Offshore Geothermal Energy Perspectives: Hotspots and Challenges

Abstract

Geothermal energy is a low-carbon and reliable energy resource capable of generating both heat and electricity from the Earth’s internal thermal energy. While geothermal development has traditionally been focused on onshore sites, offshore geothermal resources are attracting growing interest due to advancements in technology, the search for alternative baseload power, and the opportunity to repurpose decommissioned petroleum infrastructure. Recent efforts include utilizing abandoned oil and gas fields to adapt existing infrastructure for geothermal use, as well as exploring high-temperature geothermal zones such as submarine volcanoes and hotspots. Despite these initiatives, research output, scientific publications and patents remain relatively limited, suggesting that offshore geothermal technology is still in its early stages. Countries like Italy, Indonesia and Turkey are actively investigating geothermal resources in volcanic marine areas, while North Sea countries and the USA are assessing the feasibility of converting mature oil and gas fields into geothermal energy sites. These diverse strategies underscore the regional geological and infrastructure conditions in shaping development approaches. Although expertise from the oil and gas industry can accelerate technological progress in marine geothermal energy, economic challenges remain. Therefore, improving cost competitiveness is crucial for offshore geothermal energy.

1. Introduction

Over the past few decades, the global adoption of low-carbon energy has grown significantly [1,2]. Among the various low-carbon energy sources, geothermal resources stand out for their great potential worldwide as a reliable and suitable option for heat and electricity generation. Geothermal energy is derived from the Earth’s internal thermal energy, which can reach temperatures of over 200 °C [3]. Despite its benefits and potential, geothermal energy has often been overshadowed by other renewable sources like solar and wind, which are weather-dependent. Nevertheless, it is poised to play a pivotal role in the global transition to sustainable energy systems, complementing other renewable energy technologies [1].

Onshore geothermal technology and estimation resources are well established and have more than one century of development. However, expanding offshore geothermal resources’ exploitation, particularly in marine volcanic and production hot water from petroleum fields, remains a challenge and represents a new frontier in renewable energy development [4].

Offshore geothermal energy, traditionally overlooked as a viable energy source, is gaining traction due to rising energy prices, solutions to deal with offshore petroleum decommissioning infrastructure and advancements in technology, and it has gained significant attention in the energy industry over the past decade as a promising renewable energy [5,6,7,8,9]. These untapped marine resources’ potential could present competitive opportunities for companies with expertise in subsurface and offshore operations looking for expanding their renewables businesses. Hence, offshore geothermal represents a new emerging exploration frontier in renewable energy towards the ocean [9,10].

Actually, in the context of climate crisis and energy demand, limiting the consequences of human-induced global warming requires, among other strategies, actions for reducing greenhouse gas emissions through transitioning to renewable energy sources, improving energy efficiency, and implementing sustainable land management practices [11]. This must be balanced with increasing energy demand and population growth. Therefore, harnessing the vast potential of geothermal ocean resources can contribute to meeting energy needs while reducing dependence on fossil fuels, thereby addressing environmental concerns. At Conference of the Parties—COP28 in 2023, 195 signatories’ countries reached a landmark agreement indicating a gradual shift away from fossil fuels and towards renewable energy sources. The accord emphasizes the crucial need to accelerate climate action; one commitment was triple global renewable energy capacity and double the global average annual rate of energy efficiency by the year 2030. This agreement marks a significant step towards achieving global climate goals, albeit non-binding [12]. Following the same climate change solutions, the COP29 proposal endorsed renewable energy, launching the Global Energy Storage and Grids Pledge and Energy Storage Pledge [13].

The Intergovernmental Panel on Climate Change (IPCC) 6th Assessment Report (AR6) compiled global energy transition scenarios, performed by an Integrated Assessment Model (IAM). According to this report, geothermal energy is identified as a renewable energy resource capable of reducing greenhouse gas emissions. It has the potential to contribute to the reduction in greenhouse gases by up to 1.0 GtCO₂-eq year⁻¹, with costs ranging around USD 50–100 per tCO₂-eq year⁻¹ [11]. Likewise, according to the International Renewable Energy Agency (IRENA), geothermal energy “can and should play a greater role in meeting global energy needs”, contributing to counteracting climate changes and moving towards a green energy economy, for both electricity and heating and cooling [1]. According to the World Bank, the average CO₂ emission by geothermal power plants worldwide is 122 gCO₂-eqkWh⁻¹, which is from natural plutonic Earth gases, and not related to fuel combustion [14]. When the life cycle emissions are analyzed, estimations for different energies sources vary significantly. In traditional geothermal projects, life cycle emissions range from 11 to 78 gCO₂-eqkWh⁻¹. In comparison, solar photovoltaic (PV) systems exhibit a broader range, from 9 to 300 gCO₂-eqkWh⁻¹, and those of onshore wind are between 8 and 124 gCO₂-eqkWh⁻¹ [15].

Another geothermal energy characteristic is that it holds an unconventional advantage, providing low-carbon heat and electric power generation capabilities, along with the potential for making a variety of cascading uses such as direct hot water (steam), cold generation (e.g., through absorption cycles) and electricity. In addition, more recently, a diverse array of innovations has been developed for thermal energy like subsurface storage combined or not with CO₂ capture and critical mineral brine water mining [16].

Geothermal power plants are also renowned for their high-capacity characteristics, which significantly surpass those of many other renewable energy sources. Over the past decade, the global geothermal fleet has demonstrated notable capacity factors, averaging between 70% and 80%, being able to reach up to 95% [17,18,19]. This is evidenced by historical geothermal United States plants’ output [20]. This performance is largely due to the inherent geological characteristics of geothermal energy, which allows for continuous heat flow, unlike intermittent sources such as solar photovoltaic systems, which typically operate at capacity factors of only 10–15% or wind onshore with 25–40% [2,18].

The conventional use of geothermal energy (onshore plants) contributed with 5 EJ to the global primary energy supply in 2023, representing 0.8% of overall demand, including direct uses as heat and cooling. Modern bioenergy currently fulfills 7% of global primary energy demand, exceeding the contributions of hydropower, wind, and solar power, which individually account for between 1.0% to 2.2%. Thus, geothermal energy remains the fourth least-utilized renewable energy source [18].

