Theoretical Study Of Acoustoelectric Well Logging In Porous Formation

Water-saturated porous media often exhibit seismoelectric effect due to the existence of electrical double layer and a relative flow of pore fluid.The mechanism of the acoustic field and the coupling electromagnetic provides not only information about the elastic properties and electrochemical properties in formation,but also the distribution of the oil and gas storage,and the frequency of acoustic field characteristics and the excitation frequency of seismic wave field are similar.Therefore,seismoelectric effect has potential application in geophysical exploration and earthquake prediction.In this paper,the borehole seismoelectric effect response characteristics of different physical models are studied by using the electrokinetic coupling governing-equations and the quasi-stationary method.Firstly,we adopt Pride's seimoelectric coupling equation to describe the seimoelectric field in the formation outside the well.Introduced the potential function to express basic fields,we can obtain the electric field and seismic field in the well by solving the boundary equation.The calculation results show that orders of seimoelectric conversion ratio is equal to that of wireline logging,and seimoelectric effect could be used in LWD.Therefore,it is advantageous to suppress the drill collar mode wave in electric field,but the drill collar mode wave is still low with the electric field signal-to-noise ratio.Despite most of collar wave was suppressed,there are residual in the electric waveforms.In view of the acoustic velocity of drill collar is known,an LWD acoustic logging technique using dual source is put forward,the collar wave was well suppressed using that technique by controlling the range between two source and emission delay time.In what follows,theoretical modeling of LWD monopole electric waveforms are used to demonstrate the technique and its ability to suppress collar waves and enhance the Signal to Noise Ratio of formation compressional waves.Because of Water-saturated porous media often exhibit seismoelectric effect due to the existence of 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 acoustic impedance or electrochemical properties at a cylindrical interface.Both coupled and approximate methods are used to compute borehole seismoelectric fields.We carefully analyze the seismoelectric interface response since these signals show sensitivity to contrasts in electrochemical properties and elastic properties across an interface.In addition,these radiated electromagnetic waves can be used to detect a low elastic impedance or high electrical impedance interface.The existing Pride's theory is established under saturated conditions,and can not be used directly for partial saturation problems.In this paper,referring to the Warden's method,we propose here an extension of Pride's equations aiming to take into account partially porous formation.Then the responses of the seismoelectric waves are quantitatively simulated,and the influence of fluid saturation on the full waveform of acoustic field and electric field excited by the monopole source and the multipole source is discussed.Meanwhile,the characteristics of different acoustic waves or electromagnetic waves in partially saturated porous media has carried on,from the phase velocity and attenuation curve and excitation spectra,to show the effects of fluid saturation.