基于暂堵转向的干热岩水力压裂机理研究

Hydraulic fracturing technology with temporary plugging and diversion is a new trend to improve the thermal recovery efficiency of hot dry rock. The effect of high temperature and high pressure increases the plasticity of hot dry rock, and the influence of heat flow on the diversion propagation of the fracture with temporary plugging agent cannot be ignored. Thus, under the effect of THM (thermal-hydraulic-mechanical) coupling, it will cause a more complex mechanism of fracture reorientation in hot dry rock. Due to this challenge, this project will carry out the following investigations. A real crack surface is prefabricated by 3D printing technique. Then by conducting the experiment with high-speed camera monitoring, the characteristics of temporary plugging and fracture mechanics is acquired. By the use of these experimental results, a mathematical model of diverting fracturing of hot dry rock, that considers the THM damage constitutive relationship and temporary plugging characteristics, is established. After this, an efficient PGD (proper generalized decomposition) algorithm, which is based on the idea of the separation of time and space domain to reduce its dimensionality, is established. According to the mathematical model and the efficient algorithm, the quantitative relationship between the complexity index of diversion cracks (which is represented by the deep learning algorithm to describe the topology of fracture network nodes) and other factors (such as heat flow, fluid pressure and in-situ stress) is obtained. Finally, the propagation path of diversion fracture is investigated. According these three research results, the mechanism of fracture reorientation under the effect THM coupling is finally revealed, which will lay a theoretical and technical foundation for the efficient development of dry hot rocks.

暂堵转向压裂技术是提高干热岩采热效率的发展趋势之一，高温高压作用使干热岩塑性增强，热流对含暂堵剂裂缝转向扩展影响不可忽略，导致热流固多场耦合作用下干热岩人工裂缝转向扩展机理更为复杂。本项目针对这一难题，开展如下研究：采用3D打印预制真实裂缝表面，通过高速摄像机使实验可视化，查明热流固多场耦合作用下干热岩裂缝暂堵特性和含暂堵剂裂隙岩体的断裂力学特征；利用暂堵特性和断裂力学实验结果，建立考虑热流固耦合损伤断裂本构关系及暂堵特性的干热岩转向压裂数学模型，并构建时空分离降维的PGD高效求解算法；根据所构建模型及高效算法，基于深度学习算法描述转向裂缝节点拓扑结构，进而表征裂缝复杂程度，得出转向裂缝复杂指数与热流、水压和应力等因素间的定量关系，研究干热岩人工裂缝转向规律。通过研究揭示热流固耦合作用下干热岩暂堵转向压裂机理，为高效开发干热岩奠定理论和技术基础。

暂堵转向压裂技术是提高干热岩采热效率的发展趋势之一，高温高压作用使干热岩塑性增强，热流对含暂堵剂裂缝转向扩展影响不可忽略，导致热流固多场耦合作用下干热岩人工裂缝转向扩展机理更为复杂。据此，本项目首先实验研究了干热岩裂缝暂堵特性和含暂堵剂裂隙岩体的断裂力学特征，然后建立了考虑热流固耦合损伤断裂本构关系及暂堵特性的干热岩转向压裂数学模型，并构建时空分离降维的PGD高效求解算法。最后通过数值模拟研究，本项目研究取得的主要结论如下：（1）温度升高干热岩塑性逐渐增强而强度降低，高温作用产生的热应力促使干热岩内部微裂纹的产生，随着温度逐渐升高，在加载前期即可产生较多的声发射事件；同时，高温作用下干热岩断裂韧性逐渐降低，温度超过600℃，断裂韧性仅为常温状态下22.4%。（2）暂堵剂在干热岩复杂分支裂缝内分布的主要因素包括裂缝宽度、注入速率、温度及裂缝开口角度等因素；分支裂缝处流体强湍流现象有助于主裂缝内暂堵剂向分支裂缝内运动，裂缝壁面侧部位颗粒基本受湍流影响进入分支裂缝，而裂缝中心部位颗粒在主裂缝内向前流动。（3）高温条件下，干热岩人工裂缝暂堵过程中会出现明显的压力波动，温度升高，压力波动次数增多；同时，缝内高压作用下突破封堵的次数与压力波动存在明显的关系。（4）发展了基于生死弹簧单元的暂堵本构模型，构建了干热岩变形、基岩渗流、裂隙流完全耦合的三维有限元计算模型，数值模拟结果表明：影响暂堵转向压裂的主要因素为：天然裂缝拉伸与剪切强度、地应力各向异性和暂堵剂封堵位置。通过本研究揭示了热流固耦合作用下干热岩暂堵转向压裂机理，为高效开发干热岩奠定理论和技术基础。