Research Progress and Technical Challenges of Geothermal Energy Development from Hot Dry Rock: A Review

Abstract

The reserves of hot dry rock (HDR) geothermal resources are huge. The main method used to develop HDR geothermal resources is called an enhanced geothermal system (EGS), and this generally uses hydraulic fracturing. After nearly 50 years of research and development, more and more countries have joined the ranks engaged in the exploration and development of HDR in the world. This paper summarizes the base technologies, key technologies, and game-changing technologies used to promote the commercialization of HDR geothermal resources. According to the present situation of the exploration, development, and utilization of HDR at home and abroad, the evaluation and site selection, efficient and low-cost drilling, and geothermal utilization of HDR geothermal resources are defined as the base technologies. Key technologies include the high-resolution exploration and characterization of HDR, efficient and complex fracture network reservoir creation, effective microseismic control, fracture network connectivity, and reservoir characterization. Game-changing technologies include downhole liquid explosion fracture creation, downhole in-situ efficient heat transfer and power generation, and the use of CO₂ and other working fluids for high-efficient power generation. Most of the base technologies already have industrial applications, but future efforts must focus on reducing costs. The majority of key technologies are still in the site demonstration and validation phase and have not yet been applied on an industrial scale. However, breakthroughs in cost reduction and application effectiveness are urgently needed for these key technologies. Game-changing technologies remain at the laboratory research stage, but any breakthroughs in this area could significantly advance the efficient development of HDR geothermal resources. In addition, we conducted a comparative analysis of the respective advantages of China and the United States in some key technologies of HDR development. On this basis, we summarized the key challenges identified throughout the discussion and highlighted the most pressing research priorities. We hope these technologies can guide new breakthroughs in HDR geothermal development in China and other countries, helping to establish a batch of HDR exploitation demonstration areas. In addition, we look forward to fostering collaboration between China and the United States through technical comparisons, jointly promoting the commercial development of HDR geothermal resources.

1. Introduction

Geothermal energy, as a renewable and sustainable energy source, has gained increasing attention in the context of global energy transition and climate change mitigation. Geothermal resources stored in hot dry rocks (HDRs) have huge reserves and great development potential [1,2,3]. HDR refers to rock heated between 150 and 650 °C with minimal water content. Based on the MIT report, the amount of HDR resources available for human development and utilization is 30 times the oil, natural gas, and coal resources on the earth [4]. The HDR resource amount at the depth of 3–10 km was estimated by China Geological Survey (CGS). Their results show that the resource amount at the depth of 3–10 km could reach 2.5 × 10²⁵ J, which is equivalent to the heat of 860 trillion tons of standard coal. This resource is estimated to be 3400 times greater the 2020 total energy consumption on the basis of 2% of the amount of exploitable resources [5]. Several target regions for HDR geothermal resource exploration are proposed by the Institute of Geology and Geophysics, the Chinese Academy of Sciences, including South Tibet, West Yunnan (Tengchong), the southeast coast of China (Zhejiang, Fujian and Guangdong), Northeast China (Songliao basin), North China (Bohai Bay basin), and the southeast regions of Ordos Basin (Fenwei Graben).

The concept of extracting geothermal energy from HDR was first proposed in the 1970s, leading to the development of enhanced geothermal systems (EGSs). The use of an EGS is considered the main method for developing HDR geothermal resources [2,6,7]. Generally, hydraulic fracturing is used to build the flow channel of complex fracture networks in the high-temperature rock mass, which provides space and contact areas for the transport and heat exchange of heat-carrying media (e.g., water). As a result, the low-porosity, low-permeability, and high-temperature rock mass in the deep underground is built into the artificial geothermal reservoir with a high-permeability and heat-exchange area; thus, a considerable amount of heat energy can be economically extracted for utilization in the medium and long term. As shown in Figure 1, water is injected into EGS fractures through the injector and then produced through the producer. The power plant at the surface uses the produced heat energy to drive turbines in a generator. After the hot fluid transfers its heat to the power plant, the cooler fluid can again be injected into the HDR. This technology has the potential to significantly expand the geographic availability of geothermal energy, as HDR resources are more widely distributed compared to conventional hydrothermal systems [3]. However, despite decades of research and pilot projects, the commercial deployment of HDR geothermal energy remains limited due to significant technical (e.g., efficient reservoir creation), economic (e.g., drilling costs), and environmental (e.g., effective microseismic control) challenges [2,8,9].

At present, the development of HDR geothermal technology in the world is in the research and development stage of field testing. After nearly 50 years of research and development, more and more countries have joined the global exploration and development of HDR geothermal resources with the further bright prospects of commercial development [1,6]. Many countries in Europe and the United States have launched EGS research and engineering projects, such as the Fenton Hill and FORGE (Frontier Observatory for Research in Geothermal Energy) programs in the United States [10,11,12,13,14,15], the Soultz project in France [16,17,18,19], the Gonghe basin and Matouying site in China [20,21,22,23]. According to statistics, there are more than 30 EGS projects under construction or in operation in the world. Further, 16 EGS projects have realized the operation of power generation, and among which 5 EGS projects are in operation now [1,2,5]. In addition, many more EGS projects are in the early stages of demonstration.

Although a number of EGS demonstration projects have been successfully established in the world, no replicable and commercial development model of HDR geothermal engineering has been produced yet due to the limitations of key technologies [2,5,6,7,24]. Regarding the present situation in the exploration, development, and utilization of HDR at home and abroad, the base technologies, key technologies, and game-changing technologies can promote the commercialization of HDR geothermal resources. In this work, the evaluation and site selection, efficient and low-cost drilling, and geothermal utilization of HDR geothermal resources are defined as the base technologies. The key technologies include the high-resolution exploration and characterization of HDR, efficient and complex fracture network reservoir creation, effective microseismic control, fracture network connectivity, and reservoir characterization. The game-changing technologies include downhole liquid explosion fracture creation, downhole in situ efficient heat transfer and power generation, and the use of CO₂ and other working fluids for high-efficiency power generation. In addition, the technology-related research and development of HDR at home and abroad is compared and evaluated in this work. On this basis, the aim of this work is to summarize the key challenges identified throughout the discussion and highlight the most pressing research priorities. We hope these technologies can guide new breakthroughs in HDR geothermal development in China, enabling the formation a batch of HDR exploitation demonstration areas and supporting the adjustment of the national energy structure and the realization of the “dual-carbon target”. In addition, we look forward to fostering collaboration between China and the United States through technical comparisons, jointly promoting the commercial development of HDR geothermal resources.

The remainder of this paper is structured as follows: Section 2 introduces the developmental progress and trends in HDR in the world, Section 3 focuses on the technology structure tree for the development of HDR geothermal resources, Section 4 gives research suggestions for HDR besides technical breakthroughs, and conclusions are finally drawn in Section 5.

Figure 1. A schematic of the enhanced geothermal system, modified from [22].

Figure 1. A schematic of the enhanced geothermal system, modified from [22].

2. Development Progress and Trend of HDR in the World

HDR geothermal development has been studied for nearly 50 years in the world, but research is mainly conducted in several countries, such as the United States, the United Kingdom, France, Germany, Switzerland, Japan, Australia, etc. [1,4]. By the end of 2020, more than 40 EGS demonstration projects have been built worldwide [1,2,3,4,5,6,9]. The superiority and development feasibility of HDR resources are gradually recognized internationally. At present, the hardest technical challenge in terms of industrializing EGS realizing economically sustainable development. This is mainly determined by the difficulty of building a large-scale, economical, and sustainable underground heat exchange system based on existing reservoir recreation and maintenance technology [8,25]. Recently, a series of frontier research projects have been carried out worldwide to meet these technical challenges.

