页岩层水力压裂控制缝网的理论、计算和实验研究

Shale gas is unconventional natural gas, which is stored in shale in free or absorbed forms and sometimes in free fluid phase. The exploitation of shale gas has become a promising field of green energy development in China. Although great success has been achieved in shale gas revolution in North America with the technique of hydraulic fracturing, only 5~15% of the stored oil and gas could be exploited. In order to improve the production of shale oil and gas, a huge challenge accompanied with great opportunity is presented to the mechanics researchers. Based on the national major energy demand and basic mechanical problems in high-efficient exploitation of shale oil and gas, this project explores the mechanical mechanism and control of complex fracture network in hydraulic fracturing in theory, numerical simulation and experiments. The experimental platform for large objects model will be established and numerical simulation method for hydraulic fracture propagation in shale will be developed. Time-dependent anisotropic Biot constitutive model will be developed in the research. The theory for fracture initiation, branching and interaction of multiple cracks will be discussed. Multi-scale models for steadily extending fracture network and representative damage volume element will be established. A numerical simulation software for modeling hydraulic fracture network will be developed, in which both fracture mechanics and fluid mechanics can be considered. This project is with great significance and innovation and will lay a solid foundation for physical experiment model, fracture mechanics theory and numerical simulation techniques for effective exploitation of shale gas in China.

页岩气是指以吸附和游离时而还有流体相的形态状态赋存于泥页岩中的非常规天然气。页岩气开采成为我国绿色能源开发的新领域。尽管北美页岩气革命取得了成功，水力压裂是成功的开采方式，目前仅有预期产量5~15%的油气开采出来，问题出在哪里呢？由此给力学家提出了巨大的挑战和机遇。本项目结合国家重大能源需求，立足我国页岩油气高效开发中的基础力学问题，通过理论、计算和实验研究页岩水力压裂过程中复杂缝网的形成和力学控制机理。研究内容包含发展页岩人工裂缝扩展的大型物理模型实验平台和模拟方法，建立考虑时间相关性的各向异性Biot本构，建立页岩起裂、分叉及多裂缝相互作用的断裂力学理论，建立裂缝稳定性扩展创造缝网的多尺度有限元模型和代表性损伤体积单元，发展耦合断裂力学和流体力学的裂缝网扩展数值模拟软件。项目立项科学意义重大、具有创新性，将为我国页岩气的高效开发奠定坚实的物理模型实验、断裂力学理论和数值模拟技术的基础。

本项目严格执行计划内容和进度，在多孔充液弹性介质Biot本构模型、水力裂缝扩展和止裂条件、可压裂度理论模型、基于扩展有限元的流固耦合计算程序、大型物理模型试验技术、现场施工技术等方面高质量地完成了项目计划研究工作。2015年立项时确定了5项研究任务和设定的研究目标：1）页岩人工裂缝扩展的大物模实验平台和模拟；2）建立时间相关性和各向异性Biot本构模型；3）页岩起裂、分叉及多裂缝相互作用的力学准则和模拟方法；4）裂缝族稳定性扩展的多尺度有限元模型；5）耦合断裂力学和流场压力的缝网扩展模拟方法。目前上述5项研究任务均已圆满完成，并且增加了另外计划外研究内容：1）提出了水力压裂一致性条件（准则），评估裂缝簇稳定扩展状态；2）提出可压裂度理论模型，设计体积改造裂缝簇间距，指导现场韧性或者黏性压裂施工；3）发展数据驱动方法预测页岩气采收率。项目组主要成员多次赴重庆涪陵页岩气现场考察，向施工一线的工程师学习，取得了第一手工程数据和经验。因此，项目工作的理论研究紧密结合现场实际，使得本项目超预期地完成了研究目标。