In 2025, approximately 296 geothermal power plants are running worldwide, harnessing the Earth’s internal heat to generate electricity [21,22]. These plants are typically operated in onshore regions characterized by high geothermal gradients, often located within sedimentary basins or volcanic arcs, where geologic activity facilitates access to high geothermal gradient resources. The distribution of these facilities reveals a notable concentration in the West of the United States, Europe, and Asia. Notably, when examining installed capacity, the United States stands out as a leader in geothermal energy production, joined by regions in Asia and Eastern Europe. The five largest geothermal facilities in terms of installed capacity are situated across diverse counties. The United States boasts the most significant facility, The Big Geysers, with 1163 MW. Following this is the Cerro Prieto, in Mexico with 570 MW, and then the Makban plant in the Philippines, with a capacity of 458 MW. Indonesia’s Surulla facility has 330 MW, while Iceland’s Hellisheidi comes next with a capacity of 303 MW. Lastly, Kenya’s Olkaria I rounds out the list with 278 MW [21,22].

However, while geothermal energy is a proven and reliable baseload resource onshore, expanding the harness of geothermal resources towards offshore has often faced challenges related to subsurface geological complexity, the offshore operational environment, top-side equipment and costs [23]. Offshore geothermal energy may represent a largely unexploited energy resource with potential to enhance energy sustainability and deal with well-established marine petroleum infrastructure [24]. By harnessing heat from the subsurface of the ocean, it offers a consistent and reliable energy supply, complementing intermittent sources like wind or solar. Moreover, geothermal could be exploited in abandoned petroleum fields, as a brownfield project, which means that its development occurs based on sites and equipment previously used for other purposes (in this case, repurposing would take advantage of oil and gas infrastructure decommissioning, for example). On the other hand, it could be a greenfield project starting from zero, exploring high geothermal gradients areas, such as submarine volcanoes [25].

Offshore geothermal energy represents an emerging renewable source distinct from conventional onshore geothermal exploitation, offering novel opportunities for companies experienced in offshore operations. For instance, major oil and gas companies that have offshore experience, along with oil and gas services companies, are well positioned to leverage existing offshore datasets like seismic, gravimetry, magnetotelluric, petroleum wells completions, subsurface technologies, top-side infrastructure, operational expertise, and technological capabilities to address the unique challenges of harnessing geothermal energy in marine environments [23]. Although there are no offshore geothermal plants operating in the world, advanced research and pilot projects have begun in countries such as Italy, Iceland, Portugal (in the Azores), Indonesia, Norway, and the United States [26].

Therefore, this paper aims to provide a comprehensive review and brings to light hotspots and challenges for the emerging opportunities of geothermal resources sited in the offshore ambient in a global perspective. Such analysis is original, due to the scarce scientific literature on the subject, and particularly relevant for energy businesses companies considering expanding and diversifying their portfolio. It is also worthwhile for policymakers to develop proper regulatory frameworks for this emerging way to harness geothermal resources. Moreover, several offshore petroleum facilities are approaching their decommissioning process. Synergy with co-generation and/or repurpose these infrastructures for geothermal energy could represent an alternative solution for greenhouse gas emission mitigation and electricity and heat (cold) supply.

2. Method

Firstly, this paper reviewed the scientific literature to describe the typical geothermal resources that could be converted to useful energy in the offshore environment.

Then, given the relatively recent interest in the offshore geothermal development, this study develops a comprehensive literature and patent search focused on the harnessing of energy from geothermal sources in offshore environments. The aim is to determine the hotspot for technology development and application. To ensure a foundation for the research, a database of academic published articles on the development of offshore geothermal energy was compiled using the Scopus paper database [27]. For the patent search, two repositories were employed: the United States Patent and Trademark Office (USPTO) [28], managed by the U.S. Department of Commerce, and SpaceNET, a patent repository associated with the European Patent Office (EPO) [29]. Both databases were chosen for their reliability and comprehensive coverage of intellectual property worldwide. The search process employed Boolean operators to enhance flexibility and precision, allowing for the expansion or narrowing of search parameters. Keywords such as “geothermal,” “hydrothermal,” “offshore,” “ocean,” “marine,” and “seabed” were strategically combined to identify a wide range of relevant documents while minimizing unrelated results. To maintain focus on recent advancements, only articles and patents published after the year 2000 were included.

Following the initial collection of documents, a treatment process through filtering was conducted to eliminate irrelevant, tangentially related, or duplicate materials. This step ensured that the final dataset was both concise and highly relevant to this study’s objectives. Subsequently, a statistical and bibliometric analysis was performed to identify the evolution within the field of offshore geothermal energy applications.

Finally, to supplement the primary bibliographic and patent research, a secondary phase was undertaken to explore existing offshore geothermal projects, particularly those in marine regions with high geothermal temperature gradients and those which are trying to integrate oil and gas production opportunities for a synergy with geothermal energy.

3. Results

3.1. Typical Offshore Geothermal Resources

Offshore geothermal energy exploitation can be mainly approached through two primary resource types, one related to the oceanic crust and another with a sedimentary basin [10]. The first involves harnessing high-temperature energy sources in the oceanic crustal regions, particularly in proximity to volcanic formations, knolls and hydrothermal vents. This method exploits the natural geothermal activity of these underwater geologic features. The second approach involves harnessing energy from hot reservoirs and water/brine produced from sedimentary reservoirs at offshore oil and gas fields, particularly those that are close to being decommissioned [26].

Global geothermal gradient data derived from the scientific literature and several databases [26,30,31] reveals notable regions of elevated geothermal activity within the oceanic environment. As illustrated in Figure 1, the global distribution of geothermal gradients surpassing 100 °C/km stands out in several regions of the globe (red dots). The South Atlantic, for instance, has multiple locations with gradients exceeding 50 °C/km, with certain zones, particularly around volcanic islands, demonstrating values above 100 °C/km (Figure 1A). Similarly, the Tyrrhenian Sea, west of Italy, presents several points exceeding 100 °C/km, where ongoing investigations are exploring the potential for geothermal energy extraction (Figure 1B). Furthermore, the Pacific belt, extending from Indonesia to northern Japan, is punctuated by numerous locations displaying geothermal gradients exceeding 100 °C/km, suggesting a widespread and potentially exploitable geothermal resource (Figure 1C).

