超临界CO2强化采热中的流动换热及其诱发断裂滑动机制研究

Hot dry rock geothermal resource is one of the strategic energy because it is clean, renewable and has great potential for exploration. Supercritical CO2 enhanced geothermal system (CO2-EGS), which not only can improve the efficiency of heat extraction but also can storage CO2, is considered promising. One of the difficulties of this technology is to balance the efficiency of heat transfer and induction of fault slip even earthquakes. So it is critical to have a full understanding of the heat transfer characteristics and the regime of fault slip in CO2-EGS, but unfortunately, there is a lack of relative experimental work. This study aims at exploring the heat transfer characteristics of supercritical CO2 flow through fractures with various shapes and different apertures under high temperature and high pressure, based on bulk cylindrical granite specimen with a diameter of 100mm and a length of 200mm. The effects of proppant and its laying ways will also be studied. Next, numerical models of flow and heat transfer will be developed for the analysis of local and lumped heat transfer coefficients. Then, it is possible to propose a new model to predict heat transfer coefficients. Another objective of this study is to find out the influence of supercritical CO2 on the constitutive relations and strength characteristics of granite fractures and thus the constitutive models and sliding failure criterions of fractures can be obtained according to large scale triaxial experiments under high temperature and high pressure. Additionally, experiments will be conducted to study the disturbance effects of supercritical CO2 heat transfer and cooling on the fractures. Finally, an integrated research is to be made to investigate the flow and heat extraction process of supercritical CO2 and its perturbation regime of fault slip based on the in-situ simulation experiment platform established. The results of this research are expected to provide scientific basis for the design and operation of relative engineering.

干热岩地热资源潜力大、绿色可再生，是一项战略能源。超临界CO2强化地热开发(CO2-EGS)不仅可望提高综合采热效果而且能够封存CO2，前景广阔。该技术的难点之一是平衡提高换热效率与诱发断裂滑动甚至地震之间的矛盾。充分理解超临界CO2强化采热中的流动换热规律及诱发断裂滑动的机制是关键，但相关的实验研究十分缺乏。本研究通过高温高压裂隙流动换热实验研究超临界CO2在各种形状、开度裂隙中的流动换热规律，研究支撑剂及其铺设的影响；开发流动换热数值模型，分析局部与整体换热系数特征，提出新的换热系数预测模型；通过高温高压大三轴实验，研究超临界CO2作用下花岗岩裂隙本构与强度特性，得到本构模型及滑动破坏准则；实验研究超临界CO2换热冷却断裂扰滑效应；搭建CO2-EGS原位模拟实验平台，综合研究超临界CO2流动采热过程及其冷却对断层滑动的扰动机制。最后进行现场应用。研究成果可望为相关工程设计运行提供依据。

大力开发干热岩(HDR/EGS)地热资源对助力我国“双碳”目标的实现具有重大意义。本项目以CO2-EGS地热开发与诱发地震为工程背景，聚焦流动换热和扰动问题攻关。研制了CO2-EGS原位模拟实验平台、大尺寸试样裂隙流动换热等装置；系统开展了各种条件下流体（含scCO2）在干热岩裂隙中的流动换热试验、理论和数值模拟研究；开展了流体（含scCO2）作用下岩体（含花岗岩）及裂隙的力学特性研究及流体注入扰动试验研究；以Soultz-sous-Forêts和福建漳州场地为背景开展了初步应用研究。发现了现有换热系数公式出现数值振荡甚至负值的问题；提出了稳定化换热系数（OHTC）新公式；基于格林函数法提出推导换热系数的解析方法及若干具体公式；提出具有客观性的数值换热系数新概念；基于局部换热系数，表征揭示了裂隙开度、流体类型及相行为、裂隙粗糙度、支撑剂对干热岩裂隙流动换热的影响；发现试验中支撑剂类型及岩体外壁面温度对scCO2采热量影响较小；支撑剂有利于提高scCO2的采热量。得到了含scCO2花岗岩应力-温度试验拟合关系；发现不同控制条件下裂隙流体扰滑特征；提出了超临界CO2作用下花岗岩裂隙改进的Rate-State扰滑模型。建立了CO2-EGS耦合扰动模拟方法，还取得其他有价值结果。研究获得了国内外大量引用，有效促进了流动换热和扰动关键科学问题研究取得进展，部分研究成果有明确的应用价值。