Theoretical And Numerical Study Of Borehole Acoustic And Electric Field Excited By External Acoustic Sources Under Seismoelectric Effects

The So-called seismoelectric effect is the interaction between the elastic wave field and the electromagnetic field in the porous medium,Causes these two kind of geophysics field characteristic to change the physical effect in the earth medium. As the seismoelectric effect research deep day by day, Stimulate the physicist and the geophysicist interest about the coupled wave propagation mechanism research interest and the acoustic electric coupling phenomenon. At present,the seismoelectric effect research and the application have received the oil vapor industrial field more and more attention, many of the western petroleum company, the oil well logging company to shake the electricity exploration the practical application to carry on diligently.Cross-well survey one of methods can make up the insufficiency of the acoustic well logging and the ground seismism. it puts up a mutual connected bridge at the ground seismism between the acoustic well logging. The essence of cross-well survey is that, it surveys the stratum and the oil deposit characteristic between the well, makes up the insufficiency of the oil well logging crosswise detectability. Solves questions such as gathering the information type and the information abundance not balanced between the bore-hole and the well logging and so on.There are lots of seismoelectric effect research work which the acoustic source put in a well have been done. But in view of cross well survey, the acoustic electric field characteristic which the acoustic source puts outside the well in the porous stratum stimulates also lacks the understanding at present.Therefore establishment physical model aims at the actual situation, Embarks from the strict elastic waves theory and the electromagnetic theory ,solutting and the computting the acoustic electric field radiation and the dissemination inside of the well, further researching in acoustic electric field propagation rule and well acoustic electric field propagation coupling rule outside of well, As well as influence of the porous stratum parameter, To the perfect well logging interpretation, a better understanding and the consummation actual oil well logging theory and the method have important significance.This article launches the research based on above goal. This article embarks from entire frequency range's Biot theory and the Pride elastic wave - electromagnetic wave coupling equation. Separately studied acoustic electric field situations of two kind of model under seismoelectric effect. The two kind of model are the acoustic point source located outside a well and two borehole cross well.In the point-source model, First establishes the acoustic source model,derives incident acoustic pressure field which the point source stimulates and the electromagnetic field accompanied acoustic pressure field under entire frequency range situation. the non-coupling's acoustic electric field in the well and the expression of the acoustic electric field included the undetermined constant about the coupling acoustic electric field outside of the well.useing wall boundary condition determine well undetermined constant about acoustic field. Obtain the internal and external sound electric field the expression in the time spatial domain through the dual Fourier transformations, Finally through the MATLAB program, simulate well logging full wave curve.We emphasis what in the numerical simulation process is the cross well survey, also takes into consideration the peculiar circumstance which in the actual oil well logging the mine shaft ruptures. for ease of understanding and comparison, We follow the offset from to be near and to be far, Center frequency from high to low. firstly carried on the full wave numerical simulation and the analysis about near offset and high centre frequency's situation. is also the acoustic source near outside wall flank. By now worked as the acoustic pressure for the kPa magnitude, may receive to several dozens millivolts electric fields, Simultaneously carries on the multi-channel inspection, On acoustic pressure picture we can observe the compressional wave, the shear wave, the flexural wave, the Stoneley wave and so on. Meanwhile in electric field radial and axial components,in the magnetic field circumferential components can see the accompanied compressional wave, the accompanied shear wave, the accompanied flexural wave, with the accompanied Stoneley wave and so on. Meanwhile in electric field axial components, the magnetic field circumferential components may see the electromagnetism head wave. An study of the ratio of the magnitude of electric field strength to the magnitude of pressure, or REP, shows that the compressional wave has the largest REP value, the Stoneley wave the next, and the shear wave the smallest. When graduation inspection discovered that in acoustic pressure, in electric field radial components is the single-pole stimulated and the transformed wave occupies the leader status. But in electric field radial components, in the magnetic field circumferential components is the dipole occupies the dominant position. When in the high center frequency, the