However, to effectively develop offshore geothermal resources in the oceanic crust, a complete understanding of the heat flow is essential. Unlike the complex sialic composition of continental crust found onshore, the oceanic crust is primarily composed of mafic rocks and exhibits distinct thermal characteristics [31]. This difference is highlighted by the mean heat flow values: approximately 101 mW/m² over the oceanic crust compared to 65 mW/m² over the continental crust [31]. Consequently, the oceanic crust generally has higher geothermal gradients. This typical thermal characteristic underscores the significant, yet largely untapped, potential of the oceanic crust as a viable source of geothermal energy [31,33]. Geothermal systems in proximity to the magma complex are typically classified as high-temperature and high-enthalpy resources due to the elevated temperatures they exhibit [3,34]. Specifically, these systems are characterized by temperatures exceeding 200 °C, a threshold that significantly contributes to their overall energy content and potential for more-efficient power generation and direct utilization. This higher-quality thermal energy is derived from the magmatic heat source and transferred through conductive and convective processes to surrounding geological formations [34].

Sedimentary basins situated on oceanic crust and hotspots, for instance, can be considered high-temperature areas with gradients potentially exceeding 50 °C/km due to proximity to magmatic bodies. The average oceanic crust geothermal gradient reaches approximately 75 °C/km, significantly higher than the global average of 25–30 °C/km, and can surpass 100 °C/km in certain zones [31].

Specific geological settings like ocean ridges demonstrate an exceptionally high heat flow, with temperatures exceeding 300 °C. These resources can be found at relatively shallow depths, underscoring the significant thermal energy potential inherent in these offshore environments compared to continental settings [31].

Besides resource availability, advancements in geothermal energy marking a potential breakthrough for high-enthalpy geothermal applications were achieved in the Iceland Deep Drilling Project (IDDP), with well drilling in a magma-enhanced geothermal system at 2.1 km [35]. A well was drilled into molten magma, enabling controlled release of superheated, high-pressure steam at temperatures above 500 °C [35,36]. This breakthrough set a world record for geothermal heat and demonstrates potential for power generation from geothermal resources superhot areas [37].

Another potential geothermal energy offshore resource refers to the use of heat and water/brine from sedimentary reservoirs, especially those that can bring synergies with oil and gas upstream activities. Actually, most petroleum fields produce significantly more water than hydrocarbons, particularly in mature fields, where it can represent 98% of fluid production [38]. According to [39], water production has reached 250 million barrels per day globally. Under geological conditions, the extracted water can be hot enough for harnessing in geothermal plants. As explained before, sedimentary basins seated on the oceanic crust have more energy beneath and consequently much better geothermal gradients, a fact that explains the presence of hot reservoirs and water. The proposition of utilizing existing oil infrastructure for co-production of hydrocarbons and geothermal energy, during the useful life of oil production, or for single production of geothermal energy, after the oil and gas fields’ decommissioning, is being developed with a view to commissioning it for geothermal production [40]. Therefore, repurposing oil and gas wells instead of abandoning them can be considered an unconventional geothermal method under development [41].

3.2. Literature and Patent Survey Analysis

The analysis of the evolution of publications related to offshore geothermal energy reveals an intriguing trajectory, marked by fluctuations in annual output number of studies but with gradual progression over the years. The total number of articles retrieved in this research was 87. The bibliometric investigation not only indicated the evolution in quantity, but also measured the technology diffusion. Initially, it exhibited a modest beginning, with only one publication per year, reflecting its nascent phase and the limited attention it garnered in the early 2000s. However, starting from 2006, the number of publications began to rise steadily, reaching a peak of nine in 2018. This upward trend underscored the growing recognition of offshore geothermal energy as an energy exploration frontier in both technological and scientific advancements [24,25,42]. From 2021 onwards, another period of growth in related publications has been observed, further highlighting the escalating interest in this source of energy, which is mostly improved by the attention in energy transition developments and more recently associated with the opportunity for decommissioned petroleum fields [6]. This increasing focus can be attributed to the dissemination of cutting-edge innovations, whether technological, methodological, or operational. Figure 2 provides the annual evolution of publications alongside the cumulative growth between 2000 and 2025.

Moreover, the analysis of indexed keywords within offshore geothermal energy research reveals a discernible evolution in thematic publications. Early investigations predominantly concentrated on fundamental aspects, such as resource qualification, dynamics heat flow, geological structure characterization, geothermal temperature gradient, and the technical feasibility of extracting thermal energy from high-temperature geological structures, including volcanoes, seamounts, knolls, and hydrothermal vents.

However, since 2018, an outstanding increase in keywords related to the petroleum industry emerged, specifically, in the number of publications with keyword index “offshore well production” or “boreholes”. This suggests a strategic study within both academic and corporate research on this matter. The thematic shift also indicates a growing interest in exploring the potential for repurposing existing offshore oil and gas infrastructure, whether through decommissioning initiatives or by leveraging geothermal energy from associated water production (co-generation), reflecting a more pragmatic and integrated approach to offshore geothermal energy development.

This is evident in Figure 3, which illustrates the bibliometric relationship between keyword indices derived from the paper survey from 2000 to 2025. The grapho employs a color gradient where blue denotes keywords associated with older papers, while red signifies those associated with more recent. Thus, this keyword grapho approach provides a concise and intuitive way to analyze the occurrence and temporal distribution of offshore geothermal research and the state of the art.

The visualization of country-level papers’ publication and citation networks’ co-occurrence shows the concentration of research activity in few countries (Figure 4). The relative size of bubbles, as exemplified by the prominence of the United States and China, signifies the number of published papers. These two countries contribute the highest number of publications. Furthermore, color lines denote the degree of co-citation among countries, effectively highlighting international research emerging network hubs within the field.