2.1. The Progress and Trend in USA

In 2006, a comprehensive EGS review was conducted by an independent expert evaluation committee, formed by the Massachusetts Institute of Technology (MIT) and funded by the U.S. Department of Energy (U.S. DOE), to assess the potential of geothermal energy as the major energy source for the future of the United States [4]. The assessment results indicated that the industrial development of HDR geothermal resources will significantly revolutionize the global energy structure. In 2015, the U.S. DOE launched the Frontier Observatory for Research in Geothermal Energy (FORGE) program with a cumulative investment of more than USD 200 million [11,12,13,14,15]. This program aims to encourage the global geothermal research community to contribute to EGS development and ultimately provide a series of replicable EGS technology solutions and industrialization patterns for the geothermal industry. In 2016, the U.S. DOE launched the 3-year EGS Collaborative Laboratory Program (EGS Collab) with a USD 9 million investment [26,27,28,29]. The main goals of this program are to (1) improve our understanding of rock fracturing response patterns using the accessible shallow subsurface laboratories, (2) provide a medium-scale (10 m scale) experimental platform with which to validate and develop thermal–hydraulic–mechanical–chemical (THMC) coupled numerical methods, and (3) develop novel monitoring instruments for use during hydraulic fracturing. Furthermore, to reduce the cost of HDR development, the U.S. has proposed reservoir creation based on the edges and depths of existing hydrothermal systems. It is believed that this method can achieve economic benefits rapidly and develop geothermal reservoir creation technologies concomitantly [7,24]. For this purpose, the U.S. DOE has recently funded several related EGS demonstration projects (e.g., Desert Peak, Geysers, Raft River, etc.) [30,31,32,33]. These field test results indicate that this approach can rapidly increase the geothermal power generation capacity in existing hydrothermal fields.

The main mission of the FORGE program is to enable cutting-edge research and drilling and technology testing and to allow scientists to identify a replicable, commercial pathway to EGS [15]. In addition to the site itself, the FORGE effort will include robust instrumentation, data collection, and data dissemination components to capture and share data and activities occurring at FORGE in real time. All deliverables at FORGE will focus on strengthening our understanding of the key mechanisms controlling EGS success, specifically addressing how to initiate and sustain fracture networks in basement rock formations. This critical knowledge will be used to design and test a methodology for developing large-scale, economically sustainable heat exchange systems, paving the way for a rigorous and reproducible approach that will reduce industry development risk and facilitate EGS commercialization. R&D activities may include, but are not limited to, innovative drilling techniques, reservoir stimulation techniques, and well connectivity and flow-testing efforts. The site will also require the continuous monitoring of geophysical and geochemical signals. The innovative research, coupled with an equally innovative collaboration and management platform, is truly a first-of-its-kind endeavor. Now, some key technologies, including high-temperature drilling tools, new reservoir stimulation and well completion technologies, the monitoring and management of fracture networks, the prediction of induced earthquakes, stress management, and numerical simulation, have been developed at the FORGE site [14,15,34,35].

A significant seven-fold reduction in on-bottom drilling time, from 440 h to 60 h at an equivalent depth of 6000 feet, was achieved by collaborating with Texas A&M University to implement physics-based drilling approaches and use advanced drill bits. This helps reduce drilling costs, crucial for expanding the use of EGS and enhancing the commercial viability of all geothermal technologies. Up to now, the project has drilled multiple wells at the Utah site, including an injection well (Well 16B(78)-32), a production well (Well 16A(78)-32), and observation wells, reaching depths of approximately 7500 to 10,000 feet (2286 to 3048 m) in hot, low-permeability granitic rock (Figure 2).

Equipped with downhole sensors, the wells enable the real-time monitoring of temperature, pressure, and seismic activity during hydraulic stimulation and circulation tests. These wells utilize advanced directional drilling and multi-stage hydraulic fracturing techniques to enhance reservoir connectivity and permeability. The successful creation of a geothermal reservoir from scratch, through drilling two first-of-their-kind wells and performing multiple innovative stimulations to create a strong subsurface flow pathway that did not exist before, completed a critical step in making EGS technologies commercially viable.

Based on the FORGE project website (https://utahforge.com/, accessed on 16 December 2024), the research team achieved important progress in the multi-zone stimulation of hard hot granitic rock using zonal isolation technology. In 2024, FORGE built on previous successful stimulations and used multi-zone stimulation via pump-down drillable plugs with a large diameter that deviated well at high temperatures for EGS reservoir creation, enabling the expansion of EGS deployment nationwide. During the multi-zone stimulation, they deployed fiberoptic cables to continuously monitor strain, temperature, and acoustics in the subsurface, which helps understand the flow and changes in the EGS reservoir over time and monitor seismic activity. Currently in the third phase, the Utah FORGE team is drilling full-size wells spanning 15 square miles near Milford, Utah, to develop, test, and accelerate breakthroughs in EGS technologies.

Moreover, this project generates a large amount of data, including microseismic catalogs, well logs, drilling and stimulation data, etc., which are maintained in the Geothermal Data Repository (GDR). These datasets are popular downloads, and the FORGE team has also created wiki pages for key operations and data links. For more developments related to the latest research progress aside from the above-mentioned progress at the FORGE EGS project, please refer to the FORGE project website.

Recently, the U.S. Department of Energy launched the Enhanced Geothermal Shot™ and GeoVision analysis projects. The goal of the Enhanced Geothermal Shot™ is to usher in a clean energy future by bringing EGS to Americans nationwide. It aims to unlock the geothermal energy that is currently inaccessible with existing technology by creating human-made reservoirs. As part of the U.S. Department of Energy’s Energy Earthshots™, it is a department-wide effort to dramatically reduce the cost of EGS by 90%, bringing it down to USD 45 per megawatt hour by 2035. Achieving this goal will contribute significantly to reaching the targets of 100% carbon-pollution-free electricity by 2035 and net-zero emissions across the U.S. economy by 2050. This can be accomplished through accelerating research and development in multiple aspects, such as by driving down the costs of drilling, well casing, and related materials, advancing engineering techniques for more efficient well drilling, collecting better-quality data for subsurface understanding, and ensuring the proper containment of new reservoirs and geothermal fluids. The GeoVision analysis projects that, through technological improvements, geothermal electricity generation capacity has the potential to increase to more than 26 times the current level. This would increase geothermal electricity generation capacity to 60 gigawatts (GW) by 2050—providing 8.5% of all U.S. electricity generation. The analysis also finds that instituting regulatory reforms that optimize permitting timelines alone can double geothermal capacity by 2050. By achieving these objectives and improving relevant tools, technologies, and methodologies, it hopes to reduce risks and costs for developers, boost the growth potential of geothermal energy, access untapped geothermal sources, and provide the U.S. with secure, flexible energy that benefits the geothermal industry and energy consumers nationwide.

2.2. The Progress and Trend in China

The research on HDR geothermal development started relatively late in China. Some scientific research institutions and universities engaged in theoretical discussions and carried out laboratory experiments before. In 2012, the National High Technology Research and Development Program (e.g., “863” Program) launched the project of “Research on Key Technologies for Thermal Energy Development and Comprehensive Utilization of Hot Dry Rock (2012–2015)”. This project was completed with cooperation of Jilin University, Tianjin University, Tsinghua University, the Guangzhou Institute of Energy Resources, the Chinese Academy of Sciences, the Institute of Hydrology and Environmental Geology, the Chinese Academy of Geological Sciences, and the Daqing Oilfield underground branch. The success of the project demonstrated the technological feasibility of HDR volume fracturing and resource recovery, which enhanced the interest and confidence of Chinese scientific research institutions, enterprises, and institutions in the development of HDR geothermal resources. In addition, some of the EGS individual technology research results provide theoretical support for the development of HDR geothermal resources in China [36,37].

