利用随钻正交偶极子声波测井评价地层各向异性的数值研究

在随钻测井条件下,由于钻铤占据了井孔内的大部分空间,充液井孔中沿着井轴方向传播的模式波的特性与电缆测井非常不同.本文建立了随钻正交偶极子测井声学模型,采用三维有限差分方法模拟了偶极子声源在随钻条件下各向异性地层井孔内激发的声场,研究了地层的声学各向异性在随钻正交偶极子声波测井中的响应特征.数值模拟结果表明,在随钻测井条件下,对于井轴同TI地层对称轴垂直的情况,弯曲波分裂现象仍然存在,通过正交偶极子测量方式和合适的反演算法能够准确有效地确定地层的快横波方位角,可以考虑采用同正演理论相结合的反演算法来获得地层的快、慢横波速度及声学各向异性信息;对于井轴同介质对称轴呈一定夹角的TI地层井孔,情况变得非常复杂,不同井斜倾角下弯曲波的速度的变化趋势并非同对应的地层横波速度的变化趋势完全一致,不过在一定的频段内,地层横波速度仍然是弯曲波的最主要控制因素.对于本文研究的模型,当井轴同介质对称轴的夹角大于大于60°时,此时获得的弯曲波的各向异性值基本能够反映对应角度下地层横波速度的各向异性信息.

Numerical study of formation anisotropy evaluation using cross dipole acoustic LWD

Because the drill collar takes most of the space in the borehole, the mode waves in LWD conditions propagating along the borehole axis are quite different from that of wireline logging. In this work, a cross dipole acoustic LWD model was established, and the acoustic field of borehole surrounded by anisotropy in LWD conditions excited by dipole source was simulated using a three-dimensional finite difference method, and the response characteristics of formation acoustic anisotropy in cross dipole acoustic LWD were studied. Numerical results show that when the borehole axis is perpendicular to the symmetry axis of TI formation in the LWD conditions, flexural waves splitting still exist, and the fast shear wave angle can be estimated by the cross-dipole measurements and appropriated inversion method, and the velocities of fast and slow shear wave and the acoustic anisotropy information can be obtained by an appropriate inversion algorithm combined with the theoretical model. For the situation that the angle between the borehole axis and the symmetry axis is not 90? or 0?, the problem becomes very complicated. The velocity of flexural waves is inconsistent with the velocity of corresponding shear body waves with the changes of the angle between the borehole axis and formation symmetry axis. However, the velocity of shear waves is still the main controlling factor of flexural waves in certain frequency range. For the model studied in this paper, when the angle between the borehole axis and the symmetry axis is greater than 60?, the anisotropy parameter measured by flexural waves can basically indicate the true velocity anisotropy of shear waves for the corresponding angle.