In the case of the patent survey between 2000 and 2025 undertaken by this study, it revealed fifteen publications, highlighting trends in innovation and technological development towards offshore geothermal energy recovery. The patent portfolio involves an array of innovations centered on oceanic thermal energy conversion (

Table 1. Patent recovery in the survey related to geothermal offshore energy obtained from European Patent Office—EPO database and The United States Patent [29] and Trademark Office—USPTO [28] illustrating innovations for advancing this renewable energy option. Accessed on 9 February 2025.

S.Nº	Patent Title	Applicant	Country	Year Publication	Database
1.	Marine Geothermal Exploration System	Guangzhou Marine Geol Survey	China	2011	EspaceNET
2.	Deep Ocean Hydrothermal Sequence Sampler	Univ. Zhejiang	China	2007	EspaceNET
3.	Device For Measuring Geothermal Amount Of Ocean	Korea Inst. Ocean. Sci. & Tech.	South Korea	2016	EspaceNET
4.	Optimal Selection Method For Offshore Geothermal Resource Target Area	CNOOC 1	China	2024	EspaceNET
5.	Ocean Geothermal Power System Using Multi-Step Reheating Rankine Cycle	Korea Ocean Res. Dev. Inst.	South Korea	2013	EspaceNET
6.	Marine Geothermal Power Generation System With Turbine Engines	Shifferaw Tessema Dosho	USA	2013	EspaceNET
7.	Self-Powered Observation Apparatus Based On Submarine Hydrothermal Solution	Univ. Zhejiang	China	2023	EspaceNET
8.	Ocean Thermal Energy And Geothermal Energy Combined Power Generating System	Univ. Jimei	China	2012	EspaceNET
9.	Geothermal Power Generation System With Turbine Engines And Marine Gas Capture System	Shifferaw Tessema Dosho	USA	2013	EspaceNET
10.	Geothermal Power Generation System With Turbine Engines And Marine Gas Capture System	Shifferaw Tessema Dosho	USA	2013	EspaceNET
11.	Geothermal Energy And Wind Power Coupled Offshore Oil And Gas Platform Combined Power Generation System	CNOOC 1	China	2024	EspaceNET
12.	System To Extract Hydrothermal Energy From Deepwater Oceanic Sources And To Extract Resources From Ocean Bottom	Marshall Bruce	USA	2011	EspaceNET
13.	Geothermal Power Systems And Methods For Subsea Systems	Schlumberger 2	USA	2024	USPTO
14.	Geothermal Power Systems And Methods For Subsea Systems	Schlumberger 2	USA	2024	USPTO
15.	Geothermal Plant For Extracting Energy From A Geothermal Reservoir Located Below The Ocean Bottom	CGG	USA	2024	USPTO

¹ CNOOC—Chinese National Offshore Oil Company; ² Registered as OneSea, a subsidiary of Schlumberger (rebranded to SLB).

). These innovations include specialized methods, integrated systems, supportive auxiliary devices, marine heat exchangers, subsea geothermal systems, and equipment designed to efficiently capture and utilize thermal gradients present within the marine rocks for useful energy generation.

Figure 5 depicts the annual patent publication evolution since 2000. The year 2023 appears to have initiated a period of resumption research, evidenced by an increase in patent publications related to this field throughout 2024, which records the highest number of patents registered per year for the period analyzed.

Geographically, the analysis underscores the United States’s influence, accounting for 47% of the published patents. China contributes 40%, while South Korea registers 13%, indicating a concentrated innovation, as observed in the scientific papers analysis. This dominance highlights the escalating role of the United States and China in technological advancement in offshore geothermal energy.

In addition, an examination of the institutions filing these patents reveals a range of contributors. Universities and research institutes collectively account for 40%, emphasizing the continued importance of academic research in driving innovation. Petroleum and oil services companies contribute 33%, indicative of ongoing technological development within the petroleum sector. Individual applicants represent the remaining 27%, showcasing the role of independent inventors in the innovation geothermal ecosystem. As observed, 2024 has five publications, the highest number of publications for a year since 2000.

The presence of petroleum and oil service companies, such as CGG, Schlumberger (rebranded SLB) and the Chinese National Offshore Oil Company (CNOOC), further highlights the petroleum sector’s contribution to patent activity (Table 1). This suggests an effort within these companies to secure intellectual property rights related to their knowledge, positioning themselves in anticipation of knowledge transfer from the traditional oil and gas sector to geothermal energy, mainly in subsurface issues and offshore operations [43].

The frequent presence of oil and gas companies, alongside service providers, in geothermal energy patent registrations also highlights the advantageous application of shared subsurface exploration techniques. Specifically, the application of geological and geophysical investigative methodologies for subsurface exploration, drilling and production is a shared competence. As evidenced by the American Energy Agency’s report examining patents from 1978 to 2018, major oil corporations such as Chevron, Halliburton, and Schlumberger are prominent applicants, reflecting the transferability of geological and geophysical expertise [44]. In sum, analyzing patent publications offers a view of a dynamic and geographically innovative landscape, characterized by the increasing prominence of China in technological advancement in geothermal energy and the continued relevance of both academic research and corporate investment.

However, compared to the number of patents related to conventional geothermal energy, offshore geothermal technology appears to be in an earlier stage of innovation [25]. According to the IRENA Renewable Energy Patent Landscape [45], geothermal energy has accumulated over 14,000 patents published by the European Patent Office (EPO), reflecting its relatively advanced development. In contrast, the limited number of patents in the domain of offshore geothermal energy highlights the significant technological and economic barriers associated with harnessing geothermal heat in offshore environments. While conventional geothermal systems have benefited from decades of improvement and development [3], offshore applications face unique challenges, such as extreme operating conditions, higher development costs, and complex infrastructure being required, which also includes the electric power transmission [46]. These obstacles hinder the rapid advancement of offshore geothermal technology, despite its high-quality energy resources, highlighting the need for further research and investment to overcome the current limitations [23].

