热应力与超临界CO2协同作用下干热岩裂缝扩展机理及传热规律研究

Reservoir stimulation through hydraulic fracturing and efficient extraction of thermal energy are the key issues to geothermal exploitation of dry-hot rock. Super critical CO2 is supposed to have widely application in exploitation of deep geothermal energy, as it is possible to reduce the break down pressure, to form a complex fracture network and to enhance the heat recovery by use of supercritical CO2 as the fracturing and heat transfer medium. However, the study on fracture propagation mechanism under combined effect of thermal stress and CO2 state, and supercritical CO2 flow and heat transfer through rock fractures are limited. In this project, similarity simulation experiment of supercritical CO2 fracturing will be conducted to study the fracture initiation and propagation behavior in granite under high temperature, and to find the mechanism to form a complex fracture network by use supercritical CO2. Based on the experimental results, a propagation criterion will be established considering the combined effect of thermal stress and CO2 state. Then, the influence of the supercritical CO2 on pore structure and mechanical behavior of granite will be experimentally studied. Finally, an experiment will be carried out to study and to develop the supercritical CO2 flow and heat transfer model through rock fractures under coupled temperature and stress conditions. The research results of this project can provide an important theoretical support for the application of supercritical CO2 hot dry rock to enhance extraction of deep geothermal energy.

储层压裂改造和热能高效提取是干热岩地热开采的关键，超临界CO2作为压裂和传热介质具有起裂压力低、易形成复杂缝网、采热率高等优点，在干热岩开采方面具有广阔应用前景。但目前关于压裂液与储层温差产生的热应力以及超临界CO2协同作用下干热岩裂缝的扩展机理、多场耦合条件下干热岩裂缝的导流传热规律还尚未明确。因此，本项目拟通过相似模拟试验、理论分析相结合的方法，探索高温干热岩裂缝在超临界CO2驱动下的起裂扩展规律，建立热应力与超临界CO2协同作用下干热岩裂缝扩展模型，揭示干热岩复杂缝网形成机理；明确超临界CO2对干热岩孔隙特征、力学特性的影响，建立多场耦合条件下干热岩裂缝导流传热模型，揭示不同应力、温度等条件下超临界CO2在干热岩裂缝中的导流传热规律。该项目研究成果可为超临界CO2在干热岩开采技术中的应用提供重要理论支撑。

干热岩是一种稳定的清洁可再生能源，利用效率高，可提供相当于全球所有石油、 天然气和煤炭储量30倍的能源。利用超临界CO2开采干热岩地热，具有起裂压力低、采热率搞等优点。目前关于超临界CO2对干热岩孔隙、力学特性影响以及多场耦合条件下干热岩裂缝的导流传热规律还尚未明确。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。明确了超临界CO2对岩石裂缝导流能力的影响，建立了多场耦合下干热岩裂缝导流模型，并采用数值模拟方法对现场干热岩热能开采进行了模拟。结果表明，超临界二氧化碳与花岗岩相互作用后，其微观结构和力学性质会发生明显的劣化。超临界CO2浸泡后的干热岩，在破坏过程中断裂机制的变化主要是由其微观孔隙结构的变化引起的。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。通过不同应力条件下的裂缝中超临界CO2导流实验研究结果表明，裂缝渗透率随应力的增加而降低，到40MPa其渗透率将减少1-2个数量级。根据裂缝集群特征建立了多场耦合条件下干热岩裂缝导流模型。数值模拟结果显示裂缝带内注入的冷流体可以横向穿透，增加传热面积。流体在浅层温度较低的寄主岩石中失去了焓，切换井位不会提高预期的热能提取效率。