Experimental studies of electrokinetic conversions in fluid-saturated borehole models

Experimental and theoretical studies show that there are electromagnetic (E M) fields generated by seismic waves with two kinds of conversion mechanism s in a fluid-saturated, porous medium. Within a homogeneous formation, the seismic wave generates a seismoelectric field that exists only in the area disturbed by the seismic wave and whose apparent velocity is that of the se ismic wave. At an interface between differing formation properties, the gen erated seismoelectric wave is a propagating EM wave that can be detected ev erywhere. An electrode, used as a receiver on the ground surface, can detec t the propagating EM wave generated at an interface, but cannot detect the seismoelectric field generated in a homogeneous formation. When the electro de is in a borehole and close to a porous formation, it can detect both the EM waves and the seismoelectric field. Ln this paper, electrokinetic measu rements are performed with borehole models made of natural rocks or artific ial materials. Experimental results show that the Stoneley wave and other a coustic modes, excited by a monopole source in the borehole models, generat e seismoelectric fields in fluid-saturated formations. The electric compone nts of the seismoelectric fields can be detected by an electrode in the bor ehole or on the borehole wall. The amplitude and frequency of the seismoele ctric fields are related not only to the seismic wave, but also to formatio n properties such as permeability, conductivity, etc. Comparison between th e waveforms of the seismoelectric signals and acoustic logging waves sugges ts that seismoelectric well logging may explore the different properties of the formation. Electroseismic measurements are also performed with these b orehole models. The electric pulse through the electrode in the borehole or on the borehole wall induces Stoneley waves in fluid-saturated models that can be received by a monopole transducer in the same borehole. These measu rement methods (seismoelectric logging or electroseismic logging) might dir ectly apply to well logging to investigate formation properties related to the pore fluid flow.