Through this bibliometric review, offshore geothermal energy, despite being a relatively unexplored and unconventional energy model, has emerged as a potential energy frontier. The offshore geothermal has shown significant growth in recent years, as evidenced by the increasing number of patents and scientific publications in technical conferences and scientific journals, where researchers are exploring the potential of harnessing geothermal resources. The challenges, such as high costs, technical complexities, and environmental concerns, have not deterred academic innovation in this theme. Moreover, the increasing interest of the oil and gas industry on the offshore geothermal development can open up new opportunities in terms of increasing the readiness level of this option.

3.3. Geothermal Energy Recovery from Low Temperature in Oil and Gas Fields

Geothermal co-production with hydrocarbons offers an option of generating geothermal energy from existing active oil and gas wells. This process leverages the naturally hot water found within these wells, extracting its heat to produce electricity or thermal energy for immediate use or storage. Co-production employs a closed-loop system where the extracted water is reinjected back into the reservoir, minimizing environmental impact and resulting in near-zero additional carbon footprint [47]. This approach allows for the simultaneous production of both hydrocarbons and geothermal energy from a single well, effectively maximizing resource utilization and promoting a more sustainable energy landscape.

A schematic overview of repurposing petroleum is presented in Figure 6 [41]. Fluid co-production involves extracting oil and hot water or brine from multiple wells in a single field and separating them at a central facility. Given the low-to-medium quality heat obtained, the separated water is directed to a surface Organic Rankine Cycle (ORC) unit for electricity generation or to a heat exchanger for thermal energy production. After cooling, the water is reinjected into the reservoir to prevent groundwater contamination and maintain reservoir pressure, ensuring sustainable oilfield operations. This process optimizes resource utilization while minimizing environmental impact. The techniques in question can also operate within a closed-loop system, such as the GreenFire Energy’s “greenLoop” technology, as demonstrated in the “Wells2Watts” project [48], or through CeraPhi’s approach utilizing a single-well closed-loop configuration [41].

Since abandoned petroleum wells typically exhibit low-to-medium temperatures, generally below 180 °C, the application of Organic Rankine Cycle (ORC) technology is an effective solution for harnessing thermal energy in such settings [49]. The use of specialized working fluids with a lower boiling point than that of water allows the ORC system to convert thermal energy from low–medium temperature gradients that were previously deemed unsuitable for electric power generation. This capability makes ORC particularly well suited for repurposing abandoned petroleum wells, aligning with ongoing efforts to utilize depleted oil fields as sources of geothermal energy [50].

Consequently, the implementation of ORC technology not only offers an alternative mean of energy recovery but also contributes to the sustainable management and value of existing low-to-medium enthalpy subsurface resources [51,52]. In this case, an important challenge for the utilization of geothermal energy from abandoned petroleum wells lies in selecting an optimal working fluid for the ORC system. Different working fluids exhibit varying performance depending on the heat source temperature, which directly influences efficiency and power output. This variation is critical for determining the feasibility of harnessing geothermal energy from wells characterized by relatively low temperatures. Recent analyses have evaluated multiple organic fluids, assessing their net power generation, thermal efficiency, and exergy performance across a range of heat source temperatures [53]. The findings revealed that R141b achieved the highest net thermal and exergetic efficiencies. However, it also presented significant environmental concerns, such as elevated Global Warming Potential (GWP) and Ozone Depletion Potential (ODP). In contrast, n-butane delivered the greatest enhanced net power generation. Considering environmental impact alongside performance, R1233zde and R1224ydz emerged as promising alternatives, offering a balanced compromise between efficiency and reduced ecological footprint [53,54].

The United States Geothermal Technologies Office (GTO), from The United States Department of Energy (DOE), has demonstrated the technical and economic feasibility of power generation from low geothermal resources at temperatures below 150 °C. In a collaborative effort to explore the feasibility of geothermal energy co-production within existing petroleum infrastructure, the DOE, partnered with the Rocky Mountain Oilfield Testing Center (RMOTC), launched a program focused on evaluating low-temperature power generation using waste heat from oil field water streams. This initiative, formalized through a Cooperative Research and Development Agreement between Ormat Nevada, Inc., and both the DOE and RMOTC, culminated in the deployment of a pilot plant at the Teapot Dome Oil Field, also designated as Naval Petroleum Reserve No. 3 (NPR-3), located north of Casper, Wyoming, where commercial petroleum production started in the early 1920s. The core technology employed was an air-cooled, factory-integrated-mounted 250 kW Ormat Organic Rankine Cycle (ORC) power plant, engineered to harness the thermal energy of 40,000 water barrels per day at 76 °C to vaporize the working fluid, isopentane. Projections estimated a gross power output of 180 kW, resulting in a net power output of 132 kW [55]. Operated from September 2008 to February 2009, the unit generated 586 MWh of power before being decommissioned due to unforeseen operational challenges, providing valuable insights into the potential and limitations of this innovative approach to energy production [55].

The gross power potential estimated from 130,000 water barrels per day at 104 °C would be 76 MW [55]. This pilot plant demonstrates a synergistic integration of geothermal heat-to-power conversion within the oil and gas industry, offering a practical application of recovered thermal energy [55]. Importantly, the plant’s operation does not compete with or impede hydrocarbon production activities, representing a value-added, parallel energy generation system [55].

In April 2011, China’s first low-temperature geothermal power plant utilizing co-produced fluids from an oil field was put into operation in the LB Reservoir of the Huabei oil field. The plant, which ran for several months, leveraged the area’s geothermal gradient of 3.5 °C/100 m and average formation temperature of 120 °C, extracting fluids with temperatures ranging from 110 °C to 120 °C from eight wells. Given the high water cut of 97.8% in the co-produced fluids, the plant employed a 400 kW binary screw expander system to generate electricity. By the end of 2011, the cumulative energy generated by this pilot project reached approximately 310 kW, demonstrating the potential for low-temperature geothermal energy extraction from oil fields. This shows that China is increasingly focusing on novel approaches to harness geothermal energy [56]. Particular interest is surging in utilizing hot fluids co-produced from oil and gas reservoirs, alongside other unconventional geothermal resources, to stimulate power generation [57,58].

