Sustainable development of enhanced geothermal systems based on geotechnical research – a review

The world's primary energy requirements are supplied to a large extent by non-renewable energy resources, which has resulted in significant anthropogenic greenhouse gas emissions, leading to global warming and climate change. This issue can only be overcome by using low-emission energy means such as natural gas, geothermal energy, and solar energy. Although there is enormous potential for power generation with geothermal energy, to date commercial geothermal energy production has mostly been limited to geothermal areas with accessible heat sources, where significant amounts of fluids are available (conventional geothermal reservoirs). In recent decades, the geothermal industry has moved to utilize more challenging unconventional resources, and of these, enhanced geothermal systems (EGSs) have been steadily improving worldwide. Although knowledge of reservoir geomechanics has developed substantially thus far, technical challenges specific to EGS technology (high-temperature and -pressure environments, accelerated chemical characteristics) and critical environmental concerns have prevented the widespread utilization of EGSs. A number of research areas in geotechnical engineering, including understanding the coupled thermo-hydro-mechanical and chemical interactions in intact and fractured rock mass, the initiation and propagation of natural and artificially-induced rock fractures, the mechanisms of induced seismicity and mitigation techniques, and the exploration of non-water-based fracturing fluids, can provide crucial information on reservoir stimulation, long-term performance and the sustainability of EGS. This paper critically reviews the state-of-art of the technology, including major geotechnical and environmental concerns in EGS development. The current study aims to stimulate the discussion of sustainable EGS development among the geothermal community, highlighting the future research needs for exploration, stimulation, and long-term production.