Estimating Shear Wave Anisotropy From Acoustic Waves In A Borehole Surrounded By VTI Formation

Many of the rock strata are showing the anisotropic properties. People generally thought that many sedimentary rocks have many anisotropic characteristics, such as mudstones, shale. But transverse isotropy (TI is its short form), which is anisotropic in the simplest form. So-called transversely isotropic is this kind of medium that has an axis of symmetry. Properties of medium in any direction perpendicular to the axis of symmetry (such as wave velocity/slow degrees) are all the same, while properties of medium along the direction of it are different. From the perspective of strata, the most common drilling TI has two kinds: one is the case that TI axis of symmetry coincides with axis of bore. That is called VTI(The short form for Vertical Transverse Isotropy.) Another kind of TI is called the anisotropy. Under this circumstance, Properties of medium in different directions are different seen from the axis of bore. The vertical shaft of the anisotropic called HTI (The short for Horizontal Transverse Isotropy)To ensure the close relationship between the stratigraphic anisotropy and exploration and development for coal field, oil field, which usually act on the rock with high temperature and high pressure or fluid flow, thus to form anisotropy effect, especially the effect of fractures. Through the imbalance of ground stress can cause tension fissure in the orientation in the rock. That's kind of fracture can form (EDA) in certain scope. We can learn about the change of ground stress through the study of this fracture.This article did some research on the basis of the above purposes, which aim at studying of the bore transversely isotropic strata and acoustic field inside and outside bore being stimulated by the multipole source(monopole and dipole) being full of liquid.This article firstly deduced the acoustic field stimulated by multipole source in the bore and then considered the central point source in the bore as the harmomegathus vibrative small ball. The radius of small ball is far shorter than the wave length of sound. In cylindrical coordinate system, expression for direct field and reflection field being stimulated by multipole inside bore deduced by the Wave equation and Beisaier addition principle; then calculating acoustic field in the Strata outside bore. The stress-strain relationship between strata and the general form of the wave equation displace potential equation. From the equation of displacement potential bores, we can see that: of p-wave and SV layer is coupled together; SH wave is separate decoupling. So when we solve wave equation ,generally ,it is not easy to obtain independent p-wave and SV boo solution ( prospective P wave and SV wave. ) Using borehole wall boundary conditions to determine the coefficient of acoustic reflection. Through the dual Fourier transform, we can obtain Time and spatial domain acoustic expression inside and outside bore. Finally, edited by MATLAB whole wave curve and simulate the whole wave curve. This article also simulate calculate speed and the propagation characteristics of speed in TI Strata. Due to the existence of the anisotropic properties of strata, it makes speed and the group velocity of P wave, SV wave and SH wave in different directions being different with Isotropic strata and appears Transverse splitting phenomenon.In order to understand transversely isotropic strata affecting waveform, this article makes a calculation for monopole and dipole in fast formation and slow TI strata of the wave charts. From the whole wave chart, Stoneley wave of Monopole slow strata exist obvious characteristics of frequency dispersion, but quick formation of the waveform has slightly differences with isotropic stratigraphic waveform. Dipole bending wave is a kind of frequency dispersion .The wave simulated in low frequency is the critical refraction waves. That demonstrates the measured wave is transverse wave but bending wave when using low-frequency dipole source transverse wave to measure the bore.This paper made a simulated dispersion curves. I slow strata; stoneley wave has strong frequency dispersion. When the frequency is lower than some certain value (no relevant with selected strata, Speed and the group velocity in stoneley wave will be accelerated with the decrease of the frequency. That's the reason that low-frequency wave's speed is faster than the high one. Stoneley wave in TI quick strata scattered relatively weak, which speed will be accelerated with the increase of frequency. Seeing from dispersive line of dipole bending wave in strata, the phase velocity and bending curvelet group velocity in cut-off frequency place tends to be transverse wave along the axial direction spread .The biggest simulating strength in the point of Airy and the smallest group velocity while in the high frequency tends to Scholte wave speed.(namely fluid - solid interface of the wave speed stoneley )But there are some exceptions, for example, Mesaverde strata clayshale5501 is one of them. Under thisKind of circumstance, Speed and the group velocity in the limit of low-frequency bending wave are usually lower than transverse wave in strata's speed. Jingxing changes affect dispersion curves and stimulating strength curve. When jingxing R is magnified, frequency dispersion curves shall slightly move to low-frequency part and stimulating strength is decreasing. The changes of the formation parameters and the fluid velocity TI influencing on Disperse curve, among them, the changes of three parameters c1 1, c 13,c33 are slight Change c 44 is mainly manifested in the impact of high frequency area; Main effect of changes c6 6in low-frequency area. That is the reason that using the low-frequency wave's stoneley formation level inverts TI s-wave velocities; Fluid speed change effect is relatively high, because stoneley wave mainly speeds in the wave of stone in the fluid. The effect of important parameter to stoneley wave has made a detailed explanation in the sensitivity analysis. Meanwhile, many slow speed strata are analyzed the correctness of sensitivity analysis. The sensitivity of hard strata is mainly affected by the fluid speed control and other parameters on its influence become less.ditions of TI strata. In this kind of multipole acoustic stratum source when logging, it is impossible to accurately obtain the transverse wave speed value according to the time of arrival of bending wave.This paper also examines the influence of elasticity to the stoneley wave being stimulated by monopole. If the radius of the instrument is far smaller than the wavelength of sound, as to the stoneley wave in the borehole, Logging instrument can be considered as an elastic rod. Using the equivalent modulus M Tof elasticity stick to replace the elastic parameters. Observing and studying the differences of this two cases between the borehole Wells and Wells jointed the equivalent instrument modulus. The existence of well-logging instrument reduces greatly stoneley wave'speed in low-frequency. This is mainly because of stoneley wave spreading in fluid and the existence of instrument relatively reduced liquid space in Wells. Speed at high frequencies is far lower than that of low-frequency differences. Moreover, the characteristics of frequency dispersion curves in stoneley wave change with it.The superposition of waveform method is also introduced in order to make the preparation of looting shear velocity for the future.Finally, on the basis of the method for the weighted average slow degrees theorem spectrum, using stoneley wave in the low frequency inverses TI horizontal strata transverse speed and gain strata parameter c6 6 because of it. The significance of this method is slow degrees frequency dispersion curves of the weighted average equals spectrum within the scope of the frequency spectrum equals to the slow degrees of stoneley wave gained from the dispersion of the array process in time domain. Still use this method to do disperse correction for bending wave stimulated by dipole and gain transverse wave speed in vertical direction. Thus, strata parameter c 44can be obtained. The computational results show that the inversion results have small differences from forward value, which verifies the correctness of the theorem. That demonstrates the general application for this theorem on correction of disperse wave.