煤储层裂隙的地震响应机理研究

Seismic exploration has been widely used in the research of coal reservoir characteristics. However, the seismic response mechanism caused by cracks in the coal reservoir remains to be further researched. In previous research the applicant founded that the achievement of coal reservoir crack detection is mostly scattered and partial, the widely and unified understanding for coal reservoir is lacked, and the connection between the coal seam cracks and seismic response is not clear. So make a further research of the seismic response mechanism caused by coal reservoir crack is very necessary. In this project the coal samples will be taken from Shanxi, Henan, Inner Mongolia, Huainan coal mining area. P-wave, S-wave, density and porosity will be tested in the lab. The inner link between the crack growth degree and the rock physical parameters will be analyzed, and three kinds of typical coal reservoir fracture models will be extracted according to the growth degree of cracks. The change characteristics of group velocity, phase velocity, the seismic wave field and seismic gather records for different models will be researched. The seismic reflection coefficient changing with the azimuth angle and incident angle will also be analyzed in the situation of different crack’s orientation, density, opening and fillers. So the seismic response mechanism of the cracks in coal reservoir is revealed. The research results will have important theoretical significance and practical value for coal reservoir crack detection and CBM exploration.

地震勘探技术已经广泛应用于煤储层特性研究中，然而煤储层中裂隙引起的地震响应机理还有待进一步的研究。申请者在前期研究中发现煤储层裂隙检测的已有成果多是零星和局部的，缺乏广泛和统一的认识，煤层裂隙与地震响应之间的联系尚不明朗，因此开展煤储层裂隙的地震响应机理研究非常必要。课题拟从山西、河南、内蒙古、淮南等矿区实际采取煤样，进行纵横波速度、密度、孔隙度等岩石物理参数测试，揭示裂隙发育程度与岩石物理参数之间的内在联系，并依据煤样的裂隙发育程度凝练出三种代表性煤储层裂隙模型，分析不同模型中由裂隙引起的群速度和相速度变化规律以及地震波场和道集记录响应特征，研究由裂隙方位、密度、开度、填充物等因素引起的地震反射系数随方位角和入射角的变化规律，从而揭示煤储层裂隙的地震响应机理。该项目的研究将对煤储层裂隙检测和煤层气开发具有重要的理论意义与实际价值。

煤层中水和瓦斯的运移方向、含量与煤层裂隙的发育情况密切相关，在煤田勘探中，从地震数据中提取裂隙信息以掌握煤层裂隙的发育情况，对于煤层气开采、煤矿透水、瓦斯突出等灾害性事故的发生具有重要意义。本项目首先基于实验室测试数据，揭示了煤储层的岩石物理参数变化规律，获得了纵波速度、横波速度和密度之间的关系；其次，以含裂隙煤储层TTI介质的二维三分量各向异性弹性波波动方程为基础，推导出含裂隙煤储层TTI介质的应力－速度方程。采用Lebedev网格对其进行高精度的差分离散处理，避免了传统交错网格在处理各向异性介质时波场插值引起的误差，提高了模拟精度。将多轴完全匹配层吸收边界（M-PML）引入了Lebedev网格，取得了较好的边界吸收效果，同时提高了计算效率。再次，建立了不同裂隙参数条件下的煤层模型，利用数值模拟方法综合研究地震波在含裂隙煤储层中的波场分布特征及其传播规律，当模型中的裂隙参数变化时，含裂隙煤储层的弹性参数会发生明显的变化。最后，系统地研究了各向异性煤层中裂隙密度、裂隙开度、裂隙填充物等裂隙参数对AVO曲线的影响。综上所述，项目通过岩石物理测试、地震波数值模拟、AVO分析等手段，获得了煤储层岩石物理参数、不同裂隙发育程度煤储层模型中相速度和群速度特征、含裂隙煤储层弹性波场变化规律及裂隙参数引起的方位AVO变化规律，实现了煤储层裂隙参数的定量描述，为精确获得煤层气甜点区、煤储层富水区及煤层瓦斯富集带提供了依据。