Theoretical Study On Borehole P-SV-TM And SH-TE Seismoelectric Responses

Reservoirs of natural resources such as oil,water and natural gas are mostly porous media.The phenomenon in which elastic waves(acoustic waves)propagate in porous media inducing electromagnetic fields is called "seismoelectric effect".This phenomenon is closely related to the electric double layer structure and the seepage effect of pore fluid.With the improvement of weak signal detection and processing technology,the exploration method based on the seismoelectric effect has attracted more and more attention.Seismoelectric exploration receives both sound and electric waves,so the seismoelectric signal is not only sensitive to elastic properties such as porosity and permeability of the formation,but also can reflect electrical information such as conductivity.Seismoelectric logging places the source and sensors in the borehole at the same time,which has certain advantages in depth detection.In this paper,we start from the Pride acousto-electric coupling theory,the borehole seismoelectric response in porous formations is studied.The propagation characteristics of the P-SVTM excited by the centered point source and the acoustic-electric coupling wavefield of the SH-TE(Shear horizontal-Transverse electric)excited by the shear source are discussed respectively.The main contents include:The secant integral method is used to calculate the interface electromagnetic wave of the seismoelectric wave field for cylindrical double-layer porous medium,and to further understand the propagation law and the response characteristics to formation properties.We first use the Helmholtz decomposition method to deduce the basic field expressions for each medium region,and the real-axis integral method is used to simulate the time domain full wave of the sound field and electric field in the borehole.According to the distribution of body wave branch points in the double-layer pore formation,the branch points of formation body waves are analyzed.The interface electromagnetic wave waveform is obtained by calculating the secant integral of the electromagnetic wave branch point of the electric field,which shows that whenever the formation fast P wave and shear wave propagate to the interface,the interface response propagating at the speed of the formation electromagnetic wave is generated.The greater the difference in fluid salinity levels on both sides of the interface,the stronger the interface response.The sound wave passing through the salinity difference interface also produces the interface electromagnetic wave response.Aim at the model with impermeable borehole wall,the influence of impermeable borehole wall on the seismoelectric logging full-wave response,the components of body waves and mode waves is discussed.In order to simulate the formation of an impermeable borehole wall due to factors such as the thin mud cake attached to the wellbore wall and other factors leading to the blockage of pore channels at the borehole wall during the logging process,a new boundary condition(the seepage displacement of zero)is established.The wave field expression is deduced,and the full-wave response of the borehole acoustic field and electric field is simulated by the real-axis integral method.In order to further study the influence on the body waves and mode waves of the wave field,the secant integral method and the frequency-wavenumber analysis are used to analyze the components of the wave field.The results show that the impermeability of the borehole wall will obviously increase the amplitude of the Stoneley wave in the sound field,and the amplitude of the pseudo Rayleigh wave is less affected.In addition,for the body wave components of the sound field and the electric field,their excitation intensity,and the electroacoustic excitation ratio are all affected by the connectivity of the borehole pores.The borehole SH-TE seismoelectric response for the case of uniform formation and radial stratification is theoretically studied.The vertical field method and the Helmholtz decomposition method are used to deduce the basic field expression of each medium region,and the real-axis integration method is used to simulate the time domain full wave of the sound field and electric field in the borehole.The borehole sound field of a homogeneous formation has only one component of SH waves,while the elastically differential formation will excite Love mode waves with multi-order dispersion and its accompanying electric field.The relative excitation intensity of each order of Love waves is analyzed by frequency-wave number method,and the dispersion characteristics of Love mode waves and their accompanying electric fields are also calculated by Newton iteration method.The order of Love wave decreases with the decrease of the thickness of the inner porous medium,and the phase velocity of each order of Love wave decreases monotonically with the increase of frequency.The results of the sensitivity investigation to the fluid salinity show that the fluid salinity in the well hardly affects the electric field response.This special phenomenon is caused by the special excitation mode of the shear source and the low-frequency nature of the sound source(in the order of kilohertz).It is not found in other conventional excitation methods.Finally,the feasibility of combining STC and wavefield time-domain array to obtain information on the location and properties of the salinity interface next to the borehole is discussed.The borehole SH-TE seismoelectric wave fields in partially saturated porous media are studied,and the response characteristics for different soil textures are investigated.Combined with the partially saturated porous medium model based on Warden's equivalent medium theory,the Pride electrokinetic coefficients are extended to saturation-dependent forms.Firstly,the sensitivity of pore elasticity and electrical parameters to water saturation is investigated.Then the effect of saturation on borehole SH wave and its conversion electric field is simulated and investigated.Finally,starting from the equivalent medium theory,combined with the soil texture triangle,and introducing parameters such as hydraulic conductivity,the response characteristics of borehole SH-TE wave fields for different soil textures are studied.The results show that the parameters of the porous medium are sensitive to water saturation,and the electrokinetic coupling coefficient does not change monotonically with the saturation.With the increase of saturation,the amplitude of the sound field decreases monotonically,while the amplitude of the electric field first increases and then decreases.The elasticity and electrical properties for different soil textures are quite different,which have obvious effects on the attenuation,amplitude and wave speed of the SH wave and the converted electric field.Finally,the pore water is classified according to the salinity level,and the electric field response of different types of groundwater is simulated.The results show that the converted electric field is very sensitive and monotonic to groundwater salinity,which indicates that the borehole SHTE seismoelectric response is a potential method for judging groundwater contamination.In this paper,the borehole seismoelectric responses for different excitation methods and formation structures are studied.The purpose of this paper is to further explore the basic characteristics of the propagation of borehole seismoelectric waves excited by point sources and shear sources.The research results provide a certain theoretical scientific basis for in-depth understanding of borehole seismoelectric wave field characteristics and the development of new seismoelectric logging technologies.