Determination Of Electrokinetic Coupling Coefficient And Permeability In A Porous Rock Sample

Electrokinetic effect has been suggested as one of the mechanisms that causeearthquake electromagnetic signals in recent years, and it was based on this effectthat an potential logging method called seismoelectric logging was proposed tomeasure formation permeability near a borehole. But few investigation has beendone on the electrokinetic conversion phenomena of porous rocks. In this work Iconduct laboratory experiment on electrokinetic coupling coefficient of porous rockcore samples, and try to find the dependence of this coefficient on the permeability,a key paparamter of petroleum reserviors in evaluation of its recovery profit.The electrokinetic phenomena comes from the electric double layer in the fluidsaturatedmicro pore. Beside the interface of fluid-solid, the negative ions areabsorbed by solid base. So the dominate ions in the pore is positive. After establishthe electric double layer, fluid flow and current will coupling each other, and itcomes to be electrokinetic effect.The electrokinetic signal is very weak. With theaim to verify whether the signal that obtained from experiments is caused by theelectrokinetic phenomena, it is necessary to distinguish the background noise andelectrokinetic signal. In this work, the graphite which porosity is zero and do nothave electrokinetic phenomena is chosen to replace the rock sample. With thegraphite medium, we measure the background noise in the experiment and find thatthe background noise is much lower than electrokinetic signal.The experimental study consists of three measurements, the streaming potentalmeasurement, the electroosmotic measurement and the conductivity measurement. Itis an urgent task to understand the machanism behind earthquake electromagneticsignals for the setting up of an earthquake warning system. In this article, someimprovements in the measurement program and operating steps are proposed aftersome introductions of previous measurement program. In the electroosmosisexperiment, the operating frequency of the whole system is reduced to thenano-level by the change of input waveform signal in the reference terminal oflock-in amplifier, the use of a series of rank-4 low pass filter and the opening ofsynchronous filter. The signals of electroosmosis which is lower than 200mD can beobtained based on the above improvements. The measurement capability isimproved further by recording the pressure of electroosmotic signal with photomethod. By this improvement, the signal which can not be identified previous areidentified successfully. In the conductivity experiment, the interference among a fewlock-in amplifier in the circuit is eliminated by the reasonable connection ways, andthe impedance of rock can be measured by two lock-in amplifier. In this article, the electrokinetic coupling coefficient and liquid-permeability of three rock samples arecalculated. It is found that they have a good repeatability.In addition, a model of current source is proposed against the problem ofequivalent circuit of rock in the streaming potential experiment. It is verified withboth theory and experiment method simultaneously.