In 2017, the DOE established a project and funding with the primary objective of demonstrating the technical and economic feasibility of generating electricity from low-temperature geothermal fluids using binary power generation technology [59]. The project was led by the University of Dakota. For that, the project used petroleum infrastructure and abandoned wells. The target of this study was the Williston Sedimentary Basin in North Dakota, which has achieved a significant milestone in geothermal energy development with the first commercial enterprise to co-produce electricity from geothermal resources at an oil and gas well. Located in the Cedar Hills Red River B oil field, this project utilizes two 125 kW Organic Rankine Cycle (ORC) engines installed in the water stream between wellheads and heat exchangers. With formation temperatures ranging from 90 to 150 °C, the project processes approximately 29,500 barrels per day of 98 °C fluid, which is then injected back into the Red River Formation (Ordovician). Utilizing two horizontal wells for enhanced oil recovery (EOR), the project generates electricity while simultaneously optimizing oil extraction [59].

Nevertheless, according to the DOE estimations, useless oil and gas wells across the country represent an untapped energy resource with a potential to generate up to 100 GW of electricity-producing capacity. These abandoned wells can serve as sites for geothermal energy production, as the infrastructure already in place may allow for easier access to the Earth’s natural thermal energy. Utilizing these wells as geothermal sources presents an opportunity to reduce dependence on traditional renewable energy and decrease greenhouse gas emissions associated with unclean energy generation [52]. Moreover, current estimations point out that there are between 20 and 30 million abandoned or close-to-being-abandoned petroleum wells globally [60,61].

To select the hydrocarbon wells for integration with geothermal power cycles—such as dry steam, flash steam, or binary cycle plants— their petroleum recoveries should have fallen below the established economic limit, meaning that traditional oil extraction is no longer financially viable. Before repurposing these wells, it is essential to thoroughly estimate key reservoir properties, including the amount of residual thermal energy, water density, pressure, and the recoverable factor, which indicates the proportion of usable energy that can be practically extracted. This assessment provides the necessary data to determine the feasibility and potential efficiency of power cycle integration, ensuring that the conversion from petroleum to geothermal applications is both technically and economically justified [52].

Optimal conditions for repurposing petroleum wells include wells with depths around 4 km and bottom-hole temperatures exceeding 70 °C, preferably containing hydrothermal siliciclastic rocks or hot rock geological settings [47]. Besides power equipment parameters, the potential for successful conversion depends on several critical factors, such as reservoir porosity, permeability, and thermal conductivity, with preference given to wells that originally produced oil or water rather than gas. Well characteristics like vertical orientation, larger casing diameter, and excellent well integrity are crucial for successful transformation [47]. Additionally, the geothermal potential can be assessed by evaluating well productivity, with high fluid rates above 860 m³/day indicating favorable conversion prospects. Marginal or inactive wells are particularly suitable candidates, and the presence of nearby heat or electricity demand further enhances the project’s viability. Surface infrastructure, including existing facilities and utility connections, can significantly reduce implementation costs and complexity in repurposing hydrocarbon wells for geothermal energy generation [47].

A standardized methodology for identifying and assessing geothermal energy resources in decommissioned petroleum wells was detailed by Nian et al., 2018 [62]. This accessible approach integrates multiple critical factors, such as the geological and reservoir properties of candidate wells, production histories, and necessary economic considerations. Through this comprehensive analysis, the methodology supports the evaluation of recovering low-temperature waste heat from former oil and gas wellbores. The methodology’s practicality and reliability were demonstrated with its successful application in the Villafortuna oil field in Trecate, Italy, providing valuable insights and serving as a reference for deploying similar geothermal energy initiatives in other decommissioned fields [62].

Oil and gas industry data can be valuable information for geothermal exploration and development, offering multiple technical insights for potential project planning. Rock thermal conductivity values are particularly crucial, which can be derived from existing well logs such as formation resistivity, gamma ray, and density measurements [47]. Well integrity and completion data provide essential information including completed zones, perforated intervals with specific depths and formations, casing diameters, and comprehensive completion histories. While geothermal wells share significant structural similarities with petroleum wells, they have unique characteristics, especially regarding casing configurations. Casing sizes are typically determined by specific formation conditions and the characteristics of produced fluids like water or steam. Industry-standard practices include using fully cemented strings, and potentially implementing liners in the lower sections of production casings could optimize well performance and reliability in geothermal applications [47].

Finally, retrofitting preexisting abandoned production wells presents a significant opportunity to reduce the costs linked with developing a new geothermal field, as demonstrated by Davoodi et al. [51]. Notably, drilling activities alone account for approximately 51% of total project expenditure, making them the most expensive phase of development [63]. Studies by Davoodi et al. [51] have shown that by omitting the drilling stage, the overall cost budget can be reduced by around 44%. In addition to direct savings, repurposing wells also lowers capital and administrative expenses by decreasing the need for managing multiple service contracts and reducing the number of drilling operations required. Typically, drilling operations involve similar cost structures across different fields, with major expenses arising from drilling rigs, cementing, directional drilling, casing installation, drill-bit choices, and drilling fluids, which together contribute roughly 80% of the total drilling budget. Therefore, leveraging existing wells not only streamlines project management but also leads to substantial cost savings [51].

3.4. Challenges and Limitations

Repurposing abandoned oil wells presents a complex set of challenges, primarily centered on ensuring the geothermal gradient and temperature, followed by the structural integrity of wells that were built using outdated technologies for other uses. Technologically, adapting existing infrastructure to harness geothermal energy demands advanced well completion solutions. Older wells often lack the robustness required to withstand modern operational pressures and may be susceptible to chemical reactions or structural failures. Successfully adapting such wells for new applications demands advanced materials, thorough diagnostic evaluations, and detailed economic assessments to determine feasibility. The overall cost and technical approach are shaped by factors like the well’s location, depth, mass flow and the intended new technology [52].

Additionally, the process must navigate a landscape of regulatory requirements and potential government incentives, both of which affect financial viability. Broad stakeholder engagement is essential, particularly in regions where economies are heavily reliant on oil and gas, social and economic concerns must be addressed. Finally, environmental considerations, such as mitigating risks from orphaned wells and managing the challenges posed by geothermal gradients variety, are crucial to ensure the long-term success and safety of well-repurposing projects [51,64].

