centered in a borehole using a finite element model, and by making borehole and laboratory measurements. Electrokinetic signals are generated when acoustic sources move fluid in the pore space of a rock. These signals therefore contain information on the dynamic as well as the static properties of a reservoir. We have found that a reliable permeability log can be derived from electrokinetic measurements.
The finite element model combines Biot?s equations for acoustic propagation in a porous medium with a simplified form of Maxwell?s equations. In the current implementation the model computes the electrical and acoustic signals generated in and around a borehole from a source placed in the borehole, assuming 2-D geometry. The model handles the effect of mudcake as well as the presence of horizontal and radial layers. Of the various types of electrokinetic signal generated, the normalized electric field (the ratio of the electric field to the pressure of the Stoneley wave has the most significant dependence on permeability independent of porosity.
We have also constructed a simple logging tool with acoustic and electrical transducers, designed to give minimum noise or spurious electrical signals. The tool has been used to test the model results in three well-characterized water wells as well as the Callisto nuclear-tool calibration pits. Results to date show good repeatability and encouraging correlations with permeability from cores and tests. In one case we were able to invert the field data with the help of the model to obtain permeability.
Laboratory measurements have been focused on two large blocks of sandstone with widely differing permeability. After verifying that the observed electrokinetic signals originated in the sandstone, we measured electrokinetic and pressure signals which confirm experimentally that the normalized electric field is a function of permeability. The signals also corresponded qualitatively with those predicted by the model.