In September 2017, the CGS joined hands with the Natural Resources Department of Qinghai Province to drill a 236 °C high-temperature HDR body at a depth of 3705 m and explore a distribution area of 3000 km² in the Gonghe basin [21,38]. This discovery represents a major breakthrough in the exploration of HDR resources in China. The survey results also indicate that China has a great quantity of HDR geothermal resources. Thus, it is believed that realizing the safe and efficient exploitation of HDR resources will play an important role and have strategic importance for the development of science and technology, the economy, and society around the world.

Based on the above survey and research results, the National Energy Administration and other relevant government departments released the “China Geothermal Energy Development Report (2018)”. This national report specifically points out that HDR geothermal energy is an important field for future geothermal energy development and that there is an urgent need to undertake an exploration and experimental development project relating to HDR geothermal energy in Gonghe basin, Qinghai Province. In 2019, the National Key Research and Development Project “Research on Key Scientific Problems of Energy Acquisition and Utilization of Hot Dry Rock (2019–2023)” was launched by the Ministry of Science and Technology of the People’s Republic of China [37]. The 11 participating institutions, led by the Jilin University, aim to (1) advance the theoretical research on the storage, acquisition, and transfer theory of HDR energy and (2) promote the independent innovation capabilities of China in terms of HDR geothermal resources. The research results will directly serve to provide theoretical and technical support for the construction of the first HDR demonstration field in the Gonghe basin. Recently, the China Geological Survey (CGS) organized and implemented the HDR experimental production project in Gonghe basin in Qinghai Province, as shown in Figure 3. They successfully realized experimental power generation using HDR geothermal resources [39,40]. This has greatly promoted the exploration and development of HDR geothermal resources in China, allowing the achievement of a historic breakthrough.

In addition, the Hebei Provincial Coal Geology Bureau organized and implemented an HDR exploration project in 2019. This program drilled an HDR body with a temperature of 150 °C at a depth of 3965 m in Tangshan Harbor Economic Development Region [20,41,42]. This is the shallowest buried HDR drilled in the Beijing–Tianjin–Hebei region at present, representing a major breakthrough in HDR exploration in the central–eastern region of China. On the one hand, this discovery plays a positive supporting role in optimizing the energy structure, improving the atmospheric environment, and preventing and controlling pollution in Hebei province as well as the Beijing–Tianjin–Hebei region. On the other hand, it provides a significant reference for the exploration and development of HDR geothermal resources in the Beijing–Tianjin–Hebei region and even the Circum–Bohai Bay region.

2.3. The Progress and Trend in Europe

France has achieved remarkable results in the development of HDR, and the Soultz project is a typical example [43,44,45]. This project was carried out on the carboniferous granite intrusions within the Upper Rhine Graben. During the implementation of the project, a series of key experiments were successfully conducted. For example, a deep directional multi-well system was drilled, and the production well reached a depth of 5275 m (Figure 4) [44,46]. Through multiple hydraulic fracturing stimulations, the permeability of the rock was effectively enhanced, and an underground heat exchanger was created [45]. The results showed that the transmissivity of the inter-well fluid increased significantly after hydraulic fracturing, and there was no water loss or adverse environmental impacts [43]. Subsequently, a well was successfully deepened into the igneous rock at a depth of 5000 m, where the temperature exceeded 200 °C. A stimulation test was completed at this depth, and a new reservoir was created [46]. Currently, the project is further seeking to improve the heat exchange and energy extraction systems. In the future, it plans to expand the development scale, increase power generation capacity, and provide more clean energy to the surrounding areas [47].

The United Downs Geothermal Power Station project in Cornwall, the UK, is the first deep HDR geothermal power station in the country. Currently, it has made significant progress [48,49,50,51]. The main equipment has been successfully delivered and has officially entered the installation and commissioning stage and was expected to be officially put into operation by the end of 2024. This power station uses the organic Rankine cycle principle and the unique high-efficiency radial centrifugal turbine technology to convert geothermal fluids into electrical energy. At the same time, it adopts an air-cooling system, achieving zero water consumption. Two deep directional wells have been drilled, and the production well reaches a depth of 5275 m, making it the deepest well on the British mainland. After being put into production, it is expected to reduce carbon emissions by more than 6500 tons per year. In the future, based on this project, the UK plans to carry out exploration and development work in other areas with HDR resource potential in the country and gradually increase the proportion of HDR power generation in the energy structure.

In addition, in Europe, many other countries are also involved in HDR development projects [4,52,53]. Germany, Austria, and Switzerland are carrying out HDR projects in the Molasse Basin, mainly targeting the Mesozoic carbonate reservoirs. However, being affected by the geothermal background, reservoir physical properties, and lithofacies changes, some projects have not reached the expected temperature or pressure [54]. Based on summarizing experience and lessons, these countries are constantly optimizing exploration technologies and development plans [55]. In 2013, the European Commission launched the Horizon 2020 program [53]. This program has funded 11 geothermal-related projects with a total budget of €134 million. The Horizon 2020 program aims to promote the more comprehensive exploitation of geothermal resources in Europe. In 2018, the European Technology and Innovation Platform for Deep Geothermal Energy (ETIP-DG) released the European Deep Geothermal Energy Implementation Program. The total investment of €936 million was used to support the development of frontier technology and equipment development for deep geothermal energy development. The ETIP-DG program aims to promote the development and use of geothermal resources to meet most of Europe’s heat and electricity demands. In the future, European countries will further strengthen their cooperation, integrate scientific research forces and funds, jointly overcome the technical problems in HDR development—such as efficient drilling technology, accurate reservoir modeling, and methods to reduce the risk of induced earthquakes—and promote the commercialization and large-scale development of HDR.

2.4. The Progress and Trend in Korea

South Korea has long been dependent on fossil energy imports. Driven by the pursuit of energy independence and sustainable development, it is actively promoting the development of geothermal projects [56]. In 2012, South Korea launched its first HDR development project in Bonghwa County, Gyeongsangbuk-do, named the Pohang EGS project [57]. By using geophysical exploration methods such as magnetotelluric sounding and gravity measurement, it accurately delineated the areas with HDR potential. Subsequently, two exploration wells with depths of 2.1 km and 2.3 km were successfully drilled, obtaining rock samples and geothermal data. Research has found that the rock temperature at a depth of 2 km in this area reaches 150 °C, indicating good development potential [58,59]. In terms of technology research and development, South Korea actively cooperates with international research institutions, introduces advanced reservoir transformation technologies, and has mastered the fracturing parameters suitable for its own geological conditions through a large number of fracturing experiments. In terms of drilling technology, new high-temperature- and high-pressure-resistant drilling fluids are used. This effectively improves drilling efficiency and wellbore stability and reduces the development cost. A heat exchange circulation system is set up between the two exploration wells to monitor the heat exchange efficiency and fluid migration in real time. In addition, to ensure the environmental friendliness of the development process, a complete environmental impact monitoring system has been established for underground water quality and seismic activities.