A limitation when considering offshore platforms was also pointed out by Nord et al., 2012 [65] regarding the space and weight of equipment for co-production of electrical energy in petroleum platforms; this could be a challenge for retrofit in offshore operations.

In addition, to advance the exploration of offshore volcanic areas, it is essential to install a pilot experimental project to accurately verify subsurface characteristics and surface equipment. This step is critical as it constitutes a breakthrough in geothermal technology, facilitating a deeper understanding of resource potential and informing future development efforts with greater precision.

4. Offshore Geothermal Opportunities

4.1. Potential Areas for Petroleum Repurpose

Thousands of petroleum wells have been drilled in regions such as the North Sea Basin, the Gulf of Mexico, East Africa, and Brazil, many of which are now approaching the stage of abandonment [8,9,66]. A significant number of these wells produce more hot water than hydrocarbons, revealing an unexploited potential for geothermal energy. Essentially, the existing infrastructure represents a nearly complete geothermal system, with the primary missing component being the installation of power plants on top of the petroleum infrastructure [9]. Although there are currently no operational offshore geothermal power plants, numerous projects and pilot studies have been developed over the years to explore the feasibility of converting or co-generating these wells into sustainable sources of geothermal energy.

In 1998, Norway explored the possibility of producing electricity from geothermal energy at the Ula Field in the Norwegian North Sea, located at a water depth of 70 m [66,67]. The field, discovered in 1976 and commencing production in 1986, primarily produces oil from sandstone formations in the Upper Jurassic Ula Formation, with the reservoir situated at a depth of 3345 m. Additionally, production occurs from parts of the underlying Triassic reservoir at around 3450 m depth. The Ula Field produces approximately 800 m³ of water at 130 °C daily, offering a geothermal potential estimated at 10 MW [66,67]. This initiative marked Norway as a pioneer in offshore geothermal feasibility studies. However, with production currently in its late life phase and challenges in gas supply for water alternating gas (WAG) injection, the operator plans to cease production by 2028. In January 2025, the field produced 4000 barrels of oil equivalent per day compared to 17,000 barrels of water per day [68]. This indicates a notable production ratio where oil output is significantly lower than water production, which is common in mature fields undergoing water flooding. Despite the recognized geothermal potential, there have been no developments or implementation of geothermal energy reuse reported for the Ula Field to date.

Another field that has estimated geothermal potential is Ekofisk, whose production composes the well-known Brent blend benchmark. The field is in the Norwegian North Sea at a water depth of approximately 70 m. Discovered in 1969, test production was initiated in 1971, followed by regular oil production starting in 1972. Ekofisk produces oil primarily from naturally fractured chalk reservoirs, specifically the Late Cretaceous Tor Formation and the early Paleocene Ekofisk Formation [69]. Preliminary assessments by Turboden have estimated that the field could generate around 7 MW of geothermal electricity using the ORC binary power plant [66]. This estimate is based on a water flow rate of 444 L/s at a temperature of 110 °C, highlighting Ekofisk’s potential as a source for low-carbon energy production alongside its ongoing oil extraction activities [66]. At the same stage, Kristin Field is in the tail phase of oil production. With water production at 160 °C, it was estimated to be at least 1 MW gross generation from thermal water [66].

In addition to those offshore fields, there are 14 other mature offshore fields in the Norwegian North Sea with production water temperatures exceeding 84 °C that have the potential to produce geothermal energy using existing infrastructure. Actually, regarding these mature Norwegian fields, geothermal gradient and production water temperature range between 35 (Martin Linge) and 53 (Belder) avg. °C/km and 83 (Grane) and 186 °C (Tambar), respectively [66]. Reservoir depth can reach 4.3 km (Martin Linge), with many fields in the 4 km range, but there are fields below 2 km, such as Belder, Grane and Johan Sverdrup [66]. Hence, these fields could be utilized for extraction of geothermal energy without the need to deepen the wells, making it a technical option with additional retrofit.

Furthermore, if these pre-existing wells were deepened, the amount of geothermal energy generated could increase significantly due to the relatively high geothermal gradient of approximately 50 °C/km in the region. This gradient indicates that deeper wells could access even hotter water, thereby enhancing the efficiency and output of geothermal energy production from these offshore sites [66]. The transition from hydrocarbon extraction to geothermal energy production represents a significant shift in the energy landscape, particularly in places like the North Sea. As mature oil and gas fields approach the end of their productive life, innovative methods are being explored to repurpose existing infrastructure for geothermal energy development. This strategy not only addresses the challenges of energy transition, but also leverages the North Sea’s established assets to support a low-carbon future, ensuring a more sustainable and efficient use of resources [70]. Beyond the Norwegian part of the North Sea, United Kingdom areas also exhibit a geothermal gradient exceeding 50 °C/km. Notably, the Elgin and Franklin fields have production water temperatures reaching 196 °C [71].

The temperature gradient in the North Sea has been substantiated not only through data derived from petroleum wells, but also by comprehensive regional geological studies [72]. A three-dimensional thermal model was constructed to assess the present thermal conditions beneath the northern North Sea and adjacent continental regions, aiming to investigate the regional thermal regime. The model indicates that the mainland generally presents lower temperatures at a depth of 2 km, although some areas exceed 90 °C. However, at a depth of 7 km, the geothermal temperature is projected to reach up to 200 °C, with most of the region exhibiting temperatures above 130 °C [72].

In 2023, recent attempts to implement pilot studies gained significant attention. In the North Sea, the Aquarius North Sea Geothermal Consortium, led by ZeGen Energy, successfully completed a 12-month geothermal assessment project for TotalEnergies. This study, finalized last year, comprehensively evaluated the potential for geothermal energy in offshore environments by integrating subsurface, wells, and topside elements. The findings provide critical insights and guidance for incorporating geothermal power into offshore renewable energy portfolios, marking a significant step toward advancing sustainable energy solutions [73].

