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In its natural environment a rock mass is subjected to stress and temperature cycles which significantly affect the rock strength and failure behavior. Examples include tectonic stress variations and earthquake cycles, geothermal unrest before volcanic eruptions, Earth tides, seasonal water fluctuations and low-frequency stress and temperature cycles due to climate change. Underground engineering structures such as salt caverns, radioactive waste disposal facilities, mines, as well as carbon dioxide and hydrogen storage sites also experience local cyclic changes in state variables. In this study we review cyclic operational processes in mining, petroleum and geothermal industries. We apply fundamental concepts and methods from fatigue of materials to rock mechanics and geoscience to better describe and understand hydraulic fracturing, hydraulic shearing and complex mixed-mode fracturing in both laboratory- and field-scale cyclic injection operations. The review of available literature shows that control of hydraulic fractures by cyclic injection involves the following elements: (a) managing the fracture propagation path and associated damage pattern, (b) enhancing reservoir permeability to increase productivity or injectivity, and (c) mitigating induced seismicity. The importance of rock hydraulic fatigue is highlighted in the context of the energy transition, as emerging renewable energy technologies, such as Enhanced Geothermal Systems, can be made available at earlier convenience and more efficient and safer with a better understanding of the underlying processes. Thus, although largely overlooked, rock hydraulic fatigue has the potential to contribute to zero emissions climate policy goals. Highlights In this article, we address the following key questions. • How to quantify cyclic processes in subsurface energy technologies using fatigue formulations? The reduction in fatigue breakdown pressure is quantified by Wöhler curves. The permeability metrics and productivity index are quantified together with induced seismic b -values and fracture pattern complexity. • What fracture mechanisms are active during in-situ stimulation of fossil and geothermal reservoirs? Stress-induced-, stress corrosion-, and fatiguecracking mechanisms are identified and used in advanced, tailored stimulation treatments to unlock deep geothermal heat for a clean energy future. • How to optimize fatigue to protect the environment and contribute to societies' net-zero emissions goals? Cycle frequency, time exposed to maximum stress, and depressurization phases are key ingredients in fatigue methods to protect the environment while harvesting safely deep geothermal heat.
Zang et al. (Wed,) studied this question.