At the Pohang EGS site, one vertical injection well (4346 m deep; PX2) and another deviated production well (4362 m deep; PX1) were drilled into Permian granodiorite with gabbroic dykes with an expected electricity production of 1.2 MW [60]. Hydraulic stimulation began on 29 January 2016 and comprised four phases of injection with a total volume of 12,800 m³ at injection rates of 1.00 to 46.83 L/s (Figure 5). Fluid was injected into both PX1 (phases 2 and 4) and PX2 (phases 1, 3, and 4). However, the M_L 5.4 mainshock occurred at the Pohang EGS site on 15 November 2017 and was preceded and followed by foreshocks and aftershocks, respectively [57,59,60,61]. Geological and geophysical data suggest that the Pohang earthquake was induced by fluid from an enhanced geothermal system (EGS) site, which was injected directly into a near-critically stressed subsurface fault zone. The magnitude of the mainshock makes it the largest known induced earthquake at an EGS site [59,60]. This incident exposed the greater challenges for the safe and controllable development of global HDR resources.

2.5. The Progress and Trend in Japan

Japan has a small territory but is located at the junction of tectonic plates, endowing it with rich geothermal resources. Japan places significant emphasis on HDR development, allocating substantial funding for research and conducting HDR project trials in various regions. This has yielded a series of notable achievements [62]. In the past, Japan implemented two HDR projects at the Hijiori and the Ogachi EGS sites [4,63,64,65]. Combining these achievements with the recent Japan Beyond-Brittle project, Japan has achieved certain results in terms of high-temperature HDR exploration and reservoir stimulation technologies [66]. A variety of geophysical methods are used for comprehensive detection, such as magnetotelluric sounding, gravity and magnetic force measurements, etc., to determine potential HDR areas [4,63]. In terms of reservoir stimulation, Japan has been constantly trying new fracturing technologies and materials, and has developed high-temperature fracturing fluids suitable for Japan’s complex geological conditions to enhance the permeability of rocks and improve the heat exchange efficiency [63,64,65].

Recently, based on the Japan Beyond-Brittle project, Japan has been continuously improving the drilling process, using drilling equipment that can withstand high temperatures and high pressures to improve drilling efficiency and safety [66,67]. At the same time, Japan is deeply studying the environmental impacts during the HDR mining process. Through the monitoring system, it can grasp the impacts on underground water quality, surrounding seismic activities, etc., in real time during the mining process, so as to adjust the mining plan in a timely manner [62]. Japan has national targets to intensify the geothermal energy use over the short term and long term [62]. In the future, Japan will continue to combine its characteristics of multiple volcanoes and complex geothermal geological conditions to deepen technological research and development. Research objectives for the short term are reducing the exploration risk by higher-resolution geophysics and improving reservoir management [62]. In addition, research activities for the long-term target development of subduction-origin supercritical geothermal resources (Figure 6) include basic geoscientific research on subsurface supercritical conditions in volcanic regions and technological innovations for drilling and power generation with supercritical fluid [62,66,68].

Based on the analysis above, it is evident that European and American countries have made substantial scientific investments in theoretical research, technological equipment development, and pilot extraction validation related to the exploration and development of HDR geothermal resources. These efforts are gradually advancing the industrial development of HDR geothermal resources, which is expected to trigger a new energy revolution. To prevent major energy-consuming countries like China from facing restrictions in cutting-edge HDR geothermal technologies and intellectual property rights, and to avoid being constrained in future global energy pricing negotiations, it is imperative to closely follow international research frontiers and accelerate the industrial development of HDR geothermal resources as a clean energy source in China.

3. Technical Challenges and Evaluation of HDR

Based on the current status of HDR exploration, development, and utilization at home and abroad, base technologies, key technologies, and game-changing technologies are proposed to promote the commercialization of HDR geothermal resources. Figure 7 shows the technology structure tree for the development of HDR geothermal resources. Most of the base technologies already support industrial application, but future efforts must focus on reducing the cost. The majority of key technologies are still in the site demonstration and validation phases and have not yet been applied on an industrial scale. Breakthroughs in cost reduction and application effectiveness are urgently needed for these key technologies. Game-changing technologies remain at the laboratory research stage, but any breakthroughs in this area could significantly advance the efficient development of HDR geothermal resources. In addition, we conducted a comparative analysis of the respective advantages of China and the United States in some key technologies of HDR development. This facilitated collaboration between China and the United States to jointly promote the commercial development of HDR geothermal resources, contributing to global sustainable development and the achievement of carbon neutrality goals.

3.1. The Base Technology of HDR Geothermal Development

(1)

Evaluation and site selection

In 2013, the China Geological Survey initiated a project to evaluate the potential of HDR resources in China and identify demonstration target areas. The project focused on selecting exploration target areas and conducting scientific drilling in the southeastern coastal regions. Systematic geothermal geological surveys and geophysical explorations were carried out in key target areas such as Zhangzhou in Fujian, Rucheng in Hunan, Yangjiang Xinzhou in Guangdong, the Leiqiong Fault Basin, Lingshui in Hainan, and Huangshadong in Huizhou, Guangdong. In 2017, the natural resources department of Qinghai Province drilled a high-temperature HDR body at a depth of 3705 m in the Gonghe basin, with a temperature of 236 °C and a detected distribution area of 3000 km², marking a significant breakthrough in HDR resource exploration in China [36]. In 2019, the Hebei Bureau of Coal Geology organized an HDR exploration project in the Tangshan Port Economic Development Zone, where a 150 °C HDR body was drilled at a depth of 3965 m. Based on a comprehensive analysis of HDR resources discovered both domestically and internationally, combined with China’s geological tectonic background, HDR resources in China can be classified into four genetic types (Figure 8): the high-radiogenic heat-producing type, sedimentary basin type, modern volcanic type, and intense tectonic activity zone type [69].

HDR resources producing high amounts of radiogenic heat are mainly distributed in South China. Exploration in this region should focus on Late Yanshanian granite reservoirs, the thickness of insulating cap rocks, and the heat-conducting role of deep faults. Sedimentary-basin-type HDR resources are primarily found in eastern basins such as the North China and Songliao Basins. Exploration in these areas should emphasize the thickness of the overlying cover, the height of bedrock uplifts, and the impact of these regional geothermal fields. Modern-volcanic-type HDR resources are mainly distributed in areas with modern volcanic groups, such as Tengchong, Leiqiong, Changbai Mountain, and Datong. Exploration should combine regional geological tectonic backgrounds, thermal anomaly distribution characteristics, and deep geophysical detection to determine the nature of shallow low-velocity zones in the crust, providing a basis for detecting this type of HDR resource. Intense-tectonic-activity-zone-type HDR resources are associated with acidic intrusions within active fault zones that retain high amounts of radiant heat. The co-genetic characteristics of HDR and hydrothermal geothermal systems are evident, with clustered hot springs and large-scale hydrothermal geothermal fields serving as important indicators when locating this type of HDR resource.

For different types of HDR geothermal resources, it is necessary to combine geological, geophysical, and geochemical methods to carry out evaluation and site selection studies. Geothermal geological survey and resource evaluation should be carried out in the prospective areas with HDR geothermal development, the geothermal fluid in the fractures should be detected, and the favorable regions and development targets should be delineated. Geothermal geological survey and resource evaluation work specifically includes (1) the calculation of the geothermal gradient, (2) the prediction of geothermal temperature at a certain depth, (3) the measurement of in situ stress, (4) and the determination of geological characteristics, lithology, structure, fault and seismic activity, etc. Recently, an integrated method, combining geology, geophysics, and geochemistry to optimize HDR target areas, was successfully applied at the FORGE site in the United States [14,15,70]. This method obtained critical data on in situ stress, the distribution characteristics of natural fractures, reservoir permeability, geothermal gradients, and rock physical properties, accurately delineating the target HDR reservoir. In comparison, China’s techniques for optimizing HDR geothermal resource target areas and precisely evaluating resource potential remain relatively underdeveloped. These gaps are mainly reflected in the following aspects.

