基于全波形匹配的微地震震源机制反演及其应用

发布时间：2018-07-17 08:04

【摘要】：致密气、煤层气和页岩气等非常规油气资源在日益增长的低碳燃料供应方面起着越来越重要的作用。这主要得益于水平井钻井和水力压裂技术在低渗透性储层中的发展应用。水力压裂诱发的微地震能够帮助我们了解水压致裂的裂缝几何分布状况,估计经过压裂改造的岩石的体积。通过对非常规油气开采中诱发的微地震进行定位和震源机制研究,我们不仅能够刻画水压致裂的破裂过程,而且可以对钻井孔周边区域的断裂分布情况进行描述,这为我们构建断裂储层模型提供了非常重要的先验信息和约束。目前,人们常用P波初动极性法、振幅比法和波形矩张量反演方法进行地震的震源机制研究工作。由于微地震震级小,破裂释放的能量有限,仅有少数台站能够记录到震动信息,利用常规的P波初动极性和振幅比方法不能准确得到微小事件的震源机制解。所以本文使用了矩张量反演方法,其主要基于全波形匹配的思想,利用理论地震图和实际地震记录的最佳匹配拟合反演来确定震源机制解。全波形匹配方法的目标函数中不仅包含波形的振幅和相位信息,而且加入了P波初动和纵横波振幅比信息,既充分利用了波形信息,也可以对反演加以约束,避免仅靠部分波形数据反演导致的不确定性。首先,为了验证波形匹配方法的稳健性,我们使用不同地区的速度模型和震源参数进行了合成数据测试。两类模型测试结果都表明此波形匹配方法稳定可靠,可用于解决微地震的震源参数确定问题。然后,我们将波形匹配方法应用于美国犹他州中西部的Cove Fort-Sulphurdale地热资源富集区。震源机制反演结果显示,所选取地震的断层面解的走向大部分趋于南北向,与区域应力图显示的最大水平主应力优势取向—南北方向,具有较好的一致性。前人研究结果表明该地区处于近东西向的拉张应力作用下,南北方向应该是主压应力的优势取向,与基于震源机制的应力分析得到结果比较一致,符合目前人们对该地区的构造认知。为了更清楚地了解Cove Fort-Sulphurdale地热区的断裂构造特征,我们也对该区域进行了各向异性研究。横波分裂分析结果说明各向异性主要存在于浅部区域；各台站快波的极化方向指向南北向,说明如果各向异性主要是由该地区的断裂构造引起,那么该地区的主要断层或裂缝的方向应该是南北向。最后,鉴于文章讨论的震源机制反演主要是基于双力偶点源假设,它虽然简化了问题,减小了介质各向异性等不均匀性存在时反演的不确定性。然而,它得到的解并不完整,不包含体积变化和补偿线性向量偶极(CLVD)成分。对非双力偶成分的认知,特别是体变成分,对理解破裂过程、评价压裂工作效率非常重要。为此,我们将断层面解反演拓展为基于波形匹配的全矩张量反演,以确保震源机制反演结果的准确性。考虑到精度和准确性,矩张量参数的搜索间隔较小,网格搜索效率较低。为此,我们使用差分进化算法进行改进,加速反演过程。合成数据测试结果表明,与断层面反演相比,经过DE改进的矩张量反演也能够准确得到震源机制解信息,计算效率与网格搜索相比提高了几十倍。
[Abstract]:The unconventional oil and gas resources such as tight gas, coal bed gas and shale gas are playing an increasingly important role in the growing supply of low carbon fuel. This is mainly due to the development and application of horizontal well drilling and hydraulic fracturing in low permeability reservoirs. The micro earthquake induced by hydraulic fracturing can help us to understand the cracks in water pressure. We can not only describe the fracture process of hydraulic fracturing, but also describe the distribution of fracture in the surrounding area of the drilling hole, which provides us with the construction of fractured reservoir. The model provides very important prior information and constraints.
At present, P wave initial dynamic polarity method, amplitude ratio method and wave moment tensor inversion method are used to study the source mechanism of earthquake. Due to the small magnitude of the microseismic, the energy of the fracture release is limited, only a few stations can record the vibration information, and the small events can not be obtained by the conventional P wave initial motion polarity and amplitude ratio method. Therefore, this paper uses the moment tensor inversion method, which is mainly based on the idea of full waveform matching, and uses the optimal matching inversion of the theoretical seismogram and the actual seismic record to determine the source mechanism solution. The objective function of the full waveform matching method not only contains the amplitude and phase information of the wave shape, but also adds the P wave. The information of the amplitude ratio of the initial and the longitudinal and transverse waves can not only make full use of the information of the waveform, but also restrain the inversion to avoid the uncertainty caused by the inversion of the partial waveform data.
First, in order to verify the robustness of the waveform matching method, we use the velocity model and the source parameters in different regions to test the synthetic data. The two model test results show that the waveform matching method is stable and reliable, and can be used to solve the problem of the source parameters of microseismic. Then, we apply the waveform matching method to the United States. The Cove Fort-Sulphurdale geothermal resource enrichment area in central and Western Utah state. The result of seismic source mechanism inversion shows that the direction of the fault surface solution of the selected earthquake tends to North and south, and has good consistency with the north and south direction of the maximum horizontal principal stress orientation in the region. The results of previous studies show that the region is located in the region. Under the near East-West tensile stress, the north-south direction should be the dominant orientation of the main pressure stress, which is consistent with the stress analysis based on the source mechanism, which conforms to the current tectonic cognition of the region. In order to know more clearly the characteristics of the fracture structure of the Cove Fort-Sulphurdale geothermal area, we also carry out the region. The results of the shear wave splitting analysis show that the anisotropy mainly exists in the shallow region; the polarization direction of the fast waves at each station points to the north-south direction, indicating that if the anisotropy is mainly caused by the fracture structure in this area, the direction of the main faults or cracks in this area should be north-south.
In the end, the source mechanism inversion is mainly based on the double couple point source hypothesis, which simplifies the problem and reduces the uncertainty of the inversion when the anisotropy of the medium exists. However, the solution is not complete, and does not contain the volume change and compensatory linear vector dipole (CLVD) component. The cognition, especially the body variable component, is very important for understanding the fracture process and evaluating the efficiency of the fracturing work. Therefore, we extend the inversion of the fault surface to the full moment tensor inversion based on the waveform matching to ensure the accuracy of the inversion results of the source mechanism. Therefore, we use the differential evolution algorithm to improve and accelerate the inversion process. The results of synthetic data test show that, compared with the broken layer inversion, the DE improved moment tensor inversion can also get the exact information of the source mechanism solution, and the calculation efficiency is a few times higher than the grid search.
【学位授予单位】：中国科学技术大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P315.6

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