基于岩石物理实验的冻土区裂缝型水合物储层地球物理响应特征研究

Gas hydrate is an unconventional gas resource of great potential, and it draws more and more attention all over the world. Therefore, the research of gas hydrate has become a research hotspot. Gas hydrate discovered in the Qilian Mountain Permafrost which is classified into the pore-filling type and the fracture-filling type is more complicated than that discovered abroad. Because of the great anisotropy of the fracture-filling reservoirs, it is difficult to detect and recognize geophysical responses from geophysical prospecting methods. Numerical simulation based on petrophysical experiments can be useful to solve the problem. In this study, we intend to collect core samples of typical fracture-filling reservoir in the Qilian Mountain Permafrost, and conduct petrophysical experiments including acoustical properties, Nuclear Magnetic Resonance (NMR), X-diffraction to obtain rock information such as the mineral composition, acoustic wave velocity and pore distribution to assist the reconstruction of forward models of Azimuthal Amplitude Variation with Offset (Azimuthal AVO) and acoustic log. Based on rock elasticity theory and media model, Azimuthal AVO responses variation with different fracture parameters, reservoir identification sensitive factors, and effect laws of gas hydrate saturation characteristics changing with different fracture parameters will be obtained. The results of numerical simulation also will be compared with petrophysical experiment results. This study will lay the foundation of field geophysical exploration and data processing and interpretation of gas hydrate in the Qilian Mountain Permafrost.

水合物是一种潜力巨大的非常规油气资源，受到世界各国越来越多的关注，是当前国内外的研究热点。祁连山冻土区发现的水合物相比国外更为复杂，存在孔隙型和裂缝型水合物，由于裂缝型水合物储层呈现各向异性特征，使得地球物理勘探方法在其地球物理响应探测与识别方面遇到困难，而基于岩石物理实验的数值模拟可有效解决这一难题。本研究拟收集祁连山冻土区典型裂缝型水合物储层的岩样资料，开展声学性质、核磁共振、X-衍射等岩石物理实验，获取岩样的纵横波速度、孔渗、矿物组分等信息，用以指导方位AVO、声波测井方法正演模型的构建；基于岩石弹性理论及介质模型，数值模拟计算各种裂缝参数变化时的方位AVO响应规律、储层识别敏感因子、各种裂缝参数对水合物饱和度变化特征的影响规律，并与岩石物理实验结果进行对比验证。本研究可为冻土区水合物野外地球物理勘查和实测资料的处理与解释提供技术支撑。

水合物是一种储量巨大、洁净的新型替代能源，我国已在祁连山冻土区发现并钻获了实物样品。与国外极地冻土区发现的水合物相比，祁连山冻土区水合物成藏规律和控制因素更为复杂，使得野外地球物理勘探方法遇到了困难，而基于岩石物理的数值模拟是一种研究水合物储层地球物理响应特征的有效手段。本文依据祁连山冻土区水合物的实际赋存地质条件，基于不同的地球物理方法进行建模，并利用建立的模型开展数值模拟，对水合物储层的地球物理响应特征进行了探索。利用基于岩石物理模型的等效介质理论建立的正演模拟速度模型和射线追踪理论，对水合物储层孔隙度、层厚、水合物饱和度变化时的地震反射波振幅随偏移距变化(AVO)的响应特征进行模拟，结果表明孔隙度变化对AVO属性的分区较好，水合物饱和度变化对AVO属性的区分空间较小，层厚变化与AVO属性呈线性关系；由水合物储层水合物饱和度和岩性的变化对不同弹性参数敏感性的模拟结果可知，μ-E交会图识别水合物储层效果良好，μ和λ/μ进行交会可区分水合物储层岩性。基于两端元层状介质模型对裂缝充填型水合物储层的声波速度和水合物饱和度特征进行模拟，研究区钻孔地层裂缝内充填水合物体积分数的变化对纵横波速度影响较大，而裂缝倾角的变化对纵横波速度影响较小，可将利用两端元层状介质模型正演模拟得到的纵波相速度直接转换为纵波群速度；水合物赋存井段地层的水合物饱和度变化范围为14.1%-89.9%，平均值为69.4%，与现场观测结果接近。研究结果可为祁连山冻土区水合物地层测井评价与地震勘探提供理论依据和技术支撑。