1. Introduction
Energy is the cornerstone of modern societal development, and a stable energy supply is crucial for maintaining national economic growth and social stability [1,2]. However, for a long time, the global energy structure has been overly dependent on fossil fuels such as coal, oil, and natural gas [3,4,5,6]. This has not only led to the rapid depletion of resources but also caused severe environmental issues, particularly the excessive emission of greenhouse gases [7,8]. The large-scale release of carbon dioxide, methane, and other greenhouse gases has directly contributed to global warming, posing a significant threat to the survival of humanity. In response to this global challenge, the 2015 United Nations Climate Change Conference adopted the landmark Paris Agreement, which set a clear goal of limiting the global average temperature increase to within 1.5 °C. Achieving this goal urgently requires a profound transformation of the global energy structure, reducing reliance on fossil fuels and progressively transitioning to clean and renewable energy sources [9,10,11].
In this context, geothermal energy has gradually attracted widespread attention from the international community as an important renewable energy source [12,13,14]. Geothermal energy refers to the heat energy naturally generated within the Earth, which is widely distributed around the globe and is characterized by its green, low-carbon, stable, and reliable nature [15,16,17]. Compared to other renewable energy sources such as solar, wind, and hydro energy, geothermal energy has significant advantages: first, geothermal resources are abundant, with global geothermal reserves far exceeding the currently known fossil fuel reserves; second, geothermal energy is widely distributed, with potential geothermal resources available almost anywhere in the world; third, geothermal energy is a stable energy source that is minimally affected by external conditions such as seasons, weather, and day-night cycles, providing a continuous and reliable energy supply [18,19,20,21]. Among geothermal resources, Hot Dry Rock (HDR) has become a research hotspot due to its unique geological characteristics and enormous energy potential [22,23]. HDR refers to high-temperature, low-permeability rock masses buried deep within the Earth’s crust, typically located at depths of 3 to 10 km below the surface, with temperatures ranging from 150 to 650 °C. The energy reserves of HDR are extremely vast, with estimates suggesting that the energy in HDR resources at depths of 3 to 10 km worldwide is equivalent to 30 times the currently proven global fossil fuel reserves. However, due to the low porosity and permeability of HDR rock masses, containing little or no water or only small amounts of water vapor, their development and utilization face significant technical challenges [24,25]. To address the development of HDR resources, Enhanced Geothermal Systems (EGS) technology has emerged [26]. The core of EGS technology involves creating artificial fracture networks in HDR through methods such as hydraulic fracturing or chemical dissolution, allowing cold water to be injected underground and exchange heat with the high-temperature rock masses [27]. The heated water or steam is then extracted back to the surface for power generation or heating purposes. EGS systems can significantly improve the efficiency of geothermal resource development and expand the application range of geothermal energy, enabling the effective utilization of more geothermal resources. Although EGS technology shows great potential, its application worldwide is still in the experimental and exploratory stages due to its high geological requirements and considerable development costs. In the future, with continuous technological advancements and the accumulation of engineering experience, EGS technology is expected to overcome these challenges and become an indispensable part of the global energy structure. Globally, research on geothermal energy, particularly HDR, has made significant progress [28]. Countries such as the United States, Japan, Germany, and France are actively promoting the development of EGS technology and have achieved a series of important results [29]. For example, the Fenton Hill project in the United States, the Hijiori project in Japan, and the Soultz-sous-Forêts project in Germany have all accumulated valuable experience in HDR development and the application of EGS technology [30,31,32]. As the world’s largest user of geothermal energy resources, China has also increased its efforts in research and development of geothermal energy, especially HDR, in recent years. In 2017, China launched its first EGS demonstration project in the Gonghe Basin of Qinghai Province, successfully drilling HDR resources with temperatures reaching up to 236 °C, marking a significant breakthrough in China’s HDR resource development [33]. Additionally, China has achieved experimental power generation from HDR in the Matouying area of Tangshan City, further advancing the commercialization of EGS technology [34].
While prior reviews focus on technical aspects of EGS, a systematic analysis quantifying global collaboration patterns, interdisciplinary linkages, and policy-driven trends in HDR research is absent. Bibliometric analysis, as a scientific research method, enables the statistical analysis of a large volume of literature data, revealing the overall development trends, research hotspots, and technological evolution within a specific field [35,36]. Through bibliometric analysis, researchers can identify the key research directions, leading institutions, and high-impact academic contributions in a particular area, providing a scientific basis and reference for future research. In the context of geothermal energy, especially HDR, bibliometric analysis can help us understand the current state of research and uncover potential future directions and challenges. This study aims to systematically analyze the research on geothermal energy, particularly HDR, using bibliometric methods to reveal the research trends, hot topics, and future development directions in this field. By comprehensively evaluating international and Chinese studies, this paper will provide scientific references for the effective development and utilization of geothermal energy, supporting global efforts to achieve low-carbon energy transition goals. Throughout this process, the study will not only examine the theoretical advancements in geothermal energy and HDR research but also focus on key issues and development bottlenecks in technological applications. By delving deeply into the literature data, this research will identify critical areas that require attention in future studies and offer guidance and decision-making references for professionals in the field.
As the global demand for clean energy continues to rise, the development and utilization of geothermal energy, particularly HDR resources, are becoming key drivers in the transformation of the global energy structure. Through in-depth bibliometric analysis, we can gain a comprehensive understanding of the current state and trends in geothermal energy and HDR research, providing scientific guidance for future research and development. This, in turn, will support the achievement of global sustainable energy goals.
2. Data and Methodology
2.1. Data Sources
The data for this study were sourced from the Web of Science (WoS) Core Collection, one of the most comprehensive and authoritative academic literature databases globally. The WoS Core Collection includes the Science Citation Index Expanded (SCIE), Social Sciences Citation Index (SSCI), and Arts & Humanities Citation Index (A&HCI), as well as high-impact academic journals, conference proceedings, and books from around the world [37,38]. Selecting WoS as the data source ensures that the collected literature data is of high quality and broad coverage. Non-English papers, non-peer-reviewed articles, and studies unrelated to HDR were excluded. In the process of literature collection, this study employed the “Topic” search type, meaning that the titles, abstracts, author keywords, and Keywords Plus of the literature were all within the search scope. To ensure comprehensive coverage of key research related to geothermal energy and rock, the study used the keyword combination “Geothermal energy” and “Rock” for the search. This choice of keywords helps to fully capture research literature on geothermal energy development and utilization, particularly those involving HDR. The time frame for this study was set from 1996 to 2023, allowing for the reflection of the historical evolution and current development trends in geothermal energy and HDR research. Ultimately, 1764 relevant documents were retrieved, covering multiple academic fields and providing a rich data foundation to support a systematic analysis of research in the geothermal energy and HDR domain.
