套管中周向传播准SH波的传播特征

Propagation characteristics of quasi-SH waves propagating circumferentially in a casing

利用沿套管轴向偏振、周向传播的准SH波可从根本上解决声阻抗与流体接近的轻质水泥(尤其是超轻质水泥)固井质量评价的难题。在柱坐标系下计算了套管中准SH波的频散曲线,在低于300 kHz的常用超声频段,套管中可存在零阶准SH0波和一阶准SH1波,与平板中的SH0波不频散相比,准SH0波随着频率的增加速度稍有降低,出现弱频散特征;准SH1波有截止频率,其相速度随着频率的增加先急剧下降,最后逐渐趋于套管材质的横波速度。通过仿真波形提取的衰减率可知,准SH0波随着频率增加其衰减逐渐增大,这也明显与平板中SH0的衰减特征相反,准SH波轴向位移的径向分布很好解释了此衰减特征。准SH波传播的声场快照直观显示,在套管后耦合泥浆等流体介质时,准SH波没有辐射衰减,在套后耦合固体水泥时准SH波在沿着套管周向传播时向套后介质辐射剪切波,衰减明显,这可从根本上区分套后声阻抗接近的泥浆(液体,没有剪切模量,SH波没有辐射衰减)和低密度水泥(固体,有剪切模量,SH波衰减明显),从而解决了现有声阻抗类声波仪器在低密度水泥胶结评价上的不适应性。

The problem of cementing quality evaluation of the light-weight cement(especially ultra-light-weight cement)with acoustic impedance close to that of fluid can be fundamentally solved using quasi-SH waves polarized in an axial directionalong the casing and propagating circumferentially.This paper calculates the dispersion curvesof the quasi-SH wave in a casing in a cylindrical coordinate system.At the common ultrasonic frequency below 300 kHz,the zero-order quasi-SH0 wave and the first-order quasi-SH1 wave can exist in the casing.Compared with the non-dispersive SH0 wave in the plate,the speed of quasi-SH0 wave slightly decreased with the increasing frequency,showing the characteristic of weak dispersion;the quasi-SH1 wave has a cut-off frequency,its phase velocity decreased sharply with the increasing frequency,and finally approached to the shear wave velocity of the casing material.The attenuation rate extracted from the simulation waveform indicates that the attenuation of quasi-SH0 wave gradually increases with the increasing frequency,which is also obviously opposite to the attenuation characteristics of SH0 wave in the plate.The radial distribution of the axial displacement of quasi-SH wave well explains this attenuation characteristic.The snapshot of the acoustic wave field of quasi-SH wave propagation visually shows that when coupling with the fluid media such as mud behind the casing,the quasi-SH wave has no radiation attenuation.When coupling with solid cement behind the casing,the quasi-SH wave radiated shear waves towards the medium behind the casing when propagating circumferentially along the casing,showing obvious attenuation.This can fundamentally distinguish between mud(liquid,no shear modulus,SH wave without radiation attenuation)and low density cement(solid,shear modulus,obvious SH wave attenuation),thus solving the incompatibility of existing acoustic impedance logging in the evaluation of low-density cement bond.