随钻四极子横波测井钻铤尺寸优化设计的理论研究

随钻声波测井主要是采用四极子技术来测量软地层的横波速度,但在超软地层的横波测量方面仍然面临挑战.随钻四极子声源会分别激发出沿着钻铤和地层传播的螺旋模式导波,其中四极子钻铤波存在截止频率,即只有在该频率以上才能激发出钻铤波.当钻铤波截止频率降低到与地层波测量频段接近时,会干扰地层波信号的测量,尤其是在疏松的软地层这一干扰现象相当严重.数值模拟和现场实测数据分析结果表明:四极子钻铤波的截止频率主要取决于仪器接收器阵列位置的钻铤壁厚,而对声源与接收器之间的刻槽隔声体以及钻铤外地层介质不敏感.在此基础上,本文利用一种等效的随钻声波测井简化模型来分析与设计钻铤波截止频率与钻铤尺寸关系的理论图版.该理论图版可用于指导随钻四极子横波测井仪器钻铤尺寸的优化设计和数据处理,进而获得高质量的地层波信号以及准确的横波速度.

Theoretical analysis of the optimal collar size for a logging while drilling quadrupole tool

Logging-while-drilling (LWD) shear-wave velocity measurement in slow formation is mainly performed with quadrupole sources. However, this technology still faces many challenges in the ultra-slow formations. The LWD quadrupole waves consist of a formation quadrupole mode propagated along the borehole and a collar quadrupole mode propagated along collar, respectively. The collar quadrupole wave exists only above a certain frequency called the cut-off frequency. However, tool-wave contamination occurs when its cut-off frequency close to the main frequency band during the shear-wave measurement, especially in the slow formation situation. Both numerical simulations and field data analysis results show that the collar-wave cut-off frequency mainly depends on the collar thickness of receiver array section. The cut-off frequency is neither sensitive to the formation properties nor to the groove-isolator between source and receivers. Based on the sensitivity analysis of the LWD tool model, we developed an equivalent simplified approach to produce a theoretical template about the cut-off frequency versus collar sizes. The template can be used to aid the design of the LWD quadrupole tools or to guide the data processing measured by existing LWD tools, so as to obtain accurate formation shear velocity from the LWD quadrupole measurement.