2.2. Analysis Tools and Methods
In analyzing the collected literature data, this study employed a range of bibliometric tools to conduct comprehensive statistical and visual analyses [39,40]. These tools include Bibliometrix (version 4.0.0), CiteSpace (version 6.1.R3), and VOSviewer (Version 1.6.19). Bibliometrix, an open-source package developed in R (Version 4.23), is specifically designed for bibliometric analysis and visualization. It generates various bibliometric indicators, such as annual publication trends, the distribution of publications by leading countries and institutions, journal distributions, author collaboration networks, and keyword co-occurrence analyses. Through Bibliometrix, researchers can systematically analyze different dimensions of the literature data, revealing research trends and hotspots within the field. CiteSpace, a Java-based bibliometric software, is primarily used to detect research frontiers and emerging trends in scientific literature. CiteSpace analyzes citation networks, keyword co-occurrence networks, and author collaboration networks, providing a dynamic visualization of the evolution of scientific knowledge [41]. Its powerful visualization capabilities allow researchers to identify key nodes and burst hotspots in the research domain. VOSviewer is a tool specialized in constructing and visualizing bibliometric networks, particularly effective for displaying keyword co-occurrences and the collaboration relationships between institutions and authors in large-scale literature data. With VOSviewer, researchers can generate detailed network maps that visually represent the knowledge structure and academic collaboration within the research field. CiteSpace was selected for its strength in detecting emerging trends through citation burst analysis, while VOSviewer’s clustering algorithms effectively visualized keyword co-occurrence networks.
In terms of data processing and visualization, this study focused on several key dimensions. First, Keyword Analysis: By analyzing the keywords in the literature, core themes and hot topics within the research field can be identified. Keyword co-occurrence analysis reveals the relationships between different research topics and uncovers the trends in the field’s evolution. Second, Author and Institution Analysis: Researchers and research institutions are the driving forces of academic research. By analyzing the collaboration networks of authors and institutions, the study provides insights into the main research contributors and the extent of international collaboration within the field. The size of the node represents the number of publications, and the lines between nodes indicate the frequency of collaboration. Finally, Journal and Citation Analysis: Analyzing the source journals of the literature helps in understanding the main publication platforms in the research field and identifying the most influential academic journals. Citation analysis aids in determining high-impact papers and research outcomes, highlighting the important theoretical foundations and cutting-edge studies in the field. These analytical results are presented through visualization tools, allowing researchers to intuitively understand and interpret complex academic network relationships. The node network diagram not only displays the relationships between research institutions, authors, and keywords but also reveals the development trajectory and future trends of the research field. An illustration of this work is shown in Figure 1, covering 1764 publications from Web of Science (1996–2023).
3. Results and Discussion
3.1. Trend Analysis of Publishing
Figure 2 illustrates the annual publication count by country in the field of HDR geothermal research from 1996 to 2023. Overall, the study of HDR geothermal resources has undergone significant development, particularly in recent years, as this area has become a global research hotspot.
The progress in this field can be roughly divided into three main stages: First Stage (1996–2010): Low Awareness Period. During this stage, the annual publication count did not exceed 50 papers, indicating a low level of attention from researchers and the public toward HDR geothermal energy. This phase may have been characterized by technological limitations, the initial stages of resource development, and the unrecognized potential of geothermal energy. Research during this period primarily focused on foundational exploration, with scattered attention and relatively few research teams involved. Second Stage (2010–2018): Slow Development Period. From 2010 to 2018, research on HDR geothermal resources entered a slow development phase, with the annual publication count gradually increasing to an average of around 100 papers per year. This stage marked the beginning of greater interest in the potential of HDR geothermal energy, with more resources and efforts being invested in related studies. As geosciences and engineering technologies advanced, research began to shift towards applications, particularly in relation to EGS, which started gaining prominence during this period. Third Stage (2018–Present): Rapid Growth Period. Starting in 2018, research in the HDR geothermal field entered a phase of rapid growth. The annual publication count surged from 114 papers in 2018 to 281 papers in 2023, representing a significant increase. This sharp rise may be attributed to a combination of factors, including technological breakthroughs, new research findings, and the growing global demand for low-carbon and renewable energy sources. Additionally, increased public and governmental attention to climate change has further fueled the research intensity in this field. Particularly within the context of the “dual carbon” goals, the strategic importance of geothermal energy as a renewable resource has become increasingly prominent, making HDR development a crucial direction for energy transition [42,43].
This trend indicates that HDR geothermal resource research is heating up rapidly and has become a key topic in global energy research. With continued technological advancements and more countries joining HDR research, it is expected that this field will maintain its rapid development trajectory in the future. As research deepens and new technological breakthroughs and application scenarios emerge, HDR geothermal resources are likely to play an increasingly important role in the global energy structure.
3.2. Analysis of Disciplines and Research Categories
Figure 3 presents the characteristics of document types in the field of HDR geothermal research. Among the 1927 collected documents, 1764 (accounting for 91.5% of the total) are research articles. These articles provide extensive experimental data, theoretical analysis, and empirical studies, forming the primary basis for further analysis in this field. Additionally, 4.8% of the documents are review papers, which summarize and synthesize existing research findings, offering comprehensive background knowledge and research frameworks for researchers. Progress papers constitute 2.5% of the total, typically providing a quick overview of recent advances in a specific research area, showcasing the latest developments and technological breakthroughs. Early access papers account for 0.8%, representing emerging research hotspots that are in the early stages of investigation and contain cutting-edge content. Data papers, making up only 0.1% of the total, mainly provide original datasets or data analysis results, serving as valuable references for further research.
