Design Methodology of Artificial Crack -Like Reservoir for HDR Geothermal Energy Extraction ( T-Project )

该项目的目的是开发用于从干热岩石中提取地热能的人工地下裂缝的设计方法。该方法的基础是岩石断裂力学。该项目有三个研究领域；[T]岩石力学特性评价，[M]地下裂缝监测与制图，[S]地下裂缝模拟。它包括三个阶段，即第一阶段、第二阶段和第三阶段。在第一阶段，通过水力压裂对0.5 m ~ 10 m不同尺寸的岩石立方试件进行岩石破裂力学验证。二期在东八町台建立了示范田。由于大量水力压裂在地表以下近350 m处形成了一个巨大的地下裂缝，通过人工裂缝成功连接了两口井。这个地下循环系统是世界上第一个按设计建造的系统。在第三阶段，研究了地下地热条件（如温度和围压）对岩石破裂的影响，包括水与岩石之间的力学化学相互作用。通过这三个阶段，最终建立了地下储层裂缝的设计方法。每个研究领域的主要成果可以列出如下。[T]：阐明了岩石裂纹扩展的过程和机理以及岩石断裂的尺寸效应，开发了岩石断裂韧性测试的新方法。[M]：开发了井下声发射测量系统的硬件和软件。建立了利用系统监测地下裂缝的新方法。[5]：开发了用于分析多种地下地热条件下裂缝行为的模拟器，成功建立了三维应力场的原位构造应力测量方法。

The purpose of the project is for the development of the design methodology of artificial subsurface cracks for the extraction of geothermal energy from hot dry rocks. The basis of the approach is rock fracture mechanics. The project has three research areas; [T] evaluation of rock mechanical properties, [M] monitoring and mapping of subsurface cracks and [S] simulation of subsurface cracks. It consists of three phases, i.e., Phase I, II and III. In Phase I, the validation of rock fracture mechanics was made through hydraulic fracturings of rock cubic specimens with a variety of sizes from 0.5 m up to 10 m. In Phase II, a model field was set up at Higashihachimantai. A large subsurface crack was created by massive hydraulic fracturings nearly 350 m below the ground surface and two wells were successfully connected through the artificial crack. This subsurface circulation system is the first system in the world constructed as designed. In Phase III, effects of geothermal subsurface conditions, such as temperature and confining pressure, on rock fracture were investigated, including mechanochemical interactions berween water and rocks. Through these three phases, the design methodology for creating subsurface reservoir cracks was finally established. The main results of each research area can be listed as in the following. [T]: The process and the mechanism of crack growth in rocks and the size effect of rock fracture were clarified and new methods were developed for fracture toughness tests of rocks. [M]: Hardware and software of downhole AE measurement systems were developed. New methodologies utilizing the systems for monitoring of subsurface cracks were established. [S]: Simulators were developed for analyzing the crack behavior under a variety of geothermal subsurface conditions and an in-situ tectonic stress measurement method was successfully established for three-dimensional stress fields.