干热岩超临界CO2压裂的缝网扩展机理和多场耦合数值模型研究

Supercritical CO2 fracturing in hot dry rock geothermal has been paid more attention, since it has many advantages, such as low break-down pressure and easily forming a large and self-propped fracture network volume. The main problem is to predict the development of the fracture system and the created conductivity because there exists a natural fracture system in the reservoir caused by high tectonic stress causing shear propagation, and the short and long term flow behaviors of the supercritical CO2 are complex under such geological conditions with high stress, high fluid pressure, high temperature and complex loading-unloading path. Thus it is necessary to study the mechanism of supercritical CO2 fracturing in HDR geothermal within natural fracture system, and to establish a reasonable numerical model for prediction. The aim of the applied project is to experimentally study the propagation of single and cross natural fractures under shear stress during supercritical CO2 fracturing, the thermal-hydraulic-mechanic coupling effects of the HDR rock fractures under deep geological conditions and the flow characteristics of supercritical CO2 through HDR rock fractures. According to the experimental results and analysis, a numerical model for supercritical CO2 fracturing will be established, and a software with independent intellectual property will be developed. Through in-situ large scale simulation by use of the model, the fracturing failure mode and its spatial distribution, the temporal and spatial development of the fracture network, as well as the released energy during the supercritical CO2 fracturing will be studied, which can be used for safe and efficient development of the HDR geothermal.

超临界C02压裂起裂压力低，容易形成大范围的剪切自支撑体积缝网，已成为强化干热岩地热开采研究的热点之一。由于深部构造应力大，干热岩内部存在宏观的天然交叉裂缝系统，压裂扩展破坏以剪切为主，且在深部高地应力、高流体压力、高温、时间和复杂应力加卸载路径作用下超临界CO2在岩石裂缝内的流动规律更加复杂，导致裂缝系统的发展和压裂效果难以预测，所以有必要对其机理进行研究，建立合理、逻辑和科学的数值预测模型。因此，本项目拟通过实验探索干热岩剪切及交叉裂缝在超临界CO2驱动下的起裂扩展机理，岩石裂缝在干热岩地质条件下的热流固耦合行为和超临界CO2在岩石裂缝内的流动特性；建立含天然裂缝的干热岩超临界CO2压裂多场耦合数值模型，形成具有自主知识产权的软件体系；揭示现场条件下含天然裂缝干热岩超临界CO2压裂裂缝的破坏形态和分布特征、时空演化规律以及压裂过程中岩体的能量释放规律，指导干热岩地热的安全和高效开发。

水力压裂技术是干热岩地热高效开采的核心，我国水资源匮乏，以水基压裂为主的开发模式在我国必定不可持续，因此，超临界CO2作为非水基压裂液已成为强化干热岩地热开采研究的热点之一。天然裂缝和超临界CO2作用下干热岩压裂裂缝网络扩展机制和多场耦合作用下裂缝导流演化规律是研究超临界CO2强化干热岩开采的关键。因此，本项目通过理论分析、室内实验、数值建模和数值模拟等多种手段，研究了超临界CO2作用时间对干热岩矿物组成和物性的影响规律，对比分析了超临界CO2和常规压裂裂缝扩展的差异，探究了天然裂缝对干热岩压裂裂缝扩展的影响机制，明确了应力、流体压力、加卸载路径，张拉与剪切不同造缝机制、加载时间、超临界CO2作用下干热岩裂缝导流演化规律，揭示了裂缝细观结构演化及其对导流能力影响规律，建立了干热岩超临界CO2压裂缝网扩展多场耦合数学模型，基于分布耦合和并行计算原理研发了高效数值求解方法和计算程序，通过工程数值模拟研究明确了天然裂缝和超临界CO2作用下裂缝网络的扩展规律。通过本项目研究表明，超临界CO2在压裂时间尺度对干热岩物性影响较小，可忽略，其对裂缝起裂、扩展的主要影响来自超临界CO2的低黏性。和常规压裂相比，超临界CO2更容易进入裂缝尖端、以及起裂部位和裂缝尖端部位附近的孔喉,且裂缝内的流体压力较低，从而降低裂缝尖端的有效应力以及含压裂缝的屏蔽效应，但同时由于其高滤失性降低了压裂的能量，具有双面性。在低滤失条件下，相较于传统压裂，干热岩超临界CO2压裂能更容易打开天然裂缝，形成更加复杂和范围更大的裂缝网络，提升压裂改造的效果。在高滤失条件下，需提升排量来补充滤失引起的能量损失。上述结果为超临界CO2强化干热岩开采提供了理论支持。通过本项目建立的裂隙岩体多场耦合高效数值计算程序可用于干热岩储层改造和热采产能方面的设计和优化研究，同时在煤炭安全开采、非常规油气储层改造等地下工程领域具有广泛的应用前景，为进一步开发科学计算工具和商业软件提供重要的技术支撑。