➀ The United States has established a comprehensive geothermal geological database and three-dimensional geological models, leveraging long-term geological surveys and advanced geophysical exploration technologies. In contrast, China’s HDR target optimization is still in its preliminary stages, primarily relying on regional geological surveys and limited deep geophysical exploration data, resulting in a lack of systematic and high-resolution geothermal geological models.

➁ The United States employs high-precision terrestrial heat flow measurements, microseismic monitoring, and distributed fiberoptic sensing technologies to accurately identify the distribution and thermal reservoir characteristics of HDR resources. In contrast, China still faces challenges in target area localization technology, particularly in terms of the coverage and accuracy of terrestrial heat flow and geothermal gradient data. This limitation hinders the scientific and economic viability of target area selection.

➂ In the evaluation of HDR resource potential, the United States utilizes advanced numerical simulation and multi-field coupling analysis technologies, which comprehensively consider the thermal–hydraulic–mechanical–chemical (THMC) coupling effects to accurately assess resource reserves and extractability. For instance, the EGS Collab project validated the reliability of various resource evaluation methods through medium-scale experimental platforms. In contrast, China primarily relies on traditional statistical methods and empirical formulas for resource evaluation, lacking the capability for the dynamic assessment of resource potential under complex geological conditions. This limitation results in the reduced accuracy and practicality of the evaluation outcomes.

➃ Through long-term engineering practices and technological accumulation, the United States is able to estimate the extractable resources of HDR with relatively high accuracy. In contrast, China’s estimation of extractable resources still primarily relies on theoretical calculations and lacks support from actual engineering data. For example, although the China Geological Survey estimates a vast total amount of HDR resources (e.g., 2.5 × 10²⁵ J), the accuracy of the extractable resource estimates remains relatively low.

(2)

Efficient and low-cost drilling

The HDR drilling process faces challenging problems, such as low speed, short tool life, and wellbore instability due to the HDR being deep-buried, hard, and high-temperature rock mass [71,72]. Therefore, the drilling process of HDR is full of uncertainty, which seriously affects the drilling and completion schedule and cost. In addition, problems such as drilling in hard and wear-resistant formations, the thermal expansion of casing strings, mud loss, and high temperatures need to be overcome during HDR drilling. Recently, the practice of the scientific and technological exploration of HDR in Gonghe basin successfully developed the high-temperature-resistant drilling tools through the field practice [40].

Field HDR drilling adopts the downhole dynamic composite drilling process, the 240 °C high-temperature environmental protection type of clean water polymer mud system, and the mud strong refrigeration system [40]. Based on the practice of the scientific and technological exploration of HDR in Gonghe basin, the compound drilling technology system of the impactor and rotation in HDR was put forward in a preliminary way in order to effectively reduce the risk of high-temperature and hard HDR drilling. This drilling system includes the “rotary + turbodrill + impregnated diamond bit”, “rotary + churn drill + strong gauge bit”, “rotary + hydraulic impactor + strong gauge bit”, and “rotary + screw + strong gauge bit”. However, the trajectory control technology of HDR boreholes under the high-temperature condition needs further research and improvement, which is very critical for the high-slope horizontal drilling. Recently, the FORGE site in the United States successfully overcame the challenges of high-angle horizontal drilling in high-temperature hard rock, with the temperature of the HDR at the drilling level approaching 200 °C [14,15,35]. This achievement demonstrates that advanced high-temperature-resistant measurement and control-while-drilling (MWD) technologies (directional drilling technologies) are already available internationally, laying a solid foundation for staged fracturing in horizontal wells and the creation of large-volume artificial reservoirs.

In addition, there is a need to continue to study high-temperature-resistant and long-life drill bits, as well as efficient rock breaking technology and tools, and efficient and low-cost drilling technology and equipment need to be improved further. These technologies help to increase drilling rate (ROP), shorten drilling cycles, and reduce drilling costs. For example, by developing high-efficiency drill bits (such as new PDC bits) and advanced drilling fluids, the rate of penetration (ROP) can be increased to 5–10 m per hour; by improving drill bit materials (such as diamond composites) and designs, the lifespan of drill bits can be extended to over 200 h; by enhancing drilling efficiency and reducing non-productive time (NPT), the cost per unit depth can be controlled within USD 300–800 per meter; and by advancing high-efficiency drilling technologies and equipment, the drilling cycle can be shortened to 20–60 days. Recently, Fervo Energy (an energy startup located in the United States) said that it has cut drilling time by 70% [35,73]. That improvement could help reduce the high upfront cost of geothermal power. Since late 2023, the company has been delivering 3.5 MW of electricity for Google data centers in Nevada (e.g., Red EGS project). It has also signed a contract to provide 360 MW from its plant under construction near Milford to Southern California Edison, the power supplier for much of southern California, including parts of the Los Angeles area [73].

(3)

Efficient geothermal utilization

At present, organic Rankine cycle (ORC) power generation is mostly used for HDR geothermal resources, and low-boiling-point fluid is used as the circulating working medium for power generation [74,75]. The power generation equipment of ORC is relatively mature, being employed by internationally well-known manufacturers including Ormat in USA, Turboden in Italy, etc. These power generation technologies have been applied in some EGS demonstration projects, but their efficiency is constrained by the temperature of the heat source and economic viability. Projects such as the FORGE project in the United States and the Hijiori project in Japan have successfully conducted HDR power generation trials, but these remain small in scale, and widespread commercial application has yet to be achieved [1,2,3,4]. In China, breakthroughs have been made in HDR exploration in areas like the Gonghe Basin in Qinghai Province [40]. However, power generation technology is still in the experimental stage, and further improvements in efficiency and stability are needed.

To achieve the efficient utilization of HDR geothermal, the thermal parameters of the geothermal utilization system need to be optimized, and the optimal outlet temperature of the evaporator needs to be determined according to the engineering requirements. In addition, the principle of the “grade” use of thermal energy is comprehensively considered to determine the outlet temperature of the condenser to achieve the maximum power generation efficiency and carry out cascade utilization at the same time. In the future, it will be essential to integrate HDR geothermal energy with renewable energy sources such as solar and wind power to form a stable and efficient comprehensive energy system. Research should focus on supercritical CO₂ power generation systems and efficient binary cycle power generation technologies, as well as on developing high-temperature working fluids and energy conversion equipment suitable for HDR geothermal applications, to improve power generation efficiency.

In addition, the existing geothermal development models have limited heat transfer capacity. However, the productivity effects and influencing factors of different HDR development modes, such as “vertical well group”, “horizontal well”, and “cluster well group”, are still unclear. The question of whether these potential HDR development and utilization technologies can effectively increase the fluid flow, heat transfer area, and volume utilization efficiency needs further study, and improvement is required.

3.2. The Key Technology of HDR Geothermal Development

(1)

High-resolution exploration and characterization of HDR

Geophysical exploration is the main method of deep geothermal exploration. Transparency, three-dimensional visualization, and the quantification of exploration targets are the development trends in geophysical exploration technology [28,76,77,78,79]. Geophysical exploration methods are applicable to a wide range of applications, being used to (1) identify the direction and nature of various faults, (2) delineate the location of deep geothermal reservoirs, (3) identify geological structures related to the geothermal fluid, (4) investigate the distribution, size, and nature of igneous bodies, (5) monitor the hydrogeological changes of groundwater and thermal storage, and (6) determine the distribution and burial status of the underground geothermal fluid, etc. With deep geothermal resources and relatively complex geothermal systems, the source, reservoir, cap-rock, and fluid path are not as visible as they are in the “black box” [77]. Therefore, the expansion and exploration of the deep geothermal exploration methods, based on multiple field sources—such as gravity, magnetic, electric, and seismic values, and measurement—still constitute the research frontier in the field of geothermal resource exploration at home and abroad. Moreover, there is an urgent need to build a geophysical exploration technology system geared towards “transparency” and the effective exploration of deep geothermal geological structures. We must also develop key technologies that can be used for the temperature prediction and geological and geophysical modeling of deep geothermal reservoirs.