Table 1 lists the top 10 Web of Science subject categories ranked by centrality. Analyzing these subject categories provides a deeper understanding of the thematic distribution and research focus in the field of HDR geothermal resources. Earth sciences emerge as the dominant discipline in this area, with 671 articles, accounting for 38% of the total literature. This discipline plays a crucial role in geothermal energy research, reflecting its central importance in understanding the distribution, characteristics, and development potential of geothermal resources. Since 1996, research in earth sciences has steadily increased, with a centrality of 0.61, indicating its significant academic influence in the field of HDR research. Energy and fuels are the subject category with the highest number of publications, totaling 733 papers, which represent 41% of the total. This high publication volume reflects the strong interest of the energy sector in the development and utilization of HDR. As geothermal energy is recognized as a key renewable energy source with vast application potential, researchers in the energy and fuels field have devoted considerable effort to this topic. Its centrality of 0.39 indicates its importance within the research network. Environmental sciences account for 117 papers or 10% of the total literature. As the potential environmental impacts of geothermal energy development gain attention, environmental science has also taken a significant position in HDR research. These studies primarily focus on assessing the environmental impacts of geothermal development, formulating environmental protection measures, and investigating the long-term effects of geothermal exploitation on ecosystems. Although physical chemistry has fewer publications, with only 17 papers, its centrality is 0.22, indicating that this discipline plays a key role in specific research topics. These papers typically address chemical reactions, material properties, and fluid dynamics involved in geothermal energy development.
Research on HDR geothermal resources spans multiple academic disciplines, with earth sciences and energy sciences playing dominant roles. As research deepens, an increasing amount of interdisciplinary studies is driving the development of this field, providing more diverse perspectives and solutions for the comprehensive development and utilization of HDR.
3.3. Analysis of Countries, Institutions, and Authors
3.3.1. Mainly Researching Countries and Regions
This study collected a total of 1764 publications related to geothermal energy and HDR for bibliometric analysis. China is the leading country in this field, with 704 publications (accounting for 39.9% of the total), followed by the United States (292 publications, 16.5%), Germany (224 publications, 12.7%), Australia (97 publications, 5.5%), and Canada (94 publications, 5.3%). As some publications can be attributed to multiple countries, the total percentage exceeds 100%. The number of publications reflects the research activity level in each country within this field. Overall, HDR research receives more attention in developed countries, while developing countries are increasingly investing in this area. China, the United States, and Germany have high numbers of both single-country publications and international collaboration papers, highlighting their research influence in this field. Figure 4b shows the network diagram of cooperation between countries, clearly indicating close collaboration among the major countries, particularly the top three.
China is a major participant in the field of HDR geothermal energy research, with the largest node, indicating its absolute leadership in terms of publication volume. China’s lead in publications (39.9%) aligns with its 2017 National Geothermal Energy Development Plan, which allocated $2.3 billion to HDR projects [33]. China’s research primarily focuses on HDR resource assessment, development technologies, and practical applications. In recent years, China has significantly increased its research investment and output in this field, becoming a central force in global HDR research. However, despite its large volume of publications, China’s international collaboration network appears relatively loose, with more research likely concentrated on domestic collaborations. The United States is another key center for global HDR geothermal energy research, with a node size second only to China. The U.S. has a long history of research in this area, particularly leading in EGS technology. The U.S. has close collaborative relationships with other countries, with numerous and thick connections, indicating its pivotal role in global academic cooperation. The U.S. collaborates especially closely with countries such as Germany, Australia, and France, contributing significantly to the advancement of global HDR technology. Germany also holds a prominent position in the field, highlighting its important role in HDR geothermal energy research. Germany has achieved significant results in both technological development and application and has driven global research progress through extensive international collaborations. Germany’s partnerships with European countries (such as France, Italy, and Switzerland) and the United States are particularly strong, reflecting its central role in promoting geothermal research across Europe. Although Japan and Switzerland have fewer publications, they still occupy important positions in the network. These countries have a strong foundation in technological innovation and scientific research, and through collaborations with the U.S., Germany, and others, they continue to push the frontiers of HDR research. Japan focuses on geothermal power generation and resource assessment, while Switzerland has made significant contributions to the study of EGS. France and Italy also hold notable positions in the network, reflecting their important roles in European geothermal energy research. France’s Soultz-sous-Forêts project is one of the earliest EGS demonstration projects globally, providing valuable experience for HDR research worldwide. Italy has conducted in-depth research on both traditional geothermal resource utilization and the development of new technologies, maintaining close collaborative relationships with other European countries.
The global collaboration network in HDR geothermal energy research has already taken shape, as evident from Figure 4, and it shows a trend of gradual expansion. Although different countries have varied research focuses, they are collectively advancing the development and application of HDR technology through close international cooperation. As the global demand for renewable energy continues to grow, it is anticipated that these collaborative networks will further strengthen, thereby accelerating the development and utilization of HDR geothermal resources.
3.3.2. High-Yield Research Institutions and Their Collaborative Networks
Figure 5a and Table 2 present the ten most influential research institutions in the field of HDR geothermal energy research, including their respective countries, total publication counts, centrality, and the year they first began research in this domain. These institutions have played pivotal roles in global HDR geothermal energy research, contributing significantly to the field’s development through a substantial volume of high-quality research outputs. China University of Petroleum ranks first with 102 publications and a centrality of 0.13. Since entering the HDR research field in 2014, it has rapidly developed into a key player in geothermal energy research both in China and globally. The university’s research primarily focuses on the development of technologies, energy extraction, and practical applications of geothermal resources. Through collaborations with multiple domestic and international research institutions, it has continuously expanded its influence in HDR research. The Chinese Academy of Sciences (CAS), one of China’s core research institutions, ranks second with 90 publications and a centrality of 0.03. Despite its relatively lower centrality, CAS has a strong foundation in basic scientific research and technological innovation. Since beginning HDR research in 2009, CAS has made significant contributions to resource assessment, technological development, and policy studies, particularly in the field of EGS. China University of Mining and Technology ranks third with 76 publications and an h-index of 0.01. The university has been involved in HDR research since 2013, focusing on geothermal energy development, co-extraction with mineral resources, and the evaluation and modification of underground geothermal reservoirs. Although its centrality is low, its research in specific areas is of significant importance. The Helmholtz Association, Germany’s largest research institution, holds an important position in geothermal energy research, ranking fourth with 72 publications and an h-index of 0.02. Since initiating HDR research in 2002, the Helmholtz Association has achieved numerous breakthroughs in EGS technology, geothermal resource assessment, and geological engineering. Its collaborations with other international research institutions have furthered global geothermal energy research. The French National Centre for Scientific Research (CNRS), one of France’s largest research institutions, ranks ninth with 50 publications and a centrality of 0.07. CNRS began HDR research in 1997 and has made significant progress in geothermal resource development, EGS technology, and environmental impact assessments. Its collaborations with European and global research institutions have promoted the internationalization of HDR research. These research institutions, through high-quality scientific output and extensive international cooperation, have collectively advanced the field of HDR geothermal energy research. Chinese institutions stand out in terms of publication volume, reflecting China’s strong growth in global geothermal energy research. Meanwhile, research institutions from Germany, the United States, and France play crucial roles in technological innovation and international collaboration, further enhancing the connectivity and impact of the global HDR research network.
