ABSTRACT Depletion or injection into a reservoir implies stress changes and strains in the reservoir and its surroundings. This may lead to measurable time‐shifts for seismic waves propagating in the subsurface. To better understand the offset dependence of time‐shifts in the overburden, we have systematically quantified the time‐shifts of three different overburden shales in controlled laboratory tests. These experiments may be viewed as an analogue to the time‐shifts recorded from seismic field surveys. For a range of different stress paths, defined as the ratio between the horizontal and the vertical stress changes, the changes of the P‐wave velocities in different directions were measured such that the offset dependence of time‐shifts for different stress paths could be studied. The time‐shifts are stress path dependent, which is particularly pronounced at large offsets. For all stress paths, the time‐shifts exhibit a linearly decreasing trend with increasing offset, that is, a negative offset gradient. At zero offset, for which the ray path is normal to the bedding, the time‐shifts are similar for all investigated stress paths. The isotropic stress path is associated with the smallest offset gradient of the time‐shifts. In contrast, the constant‐mean‐stress path shows the largest gradient with a flip in the polarity of the time‐shifts for the largest offsets. The separate contributions from the strain and velocity changes to the time‐shifts were also quantified. The time‐shifts for the isotropic stress path are dominated by the contribution from velocity changes at all offsets. In contrast, the strain contributes significantly to the time‐shifts at small offsets for the constant‐mean‐stress path. This shows that the offset dependence in pre‐stack seismic data may be a key to understand the changes of subsurface stresses, pore pressure and strain upon depletion or injection. To utilize this knowledge from laboratory experiments, calibrated rock physics models and correlations are needed to constrain the seismic time‐shifts and to obtain an adequately updated geological model reflecting the true anisotropic nature of the subsurface. This may have important implications for improved recovery and safety, particularly in mature fields.

中文翻译：

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来自超声数据的覆盖层时间偏移的偏移依赖性

摘要 油藏的消耗或注入意味着油藏及其周围环境的应力变化和应变。这可能导致地震波在地下传播的可测量时移。为了更好地理解覆盖层时间偏移的偏移依赖性，我们在受控实验室测试中系统地量化了三种不同覆盖层页岩的时间偏移。这些实验可以被视为与地震现场调查记录的时移类似。对于一系列不同的应力路径，定义为水平和垂直应力变化之间的比率，测量不同方向的 P 波速度的变化，从而可以研究不同应力路径时移的偏移依赖性. 时移依赖于应力路径，这在大偏移时尤为明显。对于所有应力路径，时移随着偏移量的增加呈线性下降趋势，即负偏移梯度。在零偏移处，射线路径与层理垂直，所有研究的应力路径的时移相似。各向同性应力路径与时移的最小偏移梯度相关。相比之下，恒定平均应力路径显示最大的梯度，最大偏移量的时移极性翻转。应变和速度变化对时移的单独贡献也被量化。各向同性应力路径的时移主要受所有偏移处速度变化的影响。相比之下，对于恒定平均应力路径，应变对小偏移处的时移有显着贡献。这表明叠前地震数据的偏移依赖性可能是理解耗尽或注入时地下应力、孔隙压力和应变变化的关键。为了利用实验室实验中的这些知识，需要校准的岩石物理模型和相关性来约束地震时移并获得充分更新的地质模型，以反映地下的真实各向异性性质。这可能对提高采收率和安全性具有重要意义，尤其是在成熟油田。消耗或注入时的孔隙压力和应变。为了利用实验室实验中的这些知识，需要校准的岩石物理模型和相关性来约束地震时移并获得充分更新的地质模型，以反映地下的真实各向异性性质。这可能对提高采收率和安全性具有重要意义，尤其是在成熟油田。消耗或注入时的孔隙压力和应变。为了利用实验室实验中的这些知识，需要校准的岩石物理模型和相关性来约束地震时移并获得充分更新的地质模型，以反映地下的真实各向异性性质。这可能对提高采收率和安全性具有重要意义，尤其是在成熟油田。