孔隙地层震电测井波场分波分析

Component wave analysis of borehole seismoelectric wavefields in a porous formation

由于孔道的双电层结构和流体-固体耦合运动,弹性波在孔隙地层中传播时会引起电磁场.前人提出了基于这种耦合效应的震电测井方法,但对井中震电波的特性缺少细致的分析.本文对孔隙地层充流体井孔中点声源激发的震电耦合波的分波进行分析,证明了井孔流体电磁波波数不是井内震电波场支点,计算了地层电磁波支点、横波支点和快纵波支点对应的侧面波.在利用辐角原理求出震电波场函数的复极点之后,针对典型砂岩地层计算了震电波场模式波(包括泄漏模式)的频散曲线和声压、电场激发曲线.计算了各个侧面波和模式波的电场激发强度与声压激发强度的比值(电声激发比),发现纵波的电声激发比相对于横波和模式波更高,而Stoneley波电声激发比的辐角对渗透率敏感,其敏感性会随频率增大而增加,随孔隙度增大而降低.

In this work, the axisymmetric seismoelectric logging response excited by a point-pressure source is formulated as an integral in the frequency-wavenumber domain. To analyze the properties of each constituent wavepacket in the seismoelectric logs, the poles and the branch points of the integrands are examined by the complex analysis. The electromagnetic wavenumber in the borehole fluid proves to be a removable branch point. The argument principle is used to search for the complex poles. After the excitation intensity as well as the dispersion curves has been obtained for each component wave for a typical sandstone formation, the ratio of the electric excitation intensity to the acoustic excitation intensity is evaluated. Especially it is found that the argument of this ratio of the Stoneley wave is sensitive to the permeability. The excitation curves corresponding to the branch points show that the compressional wave has a higher seismoelectric conversion efficiency than the shear wave and the mode waves.