3.3.3. Main Researchers and Their Collaborative Relationships
Table 3 lists the top ten most prolific authors in the field of HDR geothermal energy research, including their affiliated institutions, the total number of publications, and the year of their first publication. These authors have played significant roles in advancing global HDR geothermal research, and their contributions have had a notable impact on the development of this field. Zhang Yanjun is the leading researcher in HDR geothermal energy, with 22 publications, ranking first among his peers. Since his first HDR-related publication in 2018, Zhang has rapidly established broad influence in this field. His research primarily focuses on the development and utilization of geothermal resources, the evaluation of HDR resources, and EGS technologies. Zhang’s work has significantly contributed to the leading position of Jilin University in this field [44]. Song ranks second with 16 publications, with his first paper published in 2015. His research covers a range of topics, including geothermal resource development technologies, dynamic analysis of geothermal reservoirs, and methods for energy extraction [45]. Ranjith PG, a researcher from Monash University in Australia, ranks third with 15 publications, with his first HDR-related research published in 2016. Ranjith PG’s work focuses on rock mechanics, geothermal energy extraction technologies, and the optimization of EGS systems. He is highly regarded in the international academic community, and his research, which often involves international collaborations, has had a profound impact on global geothermal energy development and utilization [46]. Xu Tianfu ranks fourth with 14 publications, with his first research paper published in 2018. His research centers on geothermal energy development technologies and the coupled behavior of multiphysics fields in underground reservoirs [47]. Sun Qiang is another active researcher in the HDR geothermal energy field, also with 14 publications, ranking fifth, with his first publication in 2018. His research interests include the evaluation and development of geothermal resources, geological engineering, and the optimization of EGS systems. These authors have made substantial contributions to HDR geothermal research, helping to shape the direction and advancements in this critical area of energy development [48].
3.4. Analysis of Journals and Highly Cited Literature
3.4.1. Main Published Journals and Their Influence
As shown in Table 4, Geothermics is the leading journal in the field of geothermal energy research, with 1226 publications and an h-index of 0.69, placing it at the top of the rankings [49]. Since its first publication related to HDR in 1996, this journal has become a crucial platform for global geothermal energy research, covering a wide range of topics from fundamental studies to applied developments. Although its centrality is relatively low at 0.08, the journal’s impact factor of 3.9 demonstrates its recognition and influence in the academic community. Geothermics is the preferred journal for scholars to publish their latest research findings and technological breakthroughs in geothermal energy. The International Journal of Rock Mechanics and Mining Sciences ranks second with 679 publications and an h-index of 0.38 [50]. This journal focuses on rock mechanics and mining sciences, and it has been publishing research related to HDR since 1996. With an impact factor of 7.2, the journal reflects a high level of authority and credibility in its respective fields. It provides an important academic platform for geothermal energy research, particularly in studies related to rock mechanics. The Journal of Geophysical Research: Solid Earth is ranked third, with 655 publications and an h-index of 0.37, having first published HDR-related research in 1996. The journal primarily covers research in geophysics and has an impact factor of 3.9. Although its centrality is relatively low at 0.04, it holds a significant position in geothermal resource research, especially in geophysical exploration and geological modeling. Energy ranks fourth with 561 publications and an h-index of 0.32, having published geothermal energy research since 2010. Its high impact factor of 9 reflects the journal’s substantial influence in the field of energy research. Although its centrality is 0.02, the high impact factor makes Energy a key publication platform for scholars in the energy research community, particularly in the development and utilization of renewable energy. Renewable and Sustainable Energy Reviews ranks fifth with 548 publications and an h-index of 0.31, with its first publication in 2008 [51]. This journal has an impressive impact factor of 15.9, making it one of the most influential journals in the field of renewable energy. Although it has a lower centrality of 0.01, its broad coverage of topics related to energy sustainability makes it an essential reference for geothermal energy researchers. These journals have played a pivotal role in advancing research on HDR geothermal energy. By publishing high-quality research, they have not only elevated the scientific level of geothermal energy studies but also increased global academic interest and investment in the development and utilization of geothermal resources. Looking ahead, these journals will continue to play an important role in driving technological innovation and the broader application of geothermal energy.
3.4.2. The Research Content and Influence of Highly Cited Literature
Table 5 lists the top ten most cited papers in the field of HDR geothermal energy research, including the titles, authors, total citations, annual citation rates, and publication years. These highly cited papers represent influential research outcomes in the field, serving as key references and foundational works for subsequent studies. They also reflect current research trends and cutting-edge directions. The first paper is a comprehensive review of EGS globally, making it the most cited paper in the EGS research domain. The article summarizes the current state of EGS development, technical challenges, and future prospects, becoming a crucial reference for researchers in this field. Its high citation count highlights the paper’s significance and impact on geothermal energy research. The second paper offers an in-depth discussion of the principles, technologies, and application prospects of EGS. Since its publication, it has been widely cited, underscoring its importance in the study of EGS. The third paper investigates the heat extraction performance of multilateral wells in EGS through numerical simulation. The findings of this study provide empirical support for optimizing EGS design, demonstrating its practical value in geothermal engineering research. The fourth paper explores hydraulic fracturing technology under high temperature and pressure conditions, combined with the application of micro-CT technology in HDR geothermal energy development. The research provides critical insights into understanding and optimizing rock mechanics behavior under high-temperature conditions, significantly contributing to the advancement of EGS technology. The fifth paper employs a thermal-hydraulic-mechanical coupling method based on a discrete fracture model to numerically simulate the heat extraction process in EGS. These highly cited papers cover key research topics within the EGS domain, such as heat extraction performance, hydraulic fracturing technology, thermal-hydraulic-mechanical coupling analysis, and rock mechanics behavior. These studies have not only had a broad impact on the academic community but also provided important references for practical applications, driving the development and implementation of EGS technology.