The Gulf of Mexico, a traditional oil and gas prone basin, historically characterized by intensive oil and gas extraction activities, could represent a significant opportunity for geothermal energy exploitation through the repurposing of existing infrastructure. With a legacy of over 53,000 drilled wells, including approximately 30,000 abandoned, and more than 7000 established platforms, the region offers a substantial foundation for geothermal retrofit projects [8]. Production data analysis, coupled with geothermal gradient mapping within the American sector of the Gulf, suggests the feasibility of converting offshore petroleum wells and platforms into geothermal electricity production facilities, encompassing both shallow and deep-water fields possessing adequate flow rates and temperature profiles. Initial geothermal resource assessments conducted as early as 1970 highlighted the offshore potential of the Gulf [74].

Observed geothermal gradients demonstrate considerable heterogeneity, reaching peaks of up to 100 °C/km, concentrated within zones of maximum pressure gradient alteration, while the 120 °C isogeotherm, that is at a constant temperature of 120 °C, is typically located between 2500 and 5000 m below sea level, coinciding with the upper limit of the geopressured zone. Furthermore, a maximum temperature of 273 °C has been documented at a depth of 5.859 m at studies in the 1970s, providing initial subsurface critical data for thermal and pressure regimes within the Gulf of Mexico’s geological formations [74].

Geothermal energy potential studies performed in the Galveston Bay, an area of the Gulf of Mexico, were based on data from seven offshore wells. While the geothermal gradient observed, ranging from 28 to 32 °C/km, is relatively low, the depth of the wells, ranging from 4.2 km to 3.2 km, allowed for intersection with reservoirs aquifer exhibiting temperatures between 96 and 130 °C. The production rates are substantial, with one well registering 8000 barrels of water per day and estimated bottom-hole temperature of 102 °C, potentially generating between 262,980 and 569,790 kWh annually at this water production rate. However, despite the technical feasibility, economic analyses indicate that the project is currently uncompetitive with the cost of conventionally sourced electricity in the region [9].

A geothermal gradient map was created for the Gulf of Mexico (Figure 7) to explore the feasibility project using medium-temperature oil and gas water production for power generation. Three specific areas were chosen for detailed temperature gradient estimation (Figure 7 color dots). Analysis reveals a significant variation in geothermal gradients across the region. Moving from east to west, the gradient shifts from 25 to 30 °C/km on the Alabama coast to a lower range of 15–25 °C/km in eastern Louisiana, before increasing considerably to 30–60 °C/km on the coast of Texas. This spatial variability suggests differing potentials for geothermal energy extraction within the Gulf of Mexico [75].

4.2. Potential Areas in Volcanic Geological Setting

Italy has a long history as a pioneer in geothermal energy, being home to the world’s first commercial geothermal power plant exploiting high-temperature resources on land [76]. In recent years, however, the country has focused on the innovative concept of harnessing high-temperature geothermal energy from submarine volcanoes [26,77,78]. The first country to attempt to test the offshore geothermal is also Italy, in the Marsili Project, conducted by Unione Geotermica Italiana [25]. The Southern Tyrrhenian submarine volcanic district, a relatively young geological basin that formed from the Upper Pliocene to the Pleistocene, has emerged as a new area of interest for geothermal research and development. This region, characterized by tectonic extension and the formation of numerous seamounts, has been extensively studied over the past decades, yielding an array of geological, geophysical, and geochemical data. The presence of magmatic bodies beneath the seafloor provides significant heat sources for both deep and shallow geothermal reservoirs [77].

Advances in offshore exploration technologies, originally developed for oil and gas industry operations, can also enable reliable and competitive assessment of the geothermal potential within these high-enthalpy offshore submarine systems. According to recent studies, the southern Tyrrhenian Sea represents a promising target for the future of geothermal energy in Italy, largely due to its elevated geothermal heat flow [78].

Feasibility studies in the Tyrrhenian Sea have focused on the Marsili seamount (Figure 8), situated about 100 km off Italy’s coast, which stands as the largest volcanic structure in both Europe and the Mediterranean [78]. This massive underwater volcano, rising 3500 m from the seafloor to just 489 m below sea level, spans approximately 60 km in length and 20 km in width. Geological characteristics, including a Curie isotherm depth of 4–5 km below the crest and base temperatures exceeding 600 °C, suggest the presence of significant magmatic heat sources, making Marsili a promising candidate for geothermal energy exploitation. Estimates indicate that Marsili could support a total installed capacity of up to 800 MWe, potentially delivering between 5.5 and 6.4 TWh of electricity annually, an amount that could substantially boost Italy’s renewable energy portfolio [77,78].

Following detailed studies, based on preliminary and theoretical evaluations, the Marsili geothermal field could deliver significant power output by utilizing supercritical fluids at approximately 400 °C and 10 bar pressure, with mass flow rates ranging from 20 to 100 kg/s per interconnected well group. From an energy density perspective, a 1 km³ basalt body beneath the reservoir, at 1000 °C, holds an estimated 690 TWh of thermal energy if cooled to sea temperature, given a density of 3.1 kg/m³ and a specific heat capacity of 840 J/kg °C. Cost projections, following trends in recent large geothermal plants, place the overnight investment at roughly 4000 USD/kW. Using this cost and the projected energy output over 30 years, the levelized cost of electricity (LCOE) for the Marsili field is estimated to be around 0.040 USD/kWh, highlighting its potential economic viability [77,78].

Another volcanic area investigated for geothermal energy recovery is located in Indonesia. Boasting the world’s second-largest installed geothermal capacity has become a significant focus for offshore geothermal potential, particularly in regions associated with high-enthalpy springs and volcanic activity [25,26,79]. One prominent area under investigation is the Sangihe Archipelago, located north of Sulawesi Island. This archipelago lies along a volcanic arc that has resulted from the ongoing subduction of the Philippine plate beneath the Micro-Sunda plate, a geological process that not only shapes the region’s landscape but also enriches it with geothermal energy resources. The relatively young age of this volcanic arc enhances its suitability for high-enthalpy geothermal development, as youthful volcanic regions typically exhibit more accessible and vigorous geