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

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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.