3.5. Analysis of Keywords
3.5.1. Keyword Co-Occurrence Network and Research Hotspots
Figure 6 illustrates the keyword co-occurrence network in the field of HDR geothermal energy research, highlighting the relationships among different keywords and the distribution of research hotspots. In this network, the size of the nodes represents the frequency of keyword occurrences in the literature, while the lines connecting the nodes indicate the strength of the associations between keywords. Analyzing the keyword co-occurrence network helps uncover the primary research focuses and development trends in this field. The most central keyword, “Geothermal Energy”, clearly stands as the core research theme in this domain. Studies surrounding geothermal energy encompass various aspects such as its development and utilization, resource assessment, and technological innovations. This keyword is closely linked with other significant terms like “HDR”, “EGS” and “Reservoir”, indicating researchers’ interest in diverse technological pathways for geothermal energy development. “HDR” is a critical resource type in geothermal energy development, particularly in the context of EGS. This keyword is closely connected to terms such as “Thermal Conductivity”, “Fracture”, and “Heat Transfer”, reflecting the research focus on the thermal properties and fracture networks of HDR in high-temperature environments. “Simulation” plays a crucial role in geothermal energy research, particularly in EGS and HDR studies. Simulation techniques are widely used to understand the physical and chemical processes within geothermal reservoirs, including thermal conduction, fluid flow, and stress distribution. Related keywords include “Numerical Simulation”, “Fluid Flow” and “Transport”, collectively demonstrating the application of simulation technologies in predicting and optimizing geothermal system performance. EGS has become a popular research topic in recent years, with EGS technology enhancing reservoir permeability through artificial means, thus enabling the exploitation of HDR resources. Relevant keywords such as “Hydraulic Fracturing”, “Heat Extraction” and “Sequestration” indicate the depth of research into optimizing EGS systems and their environmental impacts. The ‘Hydraulic Fracturing’ cluster (Figure 6) reflects efforts to enhance HDR permeability. The keyword “Heat Transfer” reflects the focus on the process of transferring thermal energy from underground reservoirs to the surface, a process critical to the efficiency of geothermal energy extraction. This is closely related to keywords like “Thermal Conductivity”, “Fluid Flow” and “Granite”, underscoring the importance of studying the physical and material properties that influence heat transfer processes.
3.5.2. Keyword Clustering and Emergence Analysis
The keyword clustering map in Figure 7 illustrates the clustering of different keywords in the field of geothermal energy research, particularly focusing on HDR. Each cluster represents a research theme or direction, with different colors distinguishing the clusters, and the labeled theme names indicating the main research content within each cluster. Burst analysis further reveals the frequency and temporal trends of these themes in research. The Granite, Rock, Fracture, and Thermal Conductivity cluster focuses on granite as a major rock type in HDR systems. The research themes within this cluster are primarily centered around the physical and thermodynamic properties of granite, particularly its thermal conductivity and fracture development. These studies are crucial for the development and application of EGS, as granite often constitutes a significant part of HDR reservoirs. The EGS, Hydraulic Fracturing, and Heat Extraction clusters represent research themes related to EGS, particularly focusing on enhancing the permeability of HDR reservoirs through hydraulic fracturing to improve heat extraction efficiency. This cluster covers EGS system design, implementation, and related environmental and engineering issues, making it a frontier research area in geothermal resource development. The Water-Rock Interaction, Chemical Geothermometry cluster focuses on the interaction between water and rock, especially the chemical reactions occurring in high-temperature geothermal environments. Water-rock interactions play a critical role in the formation and evolution of geothermal reservoirs, helping to understand and predict geochemical processes within geothermal systems. The Deep Borehole, Heat Exchanger, and Fluid Flow clusters involve research on deep borehole heat exchangers, emphasizing how to achieve efficient geothermal energy extraction through deep-well technology. Relevant studies include dynamic analyses of fluid flow and heat exchange processes in deep underground environments, which are crucial for improving geothermal energy extraction efficiency. The Geothermal Systems, Energy, and Power Generation cluster encompasses the overall design and application of geothermal systems, including geothermal power generation, system optimization, and integration with renewable energy sources. Research in this cluster is highly valuable for the large-scale application of geothermal energy and optimizing energy utilization. Burst analysis reveals the sudden increase in the frequency of high-impact keywords within specific time periods, indicating their heightened focus during those periods. For instance, the keyword Granite continues to be a significant research subject in HDR studies, while EGS and Hydraulic Fracturing show technological innovations and applications in geothermal resource development. The emergence of these hot topics reflects shifts in research directions and new trends in technological development. For example, the burst of interest in EGS technology highlights the strong focus on new methods for geothermal resource development, while the research on water-rock interaction underscores the importance of environmental impact assessment and sustainable resource management. Table 6 presents the strongest keyword bursts in HDR geothermal energy research from 1996 to 2023. Australian Strathbogie granite is a benchmark for HDR studies due to its uniform thermal properties, enabling reproducible experiments on fracture propagation [60]. The burst intensity of a keyword reflects a significant surge in citations during a specific period, representing a research hotspot or trend in the field during that time. For example, Systems (burst intensity: 5.92) was prominent from 2010 to 2016, indicating that research on system-level topics, including the design, optimization, and integration of geothermal systems, became a key focus. Fractured Granite Reservoir (intensity: 4.88) was concentrated between 2016 and 2019, reflecting in-depth research on the thermodynamic and mechanical properties of granite as an HDR reservoir. Groundwater Flow (intensity: 4.46) and Acoustic Emission (intensity: 4.28) gained prominence after 2016 and 2020, respectively, with the former focusing on the flow of groundwater within geothermal systems and its impact on system performance, and the latter concentrating on monitoring rock fracture behavior through acoustic emission techniques. Transport (intensity: 4.18) was notable from 2015 to 2017, related to research on heat conduction and material transport, crucial for understanding energy and material exchange within geothermal reservoirs. EGS and Hydraulic Fracture both showed strong bursts in 2019 and 2020, respectively, indicating that research on EGS technology and hydraulic fracturing techniques has gained widespread attention in recent years, becoming key technologies driving geothermal energy development.
