多极子随钻声波测井波场模拟与采集模式分析

Wave field simulation and data acquisition scheme analysis for LWD acoustic tool

应用三维非均匀交错网格有限差分程序模拟了多极子随钻声波测井的波场,考察了硬地层中单极子随钻声波测井响应及软地层中声源频率对多极子随钻声波测井的影响;计算了接收阵列波形的时域相干谱及频散特征,结果表明随钻测井仪器居中时,在硬地层中可以直接利用单极子声系得到地层横波速度,但地层纵波受到钻铤波的极大干扰.而在软地层中可以利用单极子声系直接测得地层纵波速度,但地层横波速度则需要使用工作在较低频率下的四极子随钻声波仪来测得,此时测井频段又会落入钻井噪声频带而受到影响.为此本文提出数值模拟手段结合物理实验的方法得到钻铤波的影响,在实际测井中通过信号处理方法而不是用在仪器本体上刻槽的方式来更完全地消除钻铤的影响.为了避免钻井噪声的影响,本文提出六极子随钻声波仪更适合软地层的横波测量.本文用数值实验结果来说明这些方法的可行性.

We have studied propagating wave fields of multipole acoustic LWD tool with a 3D finite difference simulator of non-uniform stagger grid. We examined monopole system response to hard formations and multipole system responses to soft formations and calculated the semblance spectra in time domain and the dispersion characteristics of these signals of array receivers. These analyses have demonstrated that, when the LWD tool is centralized properly, a monopole system can measure formation S wave velocity in hard formations while the formation P wave measurements may severely be interfered by the waves from drill collar. A quadrupole system at lower frequencies may be employed for the measurements of formation S wave velocity in soft formations. However these working frequencies for quadrupole system fall into the frequency band of drilling noise. Based on these numerical simulations, this paper proposes a numerical method for obtaining drill collar waves in conjunction with minor physical lab calibration to completely eliminate the interferences due to drilling collar during the signal processing procedure rather than making insulators of ring slots on the drill collar and on the tool. To avoid interferences of drilling noise on the quadrupole measurements for S wave velocity in soft formations, a hexpole system is proposed to replace the quadrupole system. The numerical experiments are presented to illustrate the feasibility of these proposed schemes for LWD acoustic measurements.