offset from near to far changes, in acoustic pressure field the Stoneley wave, the shear wave, the flexural wave quickly weakens along with the offset change. When the offset achieves certain distance, can only observe the compressional wave, in the corresponding electromagnetic field also can only observe the accompanied compressional wave. When in high center centre frequency and far offset we discovery that in acoustic pressure field the compressional wave, in electric field axial components the compressional wave are mainly extremely stimulated and transformed by the single-pole point source, in electric field radial component, magnetic field circumferential components are mainly stimulated and transformed by the dipole.In view of the cross well survey we emphatically inspect in the low centre frequency and far offset situation. When acoustic pressure for the kPa magnitude, can accept to several millivolt magnitude electric fields. When multipolar inspections, curve information in acoustic pressure, in the electric field axial components, in the magnetic field circumferential component are mainly stimulated and transformed by the single-pole point source. But electric field radial component is actually stimulated and transformed by the dipole. When we consider the offset change influence the acoustic electric field ,discovered that along with offset increase, in the sound electric field's compressional wave or the accompanied compressional wave to reduce. When we consider the inspection receive'influence discovered that acoutic pressure is along with the distance increases has reduced, but electric field axial component increases first, after reduces. Formation parameter's inspection has the vital significance regarding the oil gas and the geological research. Therefore with emphasis inspected the soft and hard stratum and the formation parameter influence on the acoustic electric field which inscribes to receive inside of well. In the sound pressure field compressional wave scope in soft stratum is higher than in the hard formation, but in Electromagnetic field accompanied compressional wave change rule to be just the opposite. Along with the permeability increases compressional wave amplititude increases in acoustic pressure field, but in the electromagnetic field the accompanied compressional wave rule change rule and its opposite. The tortuosity affects nearly to acoustic pressure is very small, But the electromagnetic field reduces along with the tortuosity increases. borehole fluid salinity has not affected nearly to the sound pressure, but the electromagnetic field strength decreases obviously with the borehole fluid salinity increases.In cross well model, first uses the coefficient about acoustic electric field and transmitted acoustic electric field stimulated and transformed by the point source asks to the incident acoustic electric field. Similarly using Biot and in the Pride theory and the wall boundary condition, solves the receiver in the well acoustic electric field expression. For embarks from the cross hole survey's practical significance, has studied the low basic frequency far offset separation situation with emphasis. Numerical simulation preliminary obtained,In sound pressure field we can only observe the compressional wave and the shear wave, in electric field axial component, in electric field radial component, in the magnetic field circumferential component we can only observe accompanied compressional wave and the accompanied shear wave.,and TM electromagnetic wave,When acoustic pressure are dozens of kPa, we may receive Several millivolt magnitude the electric field. When polarization inspection discovered that the wave in acoustic pressure P and in the electric field axial component are stimulated and transformed mainly by the single-pole,But the wave in electric field axial component and the magnetic field circumferential component are mainly stimulated and transformed by the dipole.Inspection point of reception to Z=0 plane distance change discovered that the compressional wave in sound pressure field reduces throughout along with the separation increases. but after the shear wave increases first, reduces. But in the electromagnetic field follows the accompanied compressional wave, the accompanied shear wave and TM electromagnetic wave are increases first, after reduces. Along with permeability increases, in the acoustic pressure compressional wave increases, but sheai wave amplititude contraction, and in electric field axial component accompanied compressional wave increases but the accompanied shear wave contraction; The hardness index has not affected nearly to the sound pressure, but the electromagnetic field along with the hardness index increases but reduces obviously.This work carries on under the seismoelectric effect background,mainly aimed at the cross well survey to study inside and outside of well acoustic electric field under two kind of model stimulation well which are the point source outside of the well and cross well. This work regarding using the acoustoelectric carries on the oil well explanation,Forecast natural earthquake as well as to Improve ground water environment examination project and so on has the important meaning, This work has certain innovation.