干热岩注冷水诱发储层围岩损伤破裂演化的机理研究

The reservoir secondary damage and permeability evolution induced by long-term cold water injection are two key problems in the development of hot dry rock resource. However, their mechanisms are not clear and lab test is still blank. In this project, the rock cold damage mechanism (RCDM) is first investigated according to the route of theoretical analysis, lab test, model establishment and numerical simulation. The research includes: (1) studying the method of establishing rock microstructure digital mode (RMDM) based on the results of CT scan; establishing the RMDM before and after the experiment; (2) designing large scale model test to investigate the influence of different temperature difference and injection flow rate on the RCDM and analyze the mechanism; (3) establishing the constitutive model and strength criterion of RCDM based on the test results and some related theories; (4) developing the data interface of the non-uniform mapping between the RMDM and finite model; developing the RCDM numerical procedure and correcting it. Finally, numerical test of RCDM is carried out to analyze the rock permeability evolution under different injection parameters. The results of this project can provide scientific theoretical basis for controlling the reservoir scale, permeability evolution and extending life time of hot dry rock project.

长期注冷水诱发储层二次破裂及渗透率演化是干热岩开发中的关键问题，但目前对其诱发致裂机理尚不清晰且室内试验较为缺乏。本项目首次按照“理论分析→室内试验→建立模型→数值模拟”的路线开展了岩石冷损伤破裂机理的系统性研究。包括：(1)研究基于CT扫描结果表征岩石细观结构数字模型的方法，建立试验前后岩样细观结构数字模型；(2)设计大尺寸物理模型试验，研究不同水岩温差、注入流速对岩石冷损伤破裂的影响并进行机理分析；(3)基于试验结果结合损伤力学、热力学和热固耦合等理论，建立岩石冷损伤破裂细观本构模型及强度准则；(4)开发细观结构数字模型与有限元模型之间非均匀性映射数据接口，编制冷损伤破裂数值程序，对比试验及经典算例，验证程序正确性并完善。最后，开展岩石冷损伤破裂特性的数值试验研究，分析不同注入条件下岩石渗透性的演化规律。本项目研究结果可为控制干热岩储层规模、渗透率演化和延长工程寿命提供科学的理论依据。

长期注冷水诱发储层二次破裂及渗透率演化是干热岩开发中的关键问题，但目前对其诱发致裂机理尚不清晰。针对该问题，本项目从室内试验（细观）角度，进行了高温花岗岩注冷水诱发破裂及渗透率变化试验研究；从场地模拟（宏观）角度，采用编制的损伤破裂程序进行干热岩开采关键参数敏感性数值分析。高温花岗岩注冷水破裂及渗透率变化试验研究结果发现：25~500℃温度范围内，试样的物理力学性质随温度上升而持续劣化。500℃急剧冷却条件下，由温度梯度诱发的热应力最大。升温使花岗岩内形成大量沿晶、晶内及穿晶裂纹，导致其渗透性增强。缓慢冷却条件下，花岗岩热破裂主要由矿物间的热膨胀各向异性所引起；急剧冷却条件下，花岗岩会形成由温度梯度诱发的热破裂。干热岩开采关键参数敏感性数值分析结果发现：储层渗透率对储层流阻影响最大；储层温度越高则出水温度保持稳定的时间越长；注入流速主要影响出水温度；注水温度增量对出水温度增量影响很小，应尽可能将抽水段设置高于注水段。以贵德盆地为例，储层温度应选200℃处，储层渗透率应被改造至10-12 m2，注水温度应选30℃，若注入流速50kg/s，则发电量为0.98~3.76MW。本项目研究结果补充了高温热岩体冷损伤破裂演化理论，对于未来超高温地热储层开发中的二次损伤、储层动态变化及漏失等工程问题具有一定理论支撑。另外，本项目基于真实场地数据建模，开展了关键参数的敏感性分析，给出了干热岩从理论到应用之间的途径，对于我国未来干热岩工程压裂及水流循环设计具有指导意义。