(2)

Efficient and complex fracture network reservoir creation

Deep HDR geothermal reservoirs have very low permeability. Thus it is critical to use appropriate permeability enhancement techniques to enable efficient fluid circulation between injection and production wells [1,2,37]. At present, hydraulic fracturing, thermal cracking, and chemical stimulation techniques are commonly used. Among these, hydraulic fracturing is widely adopted because of its fast action speed and good controllability [7,80,81]. Hydraulic fracturing usually refers to the injection of high-pressure fluid to destroy the original in situ stress field of high-temperature rock mass, thereby activating existing fractures and generating new fractures [7,8]. The aim is increase the hydraulic conductivity and heat exchange capacity of rock mass and improve the hydraulic connectivity between the injection well and the production well. In addition, chemical stimulation technology has also received extensive attention recently. This technology mainly includes injecting acid or alkali solutions into the formation at a certain fracturing pressure to achieve the desired effect, such as dissolving soluble minerals (e.g., calcite) on the fracture surface and/or sediments near the wellbore by chemical dissolution [82,83]. Recently, a field test of staged fracturing in the deep deviated well has been carried out at the FORGE site, which is expected to achieve better fracture network creation and well-to-well connectivity in the target HDR reservoir [11,15,72]. The further breakthrough of staged fracturing technology in the horizontal well is likely to be the key to the large-scale development of HDR in the future. In addition, new frequency conversion fracturing technology and waterless fracturing technology have been proposed and achieved good laboratory test results based on laboratory and modeling investigation. These new fracturing technologies need to be further validated and improved in the field [39,80,84].

(3)

Effective microseismic control

Both hydraulic fracturing and injection-production cycles have the risk of inducing earthquakes, which seriously restricts the large-scale and industrial development of deep HDR geothermal resources [59,60,61,85,86]. In terms of mitigation measures for induced seismicity, current efforts include conducting seismic risk assessments and obtaining production permits before extraction, establishing induced seismic monitoring networks, and implementing real-time risk control technology systems, such as traffic light systems [86,87,88,89,90]. Additionally, proactive statistical and physical prediction methods have been developed for induced seismicity [91,92]. To mitigate the risk of induced seismic hazards, both domestic and international HDR demonstration projects have explored strategies such as fluid injection protocols designed to limit seismic events and achieve the timely verification and calibration of injection and extraction strategies; continuous seismic activity monitoring; and the application of mitigation measures [93]. For example, in the HDR development project at the urban campus of Aalto University in Helsinki, Finland, when the traffic light system issues an orange warning, measures such as reducing fluid injection rates and adjusting injection parameters are implemented, effectively preventing the induction of larger seismic events [90]. However, it is important to note that the effectiveness of these technologies in mitigating induced seismic hazards still requires further on-site validation. Compared to the extreme difficulty of predicting natural earthquakes, forecasting induced seismic activity caused by fluid injection in HDR development is significantly easier. This is because the fluid injection process in geothermal reservoirs is controllable, and more construction parameters and downhole physicochemical conditions can be establish [86,89]. Recently, this was proved by the scientific and technological exploration of HDR in Gonghe basin [40]. The Gonghe EGS indicated that engineering measures such as the slow stopping of fracturing trucks, the expansion of the unit pumping scale, continuous pumping, and precise control of the pumping rate can effectively mitigate induced earthquakes. In an analysis of strategies used to reduce induced seismicity at the Haute-Sorne EGS project in Switzerland, it was suggested that adopting a multi-stage stimulation approach, similar to the one implemented at FORGE, could lower the risk of triggering seismic events compared to the single massive stimulation method used in previous EGS projects [94].

In the future, it is necessary to accurately characterize the deep geological structure of the target EGS site by high-precision geophysical exploration methods (e.g., 3D seismic and well imaging logging) on the basis of the in-depth analysis of the mechanism of hydraulic fracture initiation and propagation in HDR. These are very important for (1) establishing the multi-field coupled 3D geological model and evaluating the risk of induced earthquakes in real time, and (2) establishing a high-precision monitoring system, obtaining the induced seismic information, and guiding the adjustment of hydraulic fracturing parameters in real time. In addition, to achieve efficient microseismic control, it is necessary to deeply analyze the initiation and propagation mechanisms of hydraulic fractures in HDR. High-precision exploration methods, such as 3D seismic surveys and imaging logging, should be used to finely characterize the deep geological structures of the site, and a multi-field coupled 3D geological model should be established to assess induced seismic risks in real time. A high-precision real-time monitoring system should be implemented to continuously acquire induced seismic information, guiding the adjustment of hydraulic fracturing parameters (such as injection pressure, flow rate, and cumulative injection volume). Engineering measures such as gradual pump shutdown, expanding unit injection scale, continuous injection, and precise flow rate control should be adopted to mitigate induced seismicity. Additionally, there is an urgent need to establish technical systems for evaluating the safety of and disaster risks at HDR extraction sites, multidisciplinary seismic monitoring networks and analysis technologies, and traffic light systems for seismic hazard risk management.

(4)

Fracture network connectivity and reservoir characterization

The identification and description of fracture development, spatial distribution, connectivity, density, strike and other fracture factors during and after hydraulic fracturing, as well as the estimation of reservoir stimulation volume and fluid distribution, are very important tasks when evaluating the connectivity and heat exchange space (area) of the artificial fluid channels (e.g., the artificial geothermal reservoir) [28,76,77,79]. The most direct method for determining the fracture structure of a HDR reservoir is downhole imaging, which involves using technologies such as ultrasonic imaging and borehole television to obtain images of fractures on the wellbore wall [7,12]. Statistical analysis is then performed to determine the probability distribution of fracture apertures and orientations. If there are multiple boreholes, the three-dimensional spatial structure of fractures can be inferred using geostatistical methods [95]. However, due to the limited number of boreholes in HDR sites, it is difficult to determine the three-dimensional fracture structure outside the wellbore using geostatistical methods.

Since fracture propagation during HDR reservoir creation generates microseismic events, monitoring and inverting the source locations of these events can help analyze the development process and scale of fractures [26,32,78,96]. At some EGS sites, such as the Cooper Basin in Australia, practitioners have attempted to use geophysical methods based on resistivity measurements to determine the internal structure of reservoir fractures [97,98]. While this method works well for shallow fracture media, its accuracy decreases significantly with increasing reservoir depth, making it difficult to obtain high signal-to-noise ratio information for the accurate analysis of HDR fracture structures. Additionally, fracturing process simulations, which recreate the fluid–stress coupling during reservoir creation, can generate fracture networks [7,70,99]. However, due to the complex regional geological conditions influencing fracture development over geological history, such as spatiotemporal temperature variations, uneven rock layer thickness, and the superposition of multiple stresses, the process simulations can only describe fracture development under highly generalized conditions and cannot fully reconstruct small-scale fracture structures.

