Modeling Coupled Hydro-Mechanical Processes During Hydraulic Stimulation at the Bedretto Underground Laboratory
Authors: Clasen Repollés V. , Jiang D. , Rinaldi A. P. , Obermann A. , Wiemer S. , Bröker K. , Ma X. , Rosskopf M. , Gholizadeh Doonechaly N. , Hertrich M. , Maurer H. , Giardini D. , BedrettoLab team _ 
Published: August 2024

ABSTRACT:
In the framework of Enhanced Geothermal Systems (EGS), hydraulic stimulations play an important role in increasing the permeability of the host rock, facilitating more efficient heat extraction. However, these stimulations unavoidably induce reactivation of faults and earthquakes. The induced seismicity serves as proxy for the enhanced fracture network's spatial extent, but at the same time could pose a risk to nearby populations if the earthquakes' magnitude is large. To address this challenge, understanding the coupled physical processes generating fault reactivation and seismicity is crucial, and the development of forecasting tools could help mitigating the risk. Numerous numerical simulators capable of modeling complex coupled processes exist, yet their computational demands hinder their application in real-time induced seismicity forecasting. In this work, we present the development of two distinct categories of models: the first category encompasses a simplified hybrid hydromechanical model tailored for real-time applications, while the second category comprises a more detailed 3D numerical model designed to advance the fundamental understanding of the physical processes at play. By applying these models to reproduce a hydraulic stimulation at the Bedretto Underground Laboratory, we demonstrate that we can simulate first-order observations (e.g. pressure changes, seismicity rate, strain) without introducing excessive complexity.

1 INTRODUCTION
The global pursuit of sustainable and green energy solutions has intensified in response to the escalating challenges of climate change and the imperative to transition away from fossil fuels. Geothermal energy has emerged as a particularly promising alternative, drawing increased attention due to its dual capability to serve as both an energy and heating source (Coskun Avci et al., 2020). However, effective heat extraction faces challenges, as naturally occurring resources are scarce and substantial heat is found deep underground where the rock permeability and the matrix porosity are low. Enhanced Geothermal Systems (EGS) aim to improve the heat extraction efficiency, creating a more permeable fractured reservoir by hydraulically stimulating the hot dry rock (Majer et al., 2007; McClure and Horne, 2014). However, these stimulations are primarily responsible for inducing seismic events (Grigoli et al., 2017). While such events are in general small and help identify the spatial extent of the stimulated reservoir volume, single larger magnitude events may occur too. To control the occurrence of such events is the challenge, since they pose a safety risk for nearby populations. This risk is heightened, for instance, when the targeted fault zone is large and has accumulated high amounts of elastic strain (Grigoli et al., 2017). In order to shed light on the coupled physical processes generating seismicity during geothermal activities, hydraulic stimulation experiments have been conducted at the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG) in Switzerland. The unique aspect of the BULGG lies in its dense monitoring system (Plenkers et al., 2023; Obermann et al., 2024), facilitated by the experimental scale of deca- to hectometers (Hertrich et al., 2021) as well as realistic stress conditions close to real targeted reservoirs due to its overburden of more than one kilometer (Gischig et al., 2020; Ma et al., 2022).