3.5.3. Time Evolution and Future Trend Prediction of Research Hotspots
Figure 8 illustrates the evolution of major research hotspots in the field of geothermal energy and HDR over time. Different colored dots represent different research hotspots, with the size of each dot indicating the intensity of research during a specific period, and the position reflecting the year in which the hotspot emerged. By analyzing the temporal evolution of these hotspots, we can anticipate future research trends.
Early Research Stage (1996–2005): Keywords: “Geothermal Energy”, “Rock”, “Flow” and “HDR”. During this initial phase, research primarily focused on the foundational aspects of geothermal energy, including the definition of geothermal energy, heat flow characteristics, and the preliminary assessment of HDR resources. This stage laid the theoretical groundwork for subsequent technological developments through basic geological and thermodynamic studies.
Technological Development Stage (2006–2015): Keywords: “Numerical Simulation”, “Enhanced Geothermal System”, “Fracture”, and “Thermal Conductivity”. As geothermal energy development technologies matured, research hotspots gradually shifted to more specific technical issues such as numerical simulation, EGS, fracture formation and propagation, and thermal conductivity characteristics. The study of EGS became particularly prominent during this stage, with researchers exploring ways to enhance the permeability and heat exchange efficiency of subsurface heat reservoirs through artificial means.
Recent Research Stage (2016–2023): Keywords: “Permeability”, “Mechanical Behavior”, “Hydraulic Fracturing” and “Water-Rock Interaction”. Post-2016, research hotspots became more refined, focusing on technical optimization and environmental impact assessment. Keywords like “permeability”, “mechanical behavior of rocks”, “hydraulic fracturing”, and “water-rock interaction” indicate that the research focus has shifted towards improving geothermal energy extraction efficiency and mitigating environmental impacts. The diversification of research hotspots also reflects the depth and breadth of expansion in this field.
Predicted Future Trends Based on Temporal Evolution. For Keywords like “Discrete Element Method”, “Crack Propagation” and “Stress-Strain Behavior”. As understanding of subsurface heat reservoirs deepens, future research may increasingly focus on fine-tuned technological innovations, such as the application of the Discrete Element Method (DEM) for simulating complex rock structures, and the precise prediction of crack propagation and stress-strain behavior. Advances in these areas will further optimize the design and operation of EGS systems. For Keywords like “Water-Rock Interaction”, “CO2 Sequestration” and “Environmental Impact”: With growing global concern for environmental protection, future studies are likely to emphasize the environmental impact assessment of geothermal energy development, particularly in water-rock interaction and CO2 sequestration technologies. These studies will contribute to developing more sustainable geothermal energy utilization methods while minimizing potential environmental risks. For Keywords like “Organic Rankine Cycle”, “Multi-Phase Flow” and “Thermal Energy Storage”. Future research may strengthen the integration of geothermal energy with other energy technologies, such as the application of the Organic Rankine Cycle in low-temperature geothermal power generation, and the combination of multi-phase flow and thermal energy storage technologies. This multidisciplinary research will expand the role of geothermal energy in broader application scenarios. For Keywords like “Machine Learning”, “Big Data”, and “Real-Time Monitoring”. With the advancement of artificial intelligence and big data technologies, future geothermal energy research may increasingly rely on intelligent methods for real-time monitoring and data analysis. This will lead to more efficient and precise geothermal energy development processes, reducing resource waste and enhancing operational safety.
4. Future Prospects
4.1. Fracture Propagation Under Differential Stress
Geothermal energy and HDR research is expected to continue advancing toward technological optimization and system integration [61]. As EGS technology further matures, research will increasingly focus on how to effectively enhance the permeability and thermal conductivity of HDR reservoirs [62]. This will involve more sophisticated numerical simulations, advanced materials science, and in-depth studies in rock mechanics [63,64,65]. Additionally, with the growing global demand for renewable energy, the development of HDR resources is poised to become a mainstream clean energy solution, particularly in regions with energy scarcity or heavy reliance on fossil fuels [66].
Emerging research hotspots in geothermal energy are likely to center on the development of intelligent monitoring and control systems, leveraging artificial intelligence and big data analytics to optimize geothermal energy extraction processes in real time. Moreover, the application of the Discrete Element Method (DEM) and multi-scale simulation techniques will significantly enhance the understanding of subsurface fracture networks, thereby optimizing the design and operation of EGS systems [67,68,69,70]. Another potential technological breakthrough lies in improving the efficiency of low-temperature geothermal resource utilization through new technologies such as the Organic Rankine Cycle (ORC), enabling the comprehensive development of a broader range of geothermal resources [71,72,73].
4.2. The Challenge of Sustainable Development
Despite the significant progress in geothermal energy and HDR research, several challenges remain in technology development. Key obstacles include maintaining the stability of fracture systems under high-temperature and high-pressure conditions and optimizing heat extraction efficiency across diverse geological settings. Additionally, the high costs and technical complexity of geothermal drilling continue to be major bottlenecks for large-scale commercial applications. Future research will need to focus on developing more efficient and cost-effective drilling technologies, exploring methods to reduce costs, and finding interdisciplinary solutions, such as introducing materials that are more resistant to high temperatures and pressures or utilizing innovative drilling techniques [74,75]. Integrating AI/ML with geothermal engineering could optimize fracture network design. For instance, reinforcement learning algorithms might predict optimal hydraulic fracturing parameters in real time.