It is evident that using a single method is insufficient for characterizing HDR reservoir fracture networks. Therefore, further research is needed to integrate logging data, microseismic monitoring, and in situ stress observations with physical and numerical simulations to establish reservoir porosity and permeability models, achieve high-precision HDR reservoir fracture network imaging, and develop multi-data fusion methods for fine fracture network characterization. In addition, it is necessary to further develop the high-resolution characterization method of the fracture network based on multi-data fusion, and establish the use of the multi-scale flow, heat transfer, mechanical, and geochemical (THMC) coupling simulation method and technology in fractured media. To overcome the key technology problems, such as the direction difference of the plane fracture distribution, heterogeneous connectivity, a high pressure difference, a low flow rate, and a large temperature disturbance in HDR geothermal reservoirs, alternative injection and production, acid injection and dissolution, imaging of the target area, accurate perforation, and other technological measures should be investigated and improved. These key technologies are available to strengthen fluid flow and the heat transfer effect in HDR geothermal reservoirs.

3.3. The Game-Changing Technology of HDR Geothermal Development

(1)

Downhole liquid explosion fracture creation

Downhole liquid explosive fracturing technology involves injecting liquid explosives into main fractures and detonating them, utilizing the stress waves and rapid expansion of high-temperature, high-pressure gases generated by the explosion to create a network of micro-fractures, thereby enhancing formation permeability [100,101]. This technology has been successfully applied in the field of oil and gas stimulation, significantly improving the productivity of low-permeability reservoirs [102]. For example, by optimizing explosion parameters and fracture propagation ranges, some oilfields have achieved an increase greater than 30% in single-well production. However, this technology remains in the laboratory research stage in terms of HDR geothermal development [103,104,105]. Preliminary experiments indicate that explosive fracturing can improve the permeability of high-temperature hard rocks, but theoretical models, process design, and safety measures still require refinement. Current research focuses on controlling explosion energy, understanding fracture propagation mechanisms, and characterizing the high-temperature response of rocks. However, due to the lack of explosion dynamics models suitable for HDR reservoirs, practical applications face issues such as excessive fracture propagation or low energy utilization efficiency. Additionally, downhole explosions may cause wellbore damage or equipment failure, limiting their direct application in HDR reservoir stimulation [106].

The future development of downhole liquid explosive fracturing technology for HDR will focus on safety, controllability, and multi-technology integration. This includes (1) developing low-detonation-velocity, high-energy-efficiency liquid explosives (such as nano-composite explosives) to reduce the impact on the wellbore while simultaneously utilizing intelligent monitoring systems to regulate explosion energy in real time and lower operational risks. (2) It also involves combining numerical simulations and physical experiments to establish explosion fracture propagation models suitable for the high-temperature and high-pressure environments of HDR in order to optimize explosive formulations and injection processes. For example, the Cooper Basin project in Australia attempted to monitor explosion-induced fracture morphology using resistivity logging, but the accuracy of electrical exploration in deep reservoirs required improvement. (3) It also involves integrating explosive fracturing with hydraulic fracturing and acid fracturing to form comprehensive reservoir stimulation solutions. For instance, the Gonghe HDR project in China proposed an “alternate injection and production + precise perforation” process to address issues of uneven fracture distribution and low heat exchange efficiency. Finally, it involves conducting field trials in HDR demonstration projects to verify the feasibility and economic viability of the technology. (4) The FORGE EGS project, through high-precision microseismic monitoring to guide fracturing parameter adjustments, provides a reference for the engineering application of explosive fracturing technology.

(2)

Downhole in situ efficient heat transfer and power generation

At present, in situ heat transfer technologies include coaxial single-well heat transfer, gravity heat pipe, and horizontal well heat transfer, etc. Due to the limited heat exchange contact of these technologies, heat transfer in HDR is only carried out by heat conduction, and so the heat transfer rate is low. Thus, improving the thermoelectric conversion efficiency is the main bottleneck faced in terms of in situ heat transfer technology [6]. To break advance technology of forced circulation heat transfer and heat extraction in the wellbore for the deep HDR reservoir, it is necessary to study the dynamics process of fracture water circulation and the convective heat transfer mechanism in the crushed reservoir zone under different pressure and formation conditions [107,108,109]. In addition, the development of megawatt-level thermal power generation technology and prototype equipment is very important to achieve the best thermoelectric conversion efficiency possible. These research and development studies help to construct a reproducible in situ efficient heat production and power generation technology system. This is considered an important way to solve the microseismic problem induced by hydraulic fracturing and the large-scale development of HDR geothermal resources.

(3)

CO₂ and other working fluid for high-efficient power generation

Compared to traditional water-based working fluids, CO₂ possesses lower viscosity and higher thermal expansivity, enhancing its mobility in geothermal reservoirs and enabling more efficient heat transfer [22,110]. Using CO₂ as a working fluid not only improves geothermal energy extraction efficiency but also facilitates the long-term sequestration of CO₂ within reservoirs [22,110,111,112]. Particularly in low-temperature geothermal conditions, the thermal expansion effect of CO₂ significantly enhances heat exchange efficiency, leading to improved heat recovery rates [22,113,114]. It is easier to make CO₂ reach a supercritical state than water; thus, CO₂ has higher thermal efficiency than water in turbines. In addition, the CO₂-based cyclic power generation system has the beneficial characteristics of less equipment and less thermal inertia of the unit, allowing it to realize fast power load lifting and adjusting [115]. This is of great significance for adjusting power load fluctuation, balancing the supply side and demand side, and realizing multi-energy complementation. While CO₂ demonstrates notable advantages as a circulating medium for enhancing geothermal energy recovery rates and enabling carbon storage integration, its large-scale implementation remains constrained by economic barriers. The expenses linked to CO₂ capture, transportation, and subsurface infrastructure development, particularly deep drilling operations and long-term system maintenance, often surpass initial financial projections. However, by adopting integrated system optimization strategies, such as coupling geothermal systems with supplementary renewable energy inputs (e.g., solar thermal or wind-powered heating), the overall energy output and cost-efficiency of such hybrid systems can be significantly improved.

3.4. Technical Level Assessment of HDR Development in China and the United States

The United States holds a globally leading position in the exploration and development of HDR, with its FORGE project successfully achieving high-temperature hard rock horizontal drilling and multi-stage fracturing, laying the foundation for large-scale reservoir creation. In terms of efficient heat exchange and power generation technologies, the U.S. has accumulated extensive experience, having conducted multiple HDR power generation trials to validate the high efficiency and environmental friendliness of supercritical CO₂ power generation systems [1,2,4,40]. Additionally, the U.S. boasts a mature technical system for environmental monitoring and risk control, having established comprehensive seismic activity and water quality monitoring networks to ensure the safety and sustainability of the development process. The technological advantages of the U.S. are primarily reflected in fracture network characterization, real-time fracturing parameter optimization, and long-term system stability.

China has made significant progress in HDR resource exploration, particularly in the Gonghe Basin in Qinghai Province and the southeastern coastal regions, where high-temperature HDR samples have been successfully obtained through geophysical exploration and scientific drilling. In terms of reservoir stimulation technologies, China has mastered key techniques such as hydraulic fracturing and acid fracturing, but further improvements are needed in fracture network optimization and long-term stability. China’s HDR heat extraction and power generation technologies are still in the experimental stage, with small-scale heat extraction trials conducted and certain breakthroughs achieved in supercritical CO₂ power generation technology. However, power generation efficiency and commercial application still require enhancement [40]. Furthermore, China started relatively late in terms of environmental monitoring and safety assurance, but gradually established relevant monitoring systems, though environmental impact assessment technologies need further refinement.

Based on the above-mentioned comprehensive analysis, we compare the development gaps in terms of relevant technologies for HDR development at home and abroad (

Table 1. Comparison of technological development gap of HDR at home and abroad.

