各向异性地层井间弹性波层析成像及衰减补偿逆时偏移理论与方法研究

Cross-well imaging technique is the primary means to evaluate geologic structures and fracturing. It is significant for reservoir fine evaluations, dynamic monitoring, and increase of oil-gas production, etc. Due to the limitation of conventional cross-well detection in unconventional reservoirs with complex structures, in the present study we investigate the wave propagation in viscous anisotropic heterogeneous media outside the borehole. And a new means of three-dimensional imaging for reservoir structures using seismic tomography and reverse time migration is proposed by theoretical and experimental studies. First, the tube wave propagation in an anisotropic formation and the elastic wave scattering near boreholes are revealed. And we analyze the influences of formation properties and borehole effects on the acoustic responses. Second, based on the approximation theories of elastic anisotropy, the tomography of velocity fields associated with attenuation is investigated. Then, we focus on the Q-compensated elastic reverse time migration, and propose the imaging means for cracks and caves in the subsurface. Finally, according to the acoustic measurements and the imaging experiments in a scaled model, the cross-well elastic wave imaging we proposed can be validated for its reliability and accuracy. The research in this proposal has great academic significance and application values in several aspects including the development of the acoustical theories in anisotropic solids, the improvement of inversion methods for cross-well seismology in complex formations, and the optimization of newly designed down-hole sonic tools for tomography and reflection migration.

井间声波成像技术是了解井间地质构造分布和裂缝发育延伸情况的主要途径，对于储层精细评价、动态监测以及油气井增产都具有重要意义。针对目前传统成像技术在具有复杂构造的非常规储层中的局限性，本项申请从理论和实验角度研究弹性波在井外各向异性非均匀粘弹性固体中的传播特性，探索利用层析成像及逆时偏移进行井间储层结构三维评价的新方法。具体研究包括：深入研究各向异性地层井中管波的传播以及平面波的井孔散射规律，分析地层参数和井孔效应对声场的影响；基于弹性波的各向异性群速度近似理论，提出速度场和衰减率的层析成像联合反演方法；研究衰减补偿逆时偏移算法，发展含缝洞地层中的反射波成像理论；通过各向异性介质模型井声学成像物理模拟，验证弹性波井间成像方法的可靠性和精度。本项研究对于推动各向异性固体声学的发展、完善复杂储层井间成像反演技术、设计新型井下声波成像仪器等都具有积极的学术意义和潜在的应用价值。

井间地震能有效识别井外百米范围内小尺寸的沉积和缝洞，解决水力压裂和储层连通性的动态监测、岩性分析和流体预测、微构造描述等问题，最终可为油气藏建模提供精细的地质模型，为开发方案的编制提供科学依据。面对储层井间成像技术中的新问题和新挑战，在本项目中我们立足各向异性粘弹性固体声学物理和井孔声场传播机理为基础，以基于井间地震模型的弹性波层析成像及逆时偏移为研究对象，开展了相关的井孔声场理论、波场数值模拟、声学层析和逆时偏移方法、实际资料处理和缩尺实验测量等研究工作。获得了以下主要成果和认识：针对非轴对称径向分层模型，基于波动方程在边界条件的解析解，研究了井孔柱面声波导的传播规律，揭示了柱面分层边界的偏心引起的导波多模式耦合效应。建立了广义标准线性体的粘弹性模型，改进了基于不连续网格的三维时域有限差分算法，以及基于常Q模型和复频移多轴完全匹配层条件、优化了各向异性粘弹性介质的频率域有限差分算法，从而开展了非均匀各向异性衰减地层的井间地震波场数值模拟及分析。发展了井间地震的粘弹性波的幅度衰减补偿和相位畸变补偿逆时偏移算法，针对多层介质和随机缝洞模型开展了成像处理。与弹性波偏移结果对比，衰减逆时偏移可以还原界面反射幅度、并有效提高深层反射体的成像质量。基于声学走时层析研究了井间地震的速度场快速反演算法，并针对观测系统布置和地震采集数据量分析了误差和迭代收敛效率等因素。最后，针对实际资料初步开展了波场分离等信号处理工作。建立了井间地震缩尺模型并开展了超声波测量，获得井间岩石中充液孔洞的声速层析反演结果，验证了成像算法的有效性，并提出进一步的改进测量方案。这些研究结果，丰富和发展各向异性粘弹性介质声学和井孔声学理论，为解决现代井间成像理论和方法提供了物理依据和方法指导，此外还可为未来二氧化碳封隔储层的声学动态监测提供重要的学术和应用基础。