Seismoelectric wave propagation modeling in a borehole in water-saturated porous medium having an electrochemical interface

Water-saturated porous media often exhibit a seismoelectric effect due to the existence of an electrical double layer and a relative flow of pore fluid. Here we consider the seismoelectric waves in an open borehole surrounded by water-saturated porous formation which exhibits discontinuity of electrochemical properties at a cylindrical interface. We carefully analyze the seismoelectric interface response since these signals show sensitivity to contrasts in electrochemical properties across an interface. Both coupled and approximate methods are used to compute borehole seismoelectric fields. The simulation results show that the radiated electromagnetic wave from the electrochemical interface is generated due to the change of salinity in pore fluid in the porous formation. However, the elastic properties of the formation remain unchanged across such an electrochemical interface. As a result it is difficult to recognize such a change in electrochemical properties using only elastic waves. Therefore, the seismoelectric interface response is potentially used to detect the changes of the electrochemical properties in the formation.

Seismoelectric Technology in Hydrogeophysics

The seismoelectrical method(also called the Electroseismic method or seismo-electric) is based on the generation of electromagnetic fields in soils and rocks by seismic waves.Although the method is not reported to detect groundwater flow,it does measure the hydraulic conductivity,which is related to permeability and,therefore,to the potential of groundwater flow.Electroseismic technology is an effective tool for the siting of water wells。

Shear-horizontal transverse-electric seismoelectric waves in cylindrical double layer porous media

The shear-horizontal(SH) waves excited by the shear source in a borehole are easy to analyze due to the simple waveform. The borehole-side structures make the formation properties discontinuous. We consider a cylindrical double layer structure and study the borehole shear-horizontal and transverse-electric(SH-TE) seismoelectric waves. We first derive the expressions of the basic field quantities, and simulate the acoustic field and electric field using the real axis integral method. Compared with the wave fields of an infinitely homogeneous porous medium outside the borehole, the cylindrical layered structure makes the multi-mode cylindrical Love waves and their accompanying electric fields excited.Next, in order to study the interface response law of the inducing electric fields, we use the secant integral method to calculate the interface converted electromagnetic waves and analyze the causes of each component. It is found that an interface response occurs each time the SH wave impinges the interface in the layered porous medium. The results show that the SH-TE mode has a potential application for borehole-side interface detection in geophysical logs.

Theoretical and Experimental Studies of Seismoelectric Conversions in Boreholes

We present theoretical and experimental studies on the effects of formationproperties on seismoelectric conversions in fluid-filled boreholes. First, we derive thetheoretical formulations for seismoelectric responses for an acoustic source in a borehole.Then, we compute the electric fields in boreholes penetrating formations withdifferent permeability and porosity, and then we analyze the sensitivity of the convertedelectric fields to formation permeability and porosity. We also describe the laboratoryresults of the seismoelectric and seismomagnetic fields induced by an acousticsource in borehole models to confirm our theoretical and numerical developmentsqualitatively. We use a piezoelectric transducer to generate acoustic waves and a pointelectrode to receive the localized seismoelectric fields in layered boreholes and theelectric component of electromagnetic waves in a fractured borehole model. Numericalresults show that the magnitude ratio of the converted electric wave to the acousticpressure increases with the porosity and permeability increases in both fast and slowformations. Furthermore, the converted electric signal is sensitive to the formationpermeability for the same source frequency and formation porosity. Our experimentsvalidate our theoretical results qualitatively. An acoustic wave at a fracture intersectinga borehole induces a radiating electromagnetic wave.

Elimination of LWD (Logging While Drilling) Tool Modes Using Seismoelectric Data

Borehole acoustic logging-while-drilling (LWD) for formation evaluationhas become an indispensable part of hydrocarbon reservoir assessment [F. Citt ´a, C. Rus-sell, R. Deady and D. Hinz, The Leading Edge, 23 (2004), pp. 566-573]. However,the detection of acoustic formation arrivals over tool mode contamination has beena challenging problem in acoustic LWD technology. In this paper we propose a newmethod for separating tool waves from formation acoustic waves in acoustic LWD.This method is to measure the seismoelectric signal excited by the LWD acoustic waves.The LWD tool waves which propagate along the rigid tool rim can not excite any elec-tric signal. This is due to the effectively grounding of the drill string during the LWDprocess makes it impossible to accumulate any excess charge at the conductive tool —borehole fluid interface. Therefore, there should be no contribution by the tool modesto the recorded seismoelectric signals. To theoretically understand the seismoelectricconversion in the LWD geometry, we calculate the synthetic waveforms for the multi-pole LWD seismoelectric signals based on Pride’s theory [S. R. Pride, Phys. Rev. B, 50(1994), pp. 15678-15696]. The synthetic waveforms for the electric field induced by theLWD-acoustic-wave along the borehole wall demonstrate the absence of the tool mode.We also designed the laboratory experiments to collect simulated LWD monopole anddipole acoustic and seismoelectric signals in a borehole in sandstone. By analyzing thespectrum of acoustic and electric signals, we can detect and filter out the differencebetween the two signals, which are the mainly tool modes and noise.

Acousto-electric well logging by eccentric source and extraction of shear wave

The nonaxisymmetric acousto-electric field excited by an eccentric acoustic source in the borehole based on Pride seismoelectric theory is considered. It is shown that the acoustic field inside the borehole, converted electric and magnetic fields and coupled fields outside the borehole are composed of an infinitude of multipole fields with different orders. The numerical results show that both the electromagnetic waves and the seismoelectric field in the borehole, and the three components of both electric field and magnetic field can be detected. Measurements on the borehole axis will be of advantage to determining shear velocity information. The components of the symmetric and nonsymmetric acoustic and electromagnetic fields can be strengthened or weakened by adding or subtracting the two full waveforms logged in some azimuths. It may be a new method of directly measuring the shear wave velocity by using the borehole seismoelectric effect.

Theoretical Investigation of Seismic Electric Field Associated with Faulting

We present an electromagnetic model of a fault using the piezoelectric effect and the elastic dislocation theory to investigate theoretically the spatial distribution of the stress-induced charges associated with faulting. The relevant seismic electric field associated with these induced charges can be estimated quantitatively. Therefore, this simple model would provide a solid framework for additional theoretical developments on the explanations of the anomalous seismoelectric signals. The spatial distribution of the stress-induced charges around a vertical rectangular fault showed complicated characteristics. The estimation of the electric field associated with the stress-induced charges during the 1995 Kobe earthquake was consistent with the previous investigation from some reported anomalous seismic phenomena.