声波测井套管波的共振特征与固井质量评价

Resonance characteristics and cementing quality evaluation of casing wave in acoustic logging

套管井内的套管波存在固有频率,响应频谱在固有频率附近出现峰值,具有共振特征。当激发探头的主频与固有频率一致时则套管波幅度大;当激发探头的主频偏离时离固有频率则套管波幅度小,甚至没有套管波。套管波频谱峰的形状和幅度随固井I界面胶结质量变化。固井质量好,峰幅度小,形状平缓;固井质量差,峰幅度大,形状尖锐。峰的幅度决定套管波幅度(CBL),峰的形状决定套管波的形状,即套管波的波形随源距、时间的变化。利用阵列声波测井获得不同源距的波形,基于套管波的频谱峰形状提取波形中的固井质量信息,获得了新的固井质量评价曲线——共振因子(aa)。它充分利用了阵列波形中的套管波信息,随固井质量单调变化,多解性小,受到的影响因素少。套管井的水泥环参数对套管波的频谱峰影响较大。轻质水泥与水的波阻抗差异小,套管波幅度大,频谱峰尖锐,aa大,固井质量差异引起的aa变化小;高密度水泥与水的波阻抗差异大,套管波幅度小,共振峰平缓,aa小,固井质量差异引起的aa变化大。基于实验室获得的水泥环参数,利用插值方法建立了水泥环模型,然后利用二维谱研究了水泥环对aa的影响,给出aa固井质量评价的水泥密度校正方法,完善了aa曲线,形成了新的固井质量评价方法。

The casing wave of acoustic logging in a cased borehole has natural frequencies,and the casing response has resonance characteristics.If the main frequency of the excitation probe is consistent with the natural frequency of the casing wave,the excitation amplitude of the casing wave is larger;however,if the excitation frequency deviates from the natural frequency,the excitation amplitude of the casing wave is smaller.If the deviation is complete,the casing wave cannot be excited.The shape and amplitude of a single formant in the casing response spectrum vary with the cementing quality of the cementing I interface.If the cementing quality is good,then the formant amplitude is smaller and the shape is gentle.However,if the cementing quality is poor,the formant amplitude is larger and the shape is sharp.The amplitude of the formant determines the amplitude of the casing wave(CBL),and the shape of the formant determines the waveform shape of the casing wave,manifested as a change in the casing wave shape with the range from the source and time.In this study,array acoustic logging waveforms were used to convert the spectral shape of the casing wave into a resonance factor aa to evaluate the cementing quality.Compared with the CBL,the resonance factor aa was affected by fewer factors and varied monotonously with the cementing quality.The parameters of the cement ring in the casing borehole significantly influence the resonance peak of the casing wave.If the wave impedance difference between the lightweight cement and water is small,then the casing wave amplitude is large;resonance peak is sharp;and resonance factor aa is large.However,if the wave impedance difference between the high-density cement and water is large,the casing wave amplitude is small;formant peak is flat;and resonance factor aa is small.In this study,cement annulus parameters obtained from laboratory field measurements were used to establish a cement annulus density model through interpolation,and the influence of the cement annulus parameters on the size of the resonance factor aa was studied using a two-dimensional spectrum in a cased borehole and combined with the resonance factor GAL cementing quality evaluation.Finally,a cement density correction method for aa cementing quality evaluation was devised,the aa curve was improved,and a new cementing quality evaluation method was developed.