软地层横波速度的偶极随钻测井反演

在电缆测井中,可以利用偶极声场中的弯曲波反演软地层的横波速度.然而,在随钻声波测井(LWD)中,钢制钻铤的存在使得井孔结构变得复杂,同时改变了井孔声场,弯曲波也变得难以测定.此外,弯曲波与钻铤波耦合在一起,使得地层横波速度的反演变得困难.本文计算分析了随钻声场的频散曲线和激发曲线,注意到了偶极舒尔特波在较宽频带内速度频散很弱,特别是在本文研究的软地层情况下,偶极舒尔特波速度在3至25kHz的频率范围内几乎为一个常值,并且该值与地层横波速度存在一一对应关系.舒尔特波速度远小于其他模式波速度,与其他模式波在时域上易分离.相对于其他地层参数,舒尔特波对地层横波速度十分敏感.因此,它可以用来反演软地层的横波速度.

Inversion of shear wave velocity in slow formation by dipole Scholte wave during acoustic logging while drilling

Shear velocity can be inversed from the acoustic dipole flexural wave in a slow formation during wireline logging. However, the flexural wave becomes elusive during acoustic logging while drilling (LWD). The existence of the steel collar makes the borehole structure complex and changes the borehole wave field. The flexural wave is mixed with the collar wave, and cannot be used to invert shear wave velocity effectively. Dispersion and excitation analysis show the advantage of the dipole Scholte wave in LWD. In the case of slow formation studied in this paper, Scholte wave is slightly dispersive at low frequency. Its velocity is almost constant value at the high frequency range, and this value is one-to-one correspondence with the shear wave velocity of formation. The velocity of Scholte wave is much smaller than that of other modes, so it can be separated from other mode waves in the time domain. The Scholte wave velocity is much more sensitive to the shear velocity of the formation than all other formation parameters. Thus it can be used for inversion of shear wave in a slow formation.