Harnessing geothermal energy as a cornerstone of a sustainable future is rapidly gaining momentum across the globe. In Germany, a groundbreaking research initiative is poised to deepen our understanding of the complex interactions in deep geothermal reservoirs that fundamentally influence both the efficiency and safety of geothermal energy extraction. Spearheaded by Johannes Gutenberg University Mainz (JGU), the TRIGGER project delves into the intricate effects of thermal stress-induced fracturing in subsurface rock formations, a phenomenon critical to optimizing heat extraction while minimizing seismic risks.
Geothermal energy represents a vital pathway in the transition from fossil fuels to carbon-neutral energy sources. However, one of the significant challenges affecting its widespread acceptance and deployment is the potential for induced seismicity—low-magnitude earthquakes triggered by human interventions such as fluid injection and extraction. The TRIGGER project addresses this challenge by investigating how the injection of cold water into hot rock formations causes thermal stress that alters rock properties like permeability and mechanical strength. These changes subsequently influence the behavior of the geothermal reservoir, including fluid flow and fracture propagation.
The research initiative involves a multidisciplinary consortium of expert groups at Mainz University, including the Volcano Seismology group led by Professor Miriam Christina Reiss, who coordinates the project. Collaborators include teams specializing in Tectonics and Structural Geology, Geodynamics, and Metamorphic Processes. The project also partners with external institutions such as the Institute for Geothermal Resource Management (igem) in Bingen, Ruhr University Bochum, and Microstructure and Pores GmbH (MaP) in Aachen. This collaboration ensures a comprehensive approach, integrating seismology, rock mechanics, geochemistry, and computational modeling to unravel the complexities of geothermal reservoir dynamics.
At the core of TRIGGER’s research lies a fundamental question: how do substantial temperature gradients between injected cold water and native hot rock lead to fracture formation and evolution? This is crucial as fractures control permeability, the parameter dictating how efficiently thermal water can be extracted and reinjected. Understanding fracture dynamics under thermally induced stress changes also sheds light on the mechanisms that might trigger microseismic events, often undetected but potentially cumulative and impactful over time.
Geothermal reservoirs commonly lie deep underground, often beyond 1,500 meters, where temperatures escalates approximately 3 degrees Celsius per 100 meters of depth, or even 5 degrees Celsius per 100 meters in highly active regions such as the Upper Rhine Graben rift system. This pronounced geothermal gradient presents valuable energy that can be harnessed cost-effectively. However, efficient exploitation demands precise knowledge of how thermal contractions and expansions influence rock integrity during cyclic fluid injections, often resulting in temperature fluctuations exceeding 100 degrees Celsius within the reservoir.
Experimentally, the TRIGGER team is conducting elaborate laboratory tests on core samples retrieved from depths of up to 3 kilometers. These samples undergo controlled thermal, mechanical, structural, and chemical analyses to document changes induced by simulated geothermal conditions. Specialized deformation experiments involve injecting cold fluids into preheated rock samples, monitoring in real-time the onset and progression of fracturing using an array of sensitive sensors. Such meticulous characterization at the microstructural level allows researchers to quantify alterations in permeability and mechanical strength under thermal stress.
Complementing these laboratory efforts, complex computer models replicate the physical experiments and extend them to scenarios beyond laboratory constraints. These simulations allow exploration of wider parameter spaces, including variations in rock composition, temperature gradients, and fluid injection rates. This dual approach—integrating empirical data with numerical modeling—enables an unprecedented understanding of geothermal reservoir behavior over longer timescales and greater spatial dimensions, which is otherwise impossible to achieve solely through field studies.
A crucial outcome anticipated from TRIGGER is the identification of long-term impacts of thermal cycling on fracture networks and fluid- rock interactions. This knowledge could inform engineering strategies to optimize injection temperatures and schedules, maximizing heat extraction while mitigating induced seismicity risk. Such insights could pave the way for safer, more efficient geothermal operations, fostering greater public acceptance and facilitating smoother integration into national energy portfolios.
Public perception plays a decisive role in the adoption of geothermal technology. Historically, concerns about earthquakes linked to geothermal activity have hampered development efforts. TRIGGER seeks not only to advance scientific understanding but also to provide empirical evidence that supports responsible geothermal exploitation, thereby strengthening public trust. Germany’s geothermal infrastructure, exemplified by the Insheim power plant supplying renewable electricity for a decade, stands to greatly benefit from this enhanced knowledge base.
The regional specificity of geothermal systems adds complexity to this research. For instance, the Upper Rhine Graben is known for its distinctive tectonic setting and elevated geothermal gradient, making it an ideal natural laboratory. The TRIGGER project’s location at Mainz University situates it strategically close to this geologically active region, enriching the relevance and applicability of its findings. Adjacent initiatives in Rhineland-Palatinate further underscore the growing commitment to geothermal energy, with projects underway in Speyer and Wörth am Rhein.
Professor Miriam Christina Reiss, a rising figure in volcano seismology and geothermal geophysics, brings expertise that bridges fundamental earthquake science and applied geothermal research. Her academic journey—from an interdisciplinary background in English and Physics to doctorate-level seismological research—coupled with international experience at Yale University, fuels this innovative project’s success. Reiss’s vision encompasses not only unravelling the subsurface processes that govern geothermal systems but also translating these insights into actionable strategies for sustainable energy production.
As the TRIGGER project progresses, it is expected to deliver a holistic framework describing thermal fracture generation and evolution within geothermal reservoirs. Such frameworks will integrate microstructural observations, deformation mechanics, fluid dynamics, and seismicity patterns to inform geothermal engineering practices. Ultimately, this could revolutionize how geothermal reservoirs are managed, enhancing energy yields while minimizing environmental impacts.
The stakes extend beyond Germany; the methodologies and findings developed in TRIGGER could serve as a template for geothermal projects worldwide, particularly in regions with similar geological settings. By pushing the envelope in understanding thermally induced rock behavior, TRIGGER positions itself at the forefront of efforts to unlock the full potential of geothermal energy, aligning with global goals for clean, resilient, and sustainable energy systems.
In a world grappling with climate change and energy insecurity, engineering breakthroughs such as those anticipated from the TRIGGER project are indispensable. They fuse scientific rigor with practical urgency, offering hope for a future where earth’s internal heat can be accessed effectively and safely. This endeavor encapsulates the spirit of modern geosciences, blending advanced experimental techniques, computational prowess, and collaborative innovation to steward terrestrial resources responsibly.
Subject of Research: Thermal fracture formation and permeability changes in deep geothermal reservoirs induced by temperature fluctuations.
Article Title: Advancing Geothermal Energy: Unraveling Thermally Induced Fracture Dynamics in Deep Reservoirs
News Publication Date: April 2025
Image Credits: Valentin Koßmann / TRIGGER
Keywords: Geothermal energy, induced seismicity, thermal stress, fracture formation, permeability, deep reservoirs, Upper Rhine Graben, Johannes Gutenberg University Mainz, geothermal modeling, rock mechanics, microstructural analysis, geothermal sustainability
