Abstract In geothermal reservoirs, coupled thermal-hydrological-mechanical-chemical processes lead to gradual closure of fractures and consequent production decline. The objective of this study was to investigate the effects of injected fluid type on the evolution of fracture aperture/permeability at various stress levels through a series of flow-through experiments on a fractured phyllite specimen, retrieved from a production well in the Blue Mountain geothermal field. The injected fluids included deionized water, silica super- and under-saturated fluids, and the geothermal fluid extracted from the Blue Mountain geothermal field. It was found that injection of geothermal and silica super-saturated fluids led to lowest and highest permeability reduction, respectively. In addition, the degree of permeability recovery was lower in the experiment using geothermal fluid compared to the experiment using silica super-saturated fluid. On the other hand, chemical analysis of the effluent samples revealed that injection of deionized water resulted in dissolution of feldspar and quartz, while precipitation of silica occurred in the experiment using geothermal fluid. Post-test observation by scanning electron microscopy of fracture surface area in the test using silica super-saturated fluid indicated some degree of mineral precipitation. A comparison between pre- and post-test computer tomography scan images for the experiment with injected silica super-saturated showed that mostly decrease in fracture aperture occurred, with increase in fracture aperture in a few regions along the fracture surface. Finally, the Three-Element rheological model successfully predicted the fracture permeability decay. The results of this study suggested that precipitation of silica, a major problem in permeability loss of geothermal systems, can be potentially minimized by using an injection fluid that is closer to chemical equilibrium state with the host rock.

中文翻译：

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注入流体类型对地热储层裂隙岩压力相关渗透率演化的影响：实验化学力学研究

摘要 在地热储层中，热-水文-机械-化学耦合过程导致裂缝逐渐闭合，进而导致产量下降。本研究的目的是通过对从蓝山地热生产井中提取的裂隙千枚岩试样进行一系列流通实验，研究注入流体类型对不同应力水平下裂缝孔径/渗透率演变的影响。场地。注入的流体包括去离子水、二氧化硅超饱和和欠饱和流体，以及从蓝山地热田提取的地热流体。结果表明，地热和二氧化硅超饱和流体的注入分别导致最低和最高渗透率降低。此外，与使用二氧化硅过饱和流体的实验相比，地热流体实验的渗透率恢复程度较低。另一方面，流出物样品的化学分析表明，注入去离子水导致长石和石英溶解，而在使用地热流体的实验中发生了二氧化硅的沉淀。在使用二氧化硅过饱和流体的测试中，通过扫描电子显微镜对断裂表面积进行的测试后观察表明存在一定程度的矿物沉淀。对注入超饱和二氧化硅实验前和测试后计算机断层扫描图像的比较表明，大部分发生裂缝孔径减小，沿裂缝表面的少数区域裂缝孔径增加。最后，三元素流变模型成功预测了裂缝渗透率衰减。这项研究的结果表明，二氧化硅的沉淀是地热系统渗透率损失的一个主要问题，可以通过使用更接近与主岩化学平衡状态的注入流体来最小化。