孔隙地层轻质水泥套管井中偶极弯曲波的激发与传播特性

针对软地层套管井中弯曲波频散移向高频,以致超出了现行偶极声波测井仪器的激发频带的问题,本文在井外为孔隙地层时,采用Biot模型对套管井多极源激发的声波场进行了理论推导,对套管井偶极弯曲波的频散特性进行了数值模拟,考察了高密度水泥(或快速水泥)和低密度水泥(或轻质水泥)情况下的弯曲波频散,分析了不同水泥环对弯曲波主频散区和激发谱偏移的影响,重点对轻质水泥套管井中偶极源激发的模式波频散与激发谱及临界折射P、S波激发特性进行了研究,并考察了渗透率、孔隙度对弯曲波频散及衰减的影响,利用实轴积分法计算了偶极声源激发的时域全波波形.分析对比结果表明,在快速水泥情况下弯曲波频散曲线随着地层特征横波速度的减小会迅速向高频移动,随着快速水泥向轻质水泥变化,弯曲波频散曲线向高频移动将会减缓,对于特征横波速度低于1400m·s~(-1)的软地层,偶极弯曲波基础模式主频散区(或截止频率)可以由原来13kHz向低频移动至4kHz;在轻质水泥套管井中,无论是硬地层还是软地层,在目前偶极声波测井仪器声源主频激发下,接收波形中弯曲波均占主导地位.

Excitation and propagation characteristics of dipole flexural wave in low density cement cased hole

The dispersion curve of flexural wave in the soft formation cased holes always moves to high frequency region, which is obviously out of the excitation frequency range of current dipole source logging instrument. This is a challenge for dipole acoustic logging. To address this challenge, here we employed Biot and homogenization theories to investigate the multipole source acoustic field in cased hole within porous formation. The numerical calculation of flexural wave dispersion is performed in the condition of high/low density cement annulus, which demonstrate the influence of different density cement on the main frequency dispersion domain and excitation domain of flexural wave. For low density cement cased hole, not only the effects of porosity and permeability on flexural wave propagation but also the critical refraction P and S waves are studied. Besides, the dipole full waves are simulated in the time domain. The results show that the dispersion of flexural wave in high density cased hole will shift to high frequency domain rapidly, while the shifting speed obviously slow down in low density cement cased hole. For the soft formation with characteristic shear wave velocity at 1400 m·s^-1, the main frequency dispersion region and cut-off frequency of flexural wave dispersion curve shift from 13 kHz to relatively lower value 4 kHz with the lower density cement, which indicate the flexural wave may be excited at lower excitation frequency. Our research provides valuable insight, for low density cement cased hole whether in hard formation or soft formation, the flexural wave is predominant in received full wave with the main frequency excitation of current dipole logging instrument.