含偏心钻铤充液井孔中正交偶极子声波测井的数值模拟研究

Numerical study of cross-dipole acoustic logging in fluid-filled borehole with an eccentric drill collar

采用三维有限差分方法模拟了正交偶极子声源在含偏心钻铤的充液井孔中激发的声场,研究了钻铤偏心对模式波的种类、激发幅度、以及频散特征的影响.研究结果表明,钻铤偏心导致偶极子声源激发的声场的模式不唯一,观察到了除偶极模式外的单极模式波和四极模式波;钻铤偏心导致偶极模式波出现分裂现象,尤其是快速地层F2模式和慢速地层弯曲波的稍高频率的部分,且快、慢波所对应的两个方位为偏心的方位和与偏心方位垂直的方向;井孔折射横波以及快速地层F2模式的低频部分的慢度基本未受到钻铤偏心的影响,仍然能够正确反映地层的横波慢度及各向异性;对于本文研究的慢速地层井孔模型,当偏心距离l小于等于0.01 m时,弯曲波的慢度和激发幅度受钻铤偏心的影响很小,从快、慢弯曲波中提取的快、慢横波慢度基本能够反映地层的各向异性特征.

A three dimensional time-domain finite difference（TDFD） approach is utilized to investigate the types,excitation amplitude and dispersive characteristics of borehole normal modes produced by cross-dipole sources with the existence of an eccentric drill collar.The results show that an eccentric tool excites wave field of multiple azimuthal orders,where we observe monopole and quadrupole modes besides the wanted dipole modes.We also observe the wave splitting phenomenon induced by the tool eccentricity,especially at relatively high frequency,for either the F2 mode of fast formations,or the flexural mode of slow formations.The orientations of fast and slow flexural waves induced by tool eccentricity are the directions parallel or perpendicular to the eccentric direction.The borehole refracted shear waves and the low frequency range of the F2 mode are almost not affected by the tool eccentricity,and can be utilized to accurately describe the shear slowness and the shear anisotropy.For the slow formation in this study,when l≤0.01 m,the influence of collar eccentricity on flexural amplitude and dispersion is relatively small,and the extracted fast and slow shear slowness from the flexural waves can be used to assess the formation elastic anisotropy.