随钻震电波传播理论研究与地层纵横波速度反演

Measurement of the compressional (P) and shear (S) wave speeds of the rock in the drilling process is important for in-time evaluation of the elastic moduli and porosity of the formation, and for guiding the driling direction. Current technology is purely acoustic. But the acoustic signal traveling along the drilling pipe is so strong that it covers all signals from the formation, making it difficult to discern the P and S waves. Thus accurate measurement of P and S wave velocitis of the rock is impossible by pure acoustic method. In this proposal the seismoelectric waves that accompany the elastic waves in the porous rock is utilized to obtain the P and S wave speeds, so that the elastic moduli can be obtained with known density. Because the seismoelectric wave can only be generated in the porous formation, the recorded electric signal contains information of the formation rather than that of the drilling pipe. This project will focus on the following aspects. First, to investigate into the propagation behavior of the elastic waves in a cylindrically layered structure including the drilling pipe, the fluid layers in and out of the pipe, and the porous formatiuon.Secondly, to reveal the properties of the seismoelectric waves that apppear due to electrokinetic coupling. Thirdly, to study the variations of the seismoelectric signals with exitation pattern (monopole, dipole,or quadripole),frequency, elastic moduli, porosity, and salinity of the pore fluid of the porous rock. Fourthly, to determine the effective receiving mode and distance. And last, to find a set of inversion algorithms for acquiring the P and S wave speeds of the formation from the seismoelectric signals.

在钻井过程中测量地层的纵波速度和横波速度对于实时确定地层的弹性模量、孔隙度以及实现钻井地质导向具有重要意义。当前的随钻测量采用了声学方法，但在测量信号中幅度大的钻铤波掩盖了来自地层的弹性波信号，以致不能提取纵、横波速度。本项目研究利用孔隙地层中伴随弹性波的电磁场（简称'震电'）信号，实现对地层纵、横波的测量。由于震电信号只产生于孔隙地层，且具有伴随地层弹性波出现的性质，从测量到的随钻震电波信号中提取纵、横波速度，不再受到钻挺波的干扰。主要研究内容有：含液体、钻铤、孔隙地层的柱面分层介质中弹性波的传播规律；孔隙地层中由于动电耦合产生的电磁信号的传播特性；这种震电信号随声源激励方式（单极源、偶极源、四极源）、激发频率、地层弹性模量、孔隙度、孔隙流体矿化度的变化规律；为有效地测量伴随弹性波的电磁场应采用的接收方式和接收距离；从接收震电波信号反演地层纵、横波速度的信号处理和反演方法。

在钻井过程中利用声波获取地层的纵、横波速度是随钻声波测井的目标，但前期研究表明幅度大持续时间长的钻铤波会掩盖来自地层的纵波和横波信号，使得对纵横波的测量不能实现。本项目提出在钻铤上某高度位置发射声波时，在另一高度位置上接收并处理来自地层的电磁场信号，该信号来源于孔隙地层中与净剩电离子相关的动电效应，是地层中声波引起的电磁信号。由于该信号未经过钻铤，因而可望避免钻铤波的影响。. 为此，本项目从岩心动电耦合实验、弹性-电磁耦合场的理论与数值模拟、模型井波场实验观测等方面开展了随钻震电测井研究。主要成果如下：1.岩石的动电耦合系数随孔隙度和渗透率的增大而增大，随孔隙流体的矿化度增大而减小。2. 钻铤-井孔流体-向外无限伸展的地层构成一个开波导结构，钻铤导波具有多阶模式，其频散曲线与置于无限大流体中的钻铤波频散曲线几乎一致，但各阶模式导波的幅度会由于地层的存在而变化。3. 钻铤波既在钻铤中传播，又在井孔流体和地层中传播。因动电耦合效应，地层中以钻铤波速度传播的声波会在地层中引起电磁场。4. 理论和数值模拟表明，地层中以钻铤波速度传播的声波引起的电磁信号，远小于地层纵波和横波引起电磁信号。5. 模型实验表明，测量记录到的随钻声波测井信号中，钻铤波信号幅度大且持续时间长，看不到地层纵波信号和横波信号，而记录到的随钻震电信号中，钻铤波信号幅度明显降低，且持续时间减小，可提取出地层纵波信号和横波信号。6. 实验表明：随着流体矿化度的增加，震电信号减弱，这与岩心实验结果、理论模拟结果一致，表明震电测井对中、低矿化度地区适用，对高矿化度地区，我们基于波动理论和数值研究，提出了有效降低单极源钻铤波信号幅度的内刻槽方案。. 通过本项目，我们还研究了层状-孔隙地层弹性波场的互易性，研究了几种非均匀孔隙介质有效体积模量和剪切模量的求取方法。