软硬间互地层水力压裂裂缝三维延伸机理研究

In this research, as an attempt to improve the understanding of hydraulic fracturing forecasts and control in the reservoir developing engineering within interbedded soft and hard strata, physical tests (including laboratory tests, well logging and microseismic monitoring in hydraulic fracturing project sites) and numerical simulation are employed to carry out the following research. (1) The hydraulic fracturing tests in rock samples and physically equivalent material model will be conducted to obtain the basic principle of hydraulic fracture initiation and propagation in interbedded soft and hard strata. (2) Based on the correlation among background stress, hydraulic fracture development, and microseismic information, a new method combining microseismic and numerical simulation will be proposed to finely characterize hydraulic fractures. The full-scale and three-dimensional numerical model of two engineering projects will be constructed to investigate the hydraulic fracture initiation, direction and rate of expansion, and spatial scales. The well logging and microseismic data will be compared and analyzed to improve the hydraculic fracture simulation and prediction of reliability. (3) The accommodated conditions and applied scope of the proposed method in this research will be illustrated. Combining the tests of physically equivalent material model and numerical simulation, the propagation trajectory, including penetration, turning, branching, and passivation or termination, of the hydraulic fractures approaching interbedded soft and hard strata, and the function of natural existing fractures and bedding interface will be investigated. Simultaneously, the interference mechanism in multiple hydraulic fractures and rock layers will be investigated. It is expected that the research result could provide some guides for the design and evaluation of hydraulic fracturing engineering in interbedded soft and hard strata.

本项目拟通过试验（包括室内试验、水力压裂工程现场测井与微地震监测）和数值计算的相结合，以软硬间互储层水力压裂开发工程中的压裂裂缝形态预测与控制为背景：(1)开展室内岩样及相似材料模型的压裂试验，获得软硬间互条件下岩层压裂裂缝起裂与扩展的基本规律；(2)结合压裂过程中背景应力场、压裂裂缝孕育与发展、微地震信息之间的内因影响和联系，建立基于微地震与数值模拟的水力压裂裂缝精细表征方法，并构建2个实际依托工程的全尺度真三维数值模型，分析压裂裂缝的起裂、扩展方向及速率、空间规模等，并与现场测井及微地震数据进行互馈分析，提高压裂裂缝模拟与预测的可靠性；(3)界定所提出方法的适用条件和适用范围，结合相似材料模型试验和数值模拟，揭示压裂裂缝遇到软硬相间层、天然裂隙、层理时穿层、扭转、分叉、钝化或止裂等非平面扩展机理，以及缝间、层间干扰机制。尝试为软硬间互地层水力压裂工程的设计与压裂效果评价提供参考依据。

软硬间互地层不但广泛发育于坝址边坡、隧洞围岩，而且在地热、油气资源开发工程中常有涉及。受控于地应力及地层岩性的复杂性，软硬间互地层中水力压裂裂缝的三维扩展路径复杂。本项目通过实验和数值计算相结合的方法，以软硬间互储层水力压裂开发工程中的压裂裂缝形态预测与控制为背景，首先开展了室内岩样及相似材料模型的实验，获得了基本岩石力学参数及压裂裂缝起裂与扩展的基本规律；其次结合并行有限元技术及矩张量反演技术，建立了基于数值模拟与微地震的水力压裂裂缝精细表征方法；接着研究了软硬间互地层岩心的脆性特征及其对压裂裂缝的影响、研究了软硬间互地层模量、泊松比、渗透率及层厚对压裂裂缝扩展过程中应力演化的影响、研究了软硬间互地层水力压裂裂缝遇到软弱层及层理的扩展路径；最后构建了工程全尺度真三维数值模型，分析了压裂裂缝的起裂、扩展方向及速率、空间规模等，并与现场测井及微地震数据进行了互馈分析。经研究发现，软弱隔层的脆性指数如果下降20%，可有效封隔水力压裂裂缝的扩展；水力压裂裂缝遇到软硬相间层和层理时，有穿层、扭转、分叉、钝化、止裂等5种基本扩展模式；除了岩性之外，层厚、施工排量及压裂液粘度对裂缝类型及裂缝复杂性均具有重要影响。本项目提出的研究方法有一定的适用性，对于细观尺度水力压裂裂缝可实现完全定量化分析；对于工程尺度模型，受控于参数来源、计算规模及结果验证等因素影响，可实现近似量化分析。本项目的研究成果丰富了水力压裂裂缝扩展分析的研究方法和理论，为水力压裂工程的设计与压裂效果评价提供了进一步的理论和技术支撑。