套管井偶极弯曲波频散向高频偏移的特性  

Characteristics of dipole flexural wave dispersion shift to the high-frequency in cased hole

作　　者：李刚[1] 吕伟国[1] 崔志文[1] 马俊[1] 王克协[1] 谢荣华[2] 刘继生[2] 吕秀梅[2] 

机构地区：[1]吉林大学物理学院,长春130021 [2]大庆油田有限责任公司,大庆163453

出　　处：《地球物理学报》2018年第1期379-388,共10页Chinese Journal of Geophysics

基　　金：国家自然科学基金(11134011;41474098)资助

摘　　要：对套管井偶极弯曲模式波的频散特性进行了系统的数值考察、实例对比和分析.发现套管井弯曲波频散曲线随地层横波速度的降低,特别是地层横波速度小于2000m·s-1以下,会迅速移向高频区,偶极弯曲波基础模式主频散区(或截止频率)可出现在13kHz以上,以致超出了现行低频偶极子声波测井仪的激发与接收频带,这是一过去没有被研究者注意到的现象,并进一步被现场实例所证实.研究表明控制套管井弯曲波频散曲线主频散区位置的主要是钢套管的厚度和地层横波速度.对地层横波速度大于井孔流体声速的快速地层,在钢套管壁厚一定(8mm)的情况下,频散曲线主频散区可移至11kHz以上,可能出现的最大可能频域位置是同一井孔内径,井外全钢时的频散曲线上等于、小于地层横波速度那一段,这对各种地层和套管参数都是适用的.对地层横波速度小于等于井孔流体声速(1500m·s-1)的慢速地层,弯曲波频散曲线随地层横波速度的降低移向高频区的特点更为明显,可能移至16kHz以上;而套管厚度的影响,也比快速地层大的多,对地层横波速度小于1380m·s-1的慢速地层,无论用多高的频率激发,都不能在现行使用的各类套管井(壁厚6~12mm)中用偶极声波测井仪测到弯曲模式波.The numerical investigation and field example analysis of the dispersion characteristics of flexural waves in cased hole are carried out. Dispersion curves of flexural waves will shift quickly to the high frequency domain as shear wave velocity is reduced, especially when the shear velocity is less than 2000 ms-1, the main frequency dispersion region （or cut-off frequency） will appear at frequency more than 13 kHz, so much so as to exceed the frequency band of transducer and receiver in the current low frequency dipole sonic logging instrument. It is a phenomenon not noticed by the researchers, and is further confirmed by field examples. The main frequency dispersion region of dispersion curve is controlled by the thickness of the steel pipe and the shear-wave velocity of formation. When the velocity of shear wave in hard formation is greater than the velocity of fluid in borehole, and thickness of steel pipe is constant （8 mm）, main frequency dispersion region can be shifted to above 11 kHz, the possible position may be in the part of dispersion curve being less than shear wave velocity,when borehole diameter is the same as cased hole and the parameters of formation are same as steel, which is applicable to varieties of formation and parameters of cased pipe. When the shear velocity of the formation is less than or equal to the velocity of fluid （1500 ms-1） in the borehole, the shift of dispersion curve of flexural wave with shear-wave velocity reduction to high frequency area is more obvious, may shift to 16 kHz and above. The effect of the thickness of the cased pipe in the slow formation is larger than that in the hard formation when shear wave velocity is less than 1380 ms-1, no matter how high is the excitation frequency, in the current use of all kinds of cased pipe thickness 6~12 mm, the flexural wave can not be measured.

关 键 词：套管井 正交偶极 频散曲线 主频散区间 向高频偏移 

分 类 号：P631[天文地球—地质矿产勘探]