Advances in Hot Dry Rock Engineering for Extracting Heat from Earth: From Permeability Enhancement Strategies to Field Experiences in Enhanced Geothermal Systems (EGS)

ABSTRACT: Geothermal energy, sourced from the Earth's internal heat, offers a renewable solution for clean electricity production. This energy is continuously replenished by heat flowing from the Earth's core to its surface and is especially accessible through hot dry rock (HDR) formations. These formations, located deep underground with high temperatures but low permeability, require innovative engineering to extract heat. The process involves circulating a fluid to transport heat to the surface, thus enabling power generation. The key to utilizing HDR lies in the generation of a conduit path for fluid circulation through methods like hydraulic fracturing, which is the most effective technique for creating extensive surface areas for heat extraction. This approach involves inducing fractures in the rock to facilitate heat flow. Such advancements have led to the development of Enhanced Geothermal Systems (EGS), which build on the HDR concept first realized at Fenton Hill in 1977. This paper reviews significant EGS projects over the last five decades, including notable examples like Fenton Hill and Soultz-sous-Forêts, among others. Our analysis reveals that as a byproduct of stimulation, micro- and macro-fractures can be created and/or some natural fractures may reopen, which would lead to fluid retention. This evaluation underscores the potential and challenges of EGS, offering insights into optimizing geothermal energy extraction from HDR formations. 1 INTRODUCTION Geothermal energy is a pivotal aspect of renewable energy resources, characterized by its ability to provide consistent, sustainable power derived directly from the Earth's heat. Its significance lies in its vast potential to contribute to the energy mix with minimal environmental impact, offering a solution to the pressing demands for clean, reliable energy sources. Among the various technologies for harnessing geothermal energy, Hot Dry Rock (HDR) or Enhanced Geothermal Systems (EGS) stand out due to their innovative approach to extracting heat from the Earth's crust (Zhang and Zhao, 2020; Breede et al., 2013a). HDR/EGS technology capitalizes on the Earth's abundant thermal energy stored in dry underground rocks. By creating a man-made geothermal reservoir, these systems inject water into hot rocks, fracturing them to enhance permeability, thereby allowing water to circulate, absorb heat, and be extracted from another well as hot water or steam. This process effectively turns inaccessible heat resources into viable energy supplies, capable of generating electricity or providing direct heating (Zhang et al., 2019; Gong et al., 2020).