随钻四极源声波测井多模式采集测量TTI地层各向异性的研究

本文应用三维柱坐标时域有限差分方法模拟了多极子随钻声波测井的波场，提出了在横向各向同性（TI）慢速地层倾斜井中，使用四极源激发、多模式采集进行各向异性参数反演的方法.模拟结果表明：在强各向异性地层大角度倾斜井中使用四极源能够激发出多极模式波，并且井孔内的声场会随着声源方向角β（声源方向和快横波偏振方向的夹角）的改变而发生变化.使用四极子采集方式记录时，可以发现波形会分裂成快慢两种波；并且在声源方向与地层快横波偏振方向成45°时，分裂的两种波形的速度更趋近于地层快慢横波波速，用此可反演地层各向异性值大小.使用单极子采集方式记录时，其波形能量与声源方向角β大致成cos²2β的关系，在声源方向角为45°时，其波形幅度最小，用此可判断与快横波偏振方向成45°角的方向.使用AC方向（与声源同方向）偶极子采集方式记录时，当声源方向与快横波偏振方向一致时，采集到的波形幅度最小；而与快横波偏振方向垂直时，波形幅度最大，所以通过偶极子采集方式记录的波形可以判断快横波偏振方向.因此，在强各向异性倾斜井中进行随钻四极子测量时，可以使用多模式采集方式来进行地层各向异性的反演.

Inversion of anisotropy in a TTI stratum using quadrupole acoustic LWD and multimode acquisition

In this work, an acoustic Logging While Drilling (LWD) model was established and the wave fields of the borehole surrounded by a Transversely Isotropic (TI) stratum were investigated. We assumed the sonic transmitter is a quadrupole source while the receivers are monopole, dipole and quadrupole, respectively. Numerical simulation was conducted applying a 3D finite difference algorithm. The results show that in a highly deviated wellbore the acoustic fields composed of multipole mode waves vary with the azimuths of transmitters. Based on the simulations, we proposed a multimode acquisition method with the quadrupole, monopole, and the dipole modes to evaluate the formation anisotropy. First, if the quadrupole recording modes were adopted, we could observe that the waveforms separate into two arrivals due to the azimuthal anisotropy, the velocities of which tended to be faster and slower shear velocities, respectively. It would be the most prominent if the angle between the source azimuth and the fast shear wave polarization was set to 45°. Second, if the monopole recording modes were implemented, the waveform energy varied with the azimuth angle β of the quadrupole source. In particular, when the azimuth angle was 45°, the energy reached the lowest value. Third, if the dipole-recording modes were applied (e.g., in the AC direction), the wave group tended to be the smallest once the source azimuth was consistant with the fast shear wave polarization. By contrast, it turned to a peak value if the source orientation was perpendicular to that polarization direction. The multimode acquisition method could therefore be used to perform the inversion of formation anisotropy using a quadrupole source and multipole recording modes.