The environmental impact of geothermal energy development will also be a crucial focus of future research, particularly in minimizing damage to groundwater resources and surface ecosystems while maximizing geothermal energy extraction efficiency. Potential environmental risks during geothermal development include groundwater contamination, surface subsidence, and induced seismicity. Quantifying groundwater contamination risks in EGS systems should be prioritized, using tracer studies to model fluid migration pathways. Therefore, future research should aim to develop more environmentally friendly extraction technologies and robust environmental monitoring and response mechanisms to ensure the sustainability of geothermal energy development [76].
4.3. Multidisciplinary Collaboration and Application Prospects
The complexity of geothermal energy and HDR research necessitates the development of these technologies through multidisciplinary collaboration. Fields such as geology, physics, chemistry, materials science, environmental science, and engineering must work together to drive future technological breakthroughs. For instance, integrating environmental science with engineering technology can lead to a better understanding and mitigation of the environmental impacts of geothermal development, while advances in materials science may provide new materials for developing more heat- and pressure-resistant drilling equipment [77].
As technology advances and economic feasibility improves, HDR development is expected to become a commercially competitive clean energy option in the future. Achieving this goal will require not only technological breakthroughs but also policy support, market incentives, and public acceptance. Economic studies will need to comprehensively analyze development costs, energy output, market demand, and long-term benefits. Future studies should conduct Levelized Cost of Energy (LCOE) analyses for HDR projects in regions like East Africa, where geothermal potential remains underexploited due to funding gaps. Additionally, social impact assessments will be crucial in evaluating the effects of HDR development on local communities, employment, and economic growth, ensuring that this technology contributes positively to society while achieving sustainable development.
5. Conclusions
This research employed bibliometric methods alongside data-driven visualizations to perform a comprehensive, structured, and quantitative analysis of studies concerning the field of HDR geothermal research, covering the period from 1996 to 2023, leading to the following key conclusions: (1). This study conducted a bibliometric analysis of 1764 publications related to geothermal energy and hot dry rocks. The findings reveal that research from 1996 to 2023 can be divided into three distinct phases, with a rapid growth phase beginning in 2014, characterized by a significant annual increase in publications. “Multidisciplinary Geosciences” emerged as the dominant category in the Web of Science, followed by Energy & Fuels, Environmental Sciences, Physical Chemistry, and Engineering. The publication trends across different academic categories show considerable variation, with major research areas including Geology, Engineering, Environmental Science & Ecology, and Materials Science. (2). Research outcomes related to geothermal energy and hot dry rocks have been published in over 600 journals, with Geothermics being the most productive. Among the publications from more than 70 countries, approximately 72% are single-country studies, while 28% are the result of international collaborations. China leads the field, followed by the United States, Germany, and Australia. Research on hot dry rock development is gaining increasing attention in developing countries, with the Chinese Academy of Sciences being a major research institution in this area. Over 5400 authors have contributed to research on geothermal energy and hot dry rocks, with a relatively small number of single-author papers. On average, each paper has 4.6 co-authors, indicating close collaboration among countries, institutions, and researchers. (3). Research in geothermal energy and hot dry rocks encompasses various technical fields, such as EGS, hydraulic fracturing technology, and environmental impact assessments. Keyword co-occurrence analysis, cluster analysis, and burst analysis show that early research focused on the assessment and development of hot dry rock resources. As technology advanced, EGS technology and its environmental impact became research hotspots, particularly studies aimed at improving system efficiency and reducing environmental impacts. In the latest phase, the interdisciplinary application and technological integration of geothermal energy and hot dry rocks have gradually become research frontiers.
X.Q.: Writing—original draft, Funding acquisition, Visualization, Software, Conceptualization, Formal analysis, Methodology, Investigation. S.Z.: Writing—review & editing, Supervision, Resources, Project administration, Conceptualization. B.H.: Writing—review & editing, Supervision, Investigation, Formal analysis. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
The authors declare no conflicts of interest.
Footnotes
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Figure 1 A scheme of document collection and bibliometric analysis.
Figure 2 Annual publication count by country in the field of HDR geothermal research during 1996–2023.
Figure 3 The type of documents in the field of HDR geothermal research during 1996–2023.
Figure 4 Network diagram of cooperative relations between countries: (a) Sine diagram between countries; (b) National.
Figure 5 Network diagram of cooperative relations: (a) Institutions; (b) Authors.
Figure 6 Network visualization of keywords co-occurrence network.
Figure 7 Main clusters generated by keyword co-occurrence.
Figure 8 Time zone visualization analysis of keywords and clusters.
The top 10 web of science categories.
| Rank | Categories | Total Publication | Centrality | Initial Year | |
|---|---|---|---|---|---|
| No. | % | ||||
| 1 | Geosciences, Multidisciplinary | 671 | 0.38 | 0.61 | 1996 |
| 2 | Energy & Fuels | 733 | 0.41 | 0.39 | 1996 |
| 3 | Environmental Sciences | 117 | 0.10 | 0.23 | 2002 |
| 4 | Chemistry, Physical | 17 | 0.01 | 0.22 | 2007 |
| 5 | Computer Science, Interdisciplinary Applications | 20 | 0.01 | 0.21 | 2013 |
| 6 | Geography, Physical | 7 | 0.00 | 0.15 | 2007 |
| 7 | Materials Science, Multidisciplinary | 36 | 0.02 | 0.14 | 2003 |
| 8 | Engineering, Electrical & Electronic | 11 | 0.00 | 0.14 | 2000 |
| 9 | Mechanics | 114 | 0.06 | 0.12 | 1997 |
| 10 | Engineering, Chemical | 80 | 0.04 | 0.12 | 1996 |
List of top 10 institutes during 1996–2023.
| Rank | Institute | Country | Total Publications | Centrality | Initial Year | |
|---|---|---|---|---|---|---|
| No. | % | |||||
| 1 | China University of Petroleum | China | 102 | 5.7 | 0.13 | 2014 |
| 2 | Chinese Academy of Sciences | China | 90 | 5.1 | 0.03 | 2009 |
| 3 | China University of Mining & Technology | China | 76 | 4.3 | 0.01 | 2013 |
| 4 | Helmholtz Association | Germany | 72 | 4.0 | 0.02 | 2002 |
| 5 | Jilin University | China | 68 | 3.8 | 0.08 | 2014 |
| 6 | United States Department of Energy (DOE) | America | 53 | 3.0 | 0.11 | 1998 |
| 7 | China Geological Survey | China | 52 | 2.9 | 0.02 | 2009 |
| 8 | China University of Geosciences | China | 51 | 2.8 | 0.10 | 2006 |
| 9 | Centre National de la Recherche Scientifique (CNRS) | France | 50 | 2.8 | 0.07 | 1997 |
| 10 | Helmholtz-Center Potsdam GFZ German Research Center for Geosciences | Germany | 48 | 2.7 | 0.10 | 2002 |
List of the top 10 authors with published articles.
