Because of their relatively shallow volume of investigation, borehole acoustic measurements can be affected by abnormal near-wellbore conditions such as irregular calliper, drilling-induced formation damage and mud-filtrate invasion, among others. Additionally, borehole-slowness measurements inherently deliver rock elastic properties spatially averaged across the length of the multi-receiver array included in the waveform acquisition system. The consequence is that the interpretation of borehole acoustic measurements needs to account for both radial variations of elastic properties and axial spatial averaging effects across thinly laminated formations before conducting seismic-well log ties and rock physics interpretations. We introduce an inversion-based interpretation method to estimate radial shear-slowness variations from frequency-dependent slownesses in vertical wells penetrating horizontally layered formations. The inversion procedure is efficiently implemented with an optimized two-dimensional fast-forward-modelling method that simulates borehole acoustic modes in the presence of invaded and thinly laminated formations. Furthermore, the inversion method consists of two sequential steps: Firstly, layer-by-layer dispersion slownesses are processed to mitigate axial spatial averaging effects on borehole acoustic measurements; secondly, radial variations of elastic properties are estimated from inversion results obtained from the first step. The new inversion-based interpretation workflow estimates a layer-by-layer single-radial-step shear slowness model, including altered and virgin radial zones. By considering both radial and axial averaging effects, the implementation of the two-step inversion-based interpretation method in thinly laminated and radially invaded formations consistently improves the definition of shear slowness between invaded and virgin radial zones compared to traditional radial-profiling methods. The accuracy of the estimated shear slowness heavily relies on the sensitivity of dispersion modes to radial variations in formation shear slowness.

中文翻译：

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在薄层状地层中获得的钻孔声学测量的剪切慢度的径向剖面

由于其调查范围相对较浅，井眼声学测量可能会受到近井眼异常条件的影响，例如不规则的厚度、钻井引起的地层损伤和泥浆滤液侵入等。此外，井眼慢度测量本质上提供了波形采集系统中包含的多接收器阵列长度上空间平均的岩石弹性特性。结果是，在进行地震测井关系和岩石物理解释之前，井眼声学测量的解释需要考虑弹性特性的径向变化和薄层地层的轴向空间平均效应。我们引入了一种基于反演的解释方法，用于估计穿透水平层状地层的直井中与频率相关的慢度的径向剪切慢度变化。反演过程通过优化的二维快进建模方法有效地实现，该方法模拟存在侵入和薄层状地层的情况下的井眼声波模式。此外，该反演方法由两个连续步骤组成：首先，逐层处理色散慢度，以减轻轴向空间平均对钻孔声学测量的影响；其次，根据第一步获得的反演结果估计弹性特性的径向变化。新的基于反演的解释工作流程可估计逐层单径向步剪切慢度模型，包括蚀变和原始径向区域。通过考虑径向和轴向平均效应，与传统的径向剖面方法相比，在薄层和径向侵入地层中实施基于两步反演的解释方法始终改善了侵入和原始径向区域之间的剪切慢度的定义。估计的剪切慢度的准确性很大程度上取决于分散模式对地层剪切慢度径向变化的敏感性。