Effects of aligned fractures on the response of velocity and attenuation ratios to water saturation variation: a laboratory study using synthetic sandstones
Effects of aligned fractures on the response of velocity and attenuation ratios to water saturation variation: a laboratory study using synthetic sandstones
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DOI：
10.1111/1365-2478.12378
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发表时间：
2016-07
影响因子：
2.6
通讯作者：
Kelvin Amalokwu;A. Best;M. Chapman
中科院分区：
文献类型：
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作者：
Kelvin Amalokwu;A. Best;M. Chapman
P波与S波比值是重要的地震特征属性。速度比对岩石的岩石物理性质和气体的存在很敏感。衰减比也被证明是敏感的部分液体/气体饱和的存在。液体/气体饱和度与P波和S波比率之间的关系已用于区分气体饱和岩石和液体饱和岩石。对齐的裂缝在地壳中是常见的，并且引起地震各向异性和剪切波分裂。然而，部分气体/液体饱和度与P波和S波比值之间的大多数现有关系都是针对非破裂岩石的。我们目前的实验结果比较的影响，改变含水饱和度的Qs/Qp与Vp/Vs比之间的非裂缝岩石和一个含有裂缝平行于波传播方向。我们还研究了对齐的裂缝对Vp/Vs响应的影响，以改变含水饱和度，使用合成裂缝砂岩与裂缝对齐在45 °和平行于波的传播方向。结果表明，对齐的裂缝可能对观察到的趋势有显着的影响，其中一些可能并不明显。平行于波传播方向排列的裂缝可能会改变Qs/Qp与Vp/Vs比值对水饱和度的响应。剪切波分裂由于对齐裂缝的存在导致两个速度比（Vp/Vs 1和Vp/Vs 2）。对于平行于波传播方向排列的裂缝，横波分裂的流体独立性意味着Vp/Vs 1和Vp/Vs 2之间的差与含水饱和度无关。对于以倾斜角度排列的裂缝，剪切波分裂可能对水饱和度敏感，因此是频率相关的，这可能导致Vp/Vs 1和Vp/Vs 2之间的流体和频率相关差异。对齐裂缝对Vp/Vs比值的影响不仅取决于裂缝对P波和S波速度的影响，还取决于含水饱和度分布对岩石和裂缝刚度的影响，从而影响P波和S波速度。因此，由于波浪引起的流体流动，这些效应可能与频率有关。一个简单的建模研究相结合的频率依赖的裂缝岩石模型，和频率依赖的部分饱和模型被用来获得有价值的解释我们的实验观察和可能的影响，这将是有用的现场地震数据解释。
P‐wave‐to‐S‐wave ratios are important seismic characterization attributes. Velocity ratios are sensitive to the petrophysical properties of rocks and to the presence of gas. Attenuation ratios have also been shown to be sensitive to the presence of partial liquid/gas saturation. The relationship between liquid/gas saturation and P‐wave and S‐wave ratios has been used to distinguish gas‐saturated rocks from liquid‐saturated rocks. Aligned fractures are common in the Earth's crust and cause seismic anisotropy and shear wave splitting. However, most existing relationships between partial gas/liquid saturation and P‐wave and S‐wave ratios are for non‐fractured rocks. We present experimental results comparing the effects of changing water saturation on Qs/Qp versus Vp/Vs ratios between a non‐fractured rock and one containing fractures aligned parallel to wave propagation direction. We also study the effects of aligned fractures on the response of Vp/Vs to changing water saturation using synthetic fractured sandstones with fractures aligned at 45o and parallel to the wave propagation direction. The results suggest that aligned fractures could have significant effects on the observed trends, some of which may not be obvious. Fractures aligned parallel to wave propagation could change the response of Qs/Qp versus Vp/Vs ratios to water saturation from previously reported trends. Shear wave splitting due to the presence of aligned fractures results in two velocity ratios (Vp/Vs1 and Vp/Vs2). The fluid independence of shear wave splitting for fractures aligned parallel to wave propagation direction means the difference between Vp/Vs1 and Vp/Vs2 is independent of water saturation. For fractures aligned at oblique angles, shear wave splitting can be sensitive to water saturation and consequently be frequency dependent, which can lead to fluid and frequency‐dependent differences between Vp/Vs1 and Vp/Vs2. The effect of aligned fractures on Vp/Vs ratios not only depends on the fracture effects on both P‐wave and S‐wave velocities but also on the effects of water saturation distribution on the rock and fracture stiffness, and hence on the P‐wave and S‐wave velocities. As such, these effects can be frequency dependent due to wave‐induced fluid flow. A simple modelling study combining a frequency‐dependent fractured rock model, and a frequency‐dependent partial saturation model was used to gain valuable interpretations of our experimental observations and possible implications, which would be useful for field seismic data interpretation.