| Rank | Author | Institution | Total Publication | Initial Year |
|---|---|---|---|---|
| No. | ||||
| 1 | Zhang, Yanjun | Jilin University | 22 | 2018 |
| 2 | Song, Xianzhi | China University of Petroleum | 16 | 2015 |
| 3 | Ranjith, PG | Monash University | 15 | 2016 |
| 4 | Xu, Tianfu | Jilin University | 14 | 2018 |
| 5 | Sun, Qiang | Xi’an University of Science & Technology | 14 | 2018 |
| 6 | Shi, Yu | Southwest Jiaotong University | 14 | 2018 |
| 7 | Raymond, Jasmin | University of Quebec | 13 | 2018 |
| 8 | Li, Gensheng | China University of Petroleum | 13 | 2018 |
| 9 | Sass, Ingo | Technical University of Darmstadt | 9 | 2011 |
| 10 | Saar, Martin O | Swiss Federal Institutes of Technology Domain | 9 | 2018 |
Top 10 most productive journals.
| Rank | Journal | Total Publication | h-Index | Centrality | Initial Year | Impact Factor | |
|---|---|---|---|---|---|---|---|
| No. | % | ||||||
| 1 | Geothermics | 1226 | 0.69 | 58 | 0.08 | 1996 | 3.9 |
| 2 | Int J Rock Mech Min | 679 | 0.38 | 127 | 0.11 | 1996 | 7.2 |
| 3 | J Geophys Res-Sol Ea | 655 | 0.37 | 204 | 0.04 | 1996 | 3.9 |
| 4 | Energy | 561 | 0.32 | 158 | 0.02 | 2010 | 9 |
| 5 | Renew Sust Energ Rev | 548 | 0.31 | 222 | 0.01 | 2008 | 15.9 |
| 6 | Renew Energ | 537 | 0.30 | 157 | 0.03 | 2008 | 8.7 |
| 7 | Appl Therm Eng | 474 | 0.27 | 129 | 0.02 | 2009 | 6.4 |
| 8 | Tectonophysics | 460 | 0.26 | 149 | 0.07 | 1997 | 2.9 |
| 9 | Geophys Res Lett | 433 | 0.25 | 240 | 0.02 | 1998 | 5.2 |
| 10 | J Geophys Res | 417 | 0.23 | 295 | 0.02 | 1996 | / |
Top ten articles on restoration of geothermal energy from 1996 to 2023 by number of citations.
| No. | Title | Authors | Total Citations | Cited Times | Year |
|---|---|---|---|---|---|
| 1 | A global review of enhanced geothermal system (EGS) [ | Lu, SM et al. | 116 | 0.02 | 2018 |
| 2 | Enhanced geothermal systems (EGS): A review [ | Olasolo, P et al. | 72 | 0.02 | 2016 |
| 3 | Numerical simulation of heat extraction performance in enhanced geothermal system with multilateral wells [ | Song, XZ et al. | 67 | 0.02 | 2018 |
| 4 | Hydraulic fracturing under high temperature and pressure conditions with micro CT applications: Geothermal energy from hot dry rocks [ | Kumari, WGP et al. | 62 | 0.04 | 2018 |
| 5 | Numerical simulation of the heat extraction in EGS with thermal-hydraulic-mechanical coupling method based on discrete fractures model [ | Sun, ZX et al. | 61 | 0.03 | 2017 |
| 6 | An experimental investigation on thermal damage and failure mechanical behavior of granite after exposure to different high temperature treatments [ | Yang, SQ et al. | 52 | 0.02 | 2017 |
| 7 | Soil microbiomes with distinct assemblies through Numerical simulation of the heat extraction in 3D-EGS with thermal hydraulic-mechanical coupling method based on discrete fractures model [ | Yao, J et al. | 49 | 0.01 | 2018 |
| 8 | Prospects of power generation from an enhanced geothermal system by water circulation through two horizontal wells: A case study in the Gonghe Basin, Qinghai Province, China [ | Xu, TF et al. | 46 | 0.01 | 2018 |
| 9 | Study of the enhanced geothermal system (EGS) heat mining from variably fractured hot dry rock under thermal stress [ | Zhang, W et al. | 40 | 0.02 | 2019 |
| 10 | Temperature-dependent mechanical behaviour of Australian Strathbogie granite with different cooling treatments [ | Kumari, WGP et al. | 40 | 0.02 | 2017 |
Top 10 keywords with the strongest citation bursts during 1996–2023.
| Keywords | Year | Strength | Begin | End | Duration from 1996 to 2023 |
|---|---|---|---|---|---|
| Systems | 2010 | 5.92 | 2010 | 2016 | |
| Fractured granite reservoir | 2016 | 4.88 | 2016 | 2019 | |
| Groundwater flow | 2002 | 4.46 | 2016 | 2018 | |
| Acoustic emission | 2018 | 4.28 | 2020 | 2021 | |
| Transport | 2013 | 4.18 | 2015 | 2017 | |
| Geothermal systems | 2000 | 4.12 | 2017 | 2018 | |
| Australian strathbogie granite | 2018 | 3.95 | 2018 | 2021 | |
| Sequestration | 2014 | 3.92 | 2014 | 2018 | |
| Heat transfer | 1999 | 3.79 | 2010 | 2018 | |
| Shale | 2019 | 3.73 | 2019 | 2020 | |
Gray represents the low period, green represents the stable period, and purple represents the burst period.
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; Han, Bei 2 1 Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China; [email protected]
2 Key Laboratory of Urban Security and Disaster Engineering, Ministry of Education, Beijing University of Technology, Beijing 100124, China; [email protected]