High-resolution analysis of seismo-thermo-hydro-mechanical processes in fractured rocks during hydraulic stimulation: model development, validation and application

增强型地热系统(EGS)旨在通过注入和生产井眼之间的循环流体来提取储存在地壳中的热量。理想的条件通常出现在深度为2-5公里的地层中，那里的流量不足以满足商业地热发电厂的需要，而且温度很高(即&gt；&gt；100°C)。因此，高压注液，称为水力刺激，是一种常用的技术，以产生连通的裂缝网络，促进流体循环。水力刺激通常伴随着诱发的地震活动，公众可能会感受到这种活动，甚至会造成破坏。该项目的目的是提供对裂隙岩石诱发地震活动的基本了解，以提高预测和控制地震风险的能力。该项目采用了地震活动受断裂网络几何形状和地质系统中激活的热-水-力(THM)过程共同控制的假设。我们将应用离散裂隙网络(DFN)来描述结构面，并以高分辨率模拟THM过程。该项目利用最近在瑞士Grimsel试验场进行的小规模(分米)模拟试验的数据集和最先进的数值模型，以实现以下目的：1)测试高分辨率模型的有效性，以捕捉通过小规模试验观察到的地震、水力和力学过程；2)将裂缝网络的几何属性(如断裂强度、连通性、长度和空间分布)与诱发地震活动的空间、时间和震级分布联系起来；3)提出并测试一个新的最大可能震级预测模型，该模型考虑了在场地特定的地质条件和操作约束下主导的多物理过程的联合影响；4)评估小尺度(分米)高分辨率DFN模式的升级，以模拟水库尺度(公里)的实验。这一研究项目在利用高分辨率的基于物理的模型和来自独特的现场实验的高质量数据集来处理注入诱发的地震活动方面是新颖的。这项拟议的研究对促进向可再生能源供应的过渡政策具有重要意义，并有助于增进我们对诱发地震触发机制的了解。

Enhanced Geothermal Systems (EGS) aim at extracting the heat stored in the earth’s crust by circulating fluids between injection and production boreholes. Ideal conditions are typically found in formations at depths between 2-5 km, where the flow rate is not sufficient for commercial geothermal plants and where temperatures are high (i.e. >>100°C). Hence, high-pressure fluid injection, known as hydraulic stimulation, is a commonly adopted technique to generate a connected fracture network that facilitates the fluid circulation. Hydraulic stimulation is typically accompanied by induced seismicity that may be felt by the public and even cause damages. The objective of this project is to provide a fundamental understanding of induced seismicity in fractured rocks that improves the ability to forecast and control the seismic risk. This project adopts the hypothesis that seismicity is jointly controlled by the fracture network geometry and the activated thermo-hydro-mechanical (THM) processes in geological systems. We will apply Discrete Fracture Networks (DFN) to represent the structural discontinuities and model the THM processes at high resolution. This project employs the datasets from recent small‐scale (decameter) stimulation experiments at the Grimsel Test Site in Switzerland and state-of-the-art numerical models to achieve the following: 1) test the effectiveness of high-resolution models to capture the seismic, hydraulic and mechanical processes observed with small-scale experiments; 2) link the geometrical attributes of a fracture network (such as fracture intensity, connectivity, length and spatial distribution) to spatial, temporal and magnitude distribution of induced seismicity; 3) propose and test a novel maximum possible magnitude forecast model that regards the joint impact of multiphysics processes dominating at site-specific geological conditions and operational constraints; 4) assess upscaling of the small-scale (decameter) high-resolution DFN models to simulate the reservoir-scale (kilometer) experiments. This research project is novel in addressing the injection-induced seismicity by high-resolution physics-based models and high-quality datasets derived from unique in-situ experiments. The proposed research has significant implications to promote the transition policy towards a renewable energy supply and contributes to advancing our knowledge about the triggering mechanisms of induced earthquakes.