Technological Type	Technological Name	Technological Development Gap	Technical Application
Base technologies	Evaluation and site selection	Synchronization	Industrialization
	Efficient and low-cost drilling	Run after	Industrialization
	Efficient geothermal utilization	Run after	Industrialization
Key technologies	High-resolution exploration and characterization of HDR	Run after	Pilot
	Efficient and complex fracture network reservoir creation	Run after	Pilot
	Effective microseismic control	Synchronization	Pilot
	Fracture network connectivity and reservoir characterization	Synchronization	Pilot
Game-changing technologies	Downhole liquid explosion fracture creation	Synchronization	Laboratory
	Downhole in situ efficient heat transfer and power generation	Synchronization	Pilot
	CO2 and other working fluid for high-efficient power generation	Synchronization	Laboratory

). Overall, compared with international standards, the research on HDR geothermal development in China is continuously narrowing the gap in the development of multiple individual technologies. Theoretical and laboratory research has reached the international parallel-running stage, and the construction of HDR geothermal demonstration projects is gradually shifting from following to parallel—running with international efforts. However, there are notable gaps between China and the U.S. in fracture network optimization, efficient heat exchange technologies, and environmental monitoring and risk control. The U.S. holds a clear advantage in the maturity of reservoir stimulation and power generation technologies, while China demonstrates strong potential in resource exploration and preliminary trials. In the future, China and the U.S. can promote mutual progress in HDR technology by strengthening technical exchange and joint research and development. For example, China can learn from the U.S. in real-time fracturing parameter optimization and environmental monitoring, while the U.S. can collaborate with China to explore new reservoir stimulation technologies suitable for complex geological conditions. Additionally, the two countries can engage in in-depth cooperation in supercritical CO₂ power generation systems and multi-energy complementary technologies, jointly advancing the commercial application of HDR geothermal resources.

Based on the above comparison of existing technologies for HDR geothermal resource development, we conclude that the key challenges currently hindering the commercialization of HDR development include (1) the significant drilling depth (3000–6000 m) and high temperatures (>200 °C) of HDR, requiring advanced drilling tools and fluids. However, the current heat resistance of drill bits and drilling fluids is insufficient, resulting in low drilling efficiency and high costs. (2) HDR reservoirs are dense and have low permeability, necessitating the creation of artificial fracture networks through technologies like hydraulic fracturing. However, the prediction and monitoring of fracture propagation remain underdeveloped, making precise control difficult to achieve. (3) The development of HDR may induce microseismic events, posing risks to engineering safety and social acceptance.

To better advance the commercialization of HDR development, the most urgent research priorities include (1) establishing resource evaluation models based on big data and artificial intelligence to enhance the scientific and economic viability of target area selection; (2) developing high-temperature-resistant (>300 °C) drill bits, drilling fluids, and downhole tools to improve drilling efficiency and tool longevity; (3) investigating fracture propagation mechanisms under multi-field coupling (thermal–hydraulic–mechanical–chemical) conditions while developing real-time monitoring and optimization technologies to create efficient and complex fracture network reservoirs; and (4) developing predictive models and control measures for induced seismicity to ensure the environmental safety of HDR development.

4. Research Suggestions for HDR

Besides the technical breakthrough mentioned above, the following issues should be considered when developing and utilizing high-temperature HDR resources.

4.1. Combined Heating and Power Generation

There are many thermal anomalies regions in western China due to the special geological conditions of the Qinghai Tibet Plateau, where the 200 °C HDR sites could be obtained within a depth of less than 4000 m. For example, Qinghai Gonghe basin is two hours away from Xining, with convenient transportation and power grid conditions. It is situated within the Chinese western Clean Energy Corridor adjacent to the Longyangxia Hydropower Station on the Yellow River, featuring a wind power station, a solar photovoltaic power station, and a photothermal power station. If the commercial-scale HDR in Gonghe basin is successfully developed, it will be of significant importance for the combination of water–wind–solar power generation. Compared with the other climate-sensitive clean energy (e.g., wind energy, solar energy, and/or hydro-power electricity), the energy stored in the HDR is more flexible. As a result, geothermal energy plays an important role in considering power grid peak adjustment. Therefore, in terms of economic calculation, the power generation price of HDR should be higher, which can be distinctively considered according to the peak load electricity price.

The HDR at the bottom of the medium/low-temperature sedimentary basin in the “three-North” areas (North China, Northeast China, and Northwest China) is a good alternative to fossil fuel for district heating after reservoir recreation. For example, a geothermal well drilled at the Matouying field in the North China had a geothermal temperature of only 180 °C at a depth of more than 5000 m. Given the cost of well drilling and fracturing, the economics of generating electricity is poor. However, the utilization of HDR in district heating for residential regions, factories, and/or ecological agriculture can effectively alleviate air pollution and smog in winter. Additionally, district heating in the three-north areas in China is a multi-trillion market, which suggests the economic and environmental benefits of medium-low temperature HDR geothermal resources for heating.

4.2. Industry–University–Research Cooperation

HDR geothermal energy development and utilization is a technology-intensive system. Generally, engineering implementations need to start from theoretical study and feasibility assessment. However, theoretical study is often disconnected from the actual project due to the complexity of the subsurface HDR geothermal reservoir. Therefore, it is necessary to closely combine theoretical study with the pilot test and field test to promote the development of EGS technology. This means the development of the HDR geothermal resource need to follow the “Industry–University–Research cooperation” model, which contributes to the development and improvement of EGS technology. Based on the “Industry–University–Research cooperation” model, we will continue to improve theoretical cognition, break through key technologies, and enhance the economic and application value of HDR geothermal resources. There is no doubt that this research model is beneficial to promoting the commercial exploitation of HDR geothermal resources, which is very important for achieving the “carbon peak” and “carbon neutrality” goals in China.

4.3. Increasing Government Investment in R&D and Demonstration

The process of creating and sustaining EGS reservoirs is still in its initial stage all around the world. Due to the huge engineering efforts and economic risks, government support is the most important way to achieve the sustainable commercial development of HDR geothermal resources. We can conduct and encourage more national R&D projects and field large-scale demonstration projects (e.g., high-temperature granite in west China, and the medium/low-temperature carbonate/volcanic rock in three-North China), based on a comprehensive understanding of the international experience and the geothermal engineering conditions in China.

5. Conclusions

Developed countries have adopted a series of key technology research and engineering practices in response to government guidance, promoting the commercial development of HDR geothermal resources. Now, relatively complete HDR geothermal development and utilization systems have been established in these countries. The U.S. Department of Energy’s Frontier Observatory for Research in Geothermal Energy (FORGE) program was initiated to develop and test techniques to create, monitor, and sustain the EGS reservoir, which was expected to be a milestone in EGS engineering. However, with only a dozen years of research history for HDR in China, there are still some limitations in the technology, engineering practice, R&D investment, etc. In addition, more effort should be focused on reservoir creation and control technologies (e.g., staged fracturing in deep deviated wells or horizontal wells). China is relatively late in reaching the reservoir creation stage of its HDR development project, thus requiring researchers and engineers to better understand the international theories and techniques. There are significant challenges to achieving the commercial-scale development and utilization of HDR geothermal resources. Geothermal energy, like hydro-power energy, wind energy, solar energy, and nuclear energy, is non-carbon-based energy. It is believed that geothermal energy is of great significance for China in terms of achieving the goal of carbon neutrality. Handling scientific problems and key technologies is the most important pathway to achieving the successful development and utilization of HDR geothermal resources. Once the bottleneck problems are solved, the development of HDR geothermal resources will make a significant contribution to meeting the energy demand of China.