随钻声波测井FDTD模拟及钻铤波传播特性研究

幅度大的钻铤波掩盖地层信号是困扰随钻声波测井技术的难题,认识钻铤波特性对于消除或有效降低钻铤波至关重要.记录岩石动电效应引起电磁场的随钻动电测井,被认为有望彻底解决钻铤波干扰问题.本文采用有限差分法,模拟不同钻铤、不同井孔结构的随钻声场,对比分析了单极源钻铤波的传播特性,阐明存在伴随钻铤波动电信号的原因.结果表明:钻铤声波在沿钻铤传播时向外部介质辐射能量,透过井壁进入地层的具有视钻铤波速度的声波,与地层纵横波一样可发生动电转化,因而可导致随钻动电测井时产生钻铤波速度的电磁信号.计算还表明,高频情况下,随钻声波测井钻铤波呈现两阶模式:具有低频截止频率的高阶钻铤波幅度较小,其速度略低于钻铤纵波速度;无截止频率的低阶钻铤波幅度较大,其波速在高频时甚至低于钻铤横波速度.这种频散特性和多阶模式特性,是径向多分层开波导结构的导波属性.

FDTD simulation and analysis of the collar wave propagation in acoustic logging while drilling

It is still an outstanding problem in acoustic logging while drilling(LWD) technique of the large-amplitude collar wave covering the formation waves, thus cognition of the characteristics of the collar wave is essential for eliminating or effectively reducing that wave. The seismoelectric LWD that records the electromagnetic fields induced by acoustic waves in the formation is expected to solve this problem thoroughly. In this study, a finite-difference time-domain algorithm is implemented to simulate the acoustic LWD waves in various collars and borehole models. Based on the propagation characteristics of the collar wave excited by a monopole source, the reason is addressed of the electric field accompanying the collar wave in seismoelectric LWD. It is found that the collar wave radiates energy into the outer media continually when propagating along the collar. Part of the energy with an apparent collar-wave velocity goes into the formation through the borehole wall, which induces electric fields like the formation compressional and shear waves thus can cause a signal of with collar-wave velocity in the seismoelectric LWD. In addition, there are two collar wave groups in the high-frequency acoustic LWD full waveforms. The high-order one having smaller amplitude cuts off in low frequency and it is slightly slower than the compressional wave in the collar. The low-order one having larger amplitude exists in the whole frequency range and it is even slower than the shear wave in the collar. The dispersion and multiple modes are the characteristics of guided waves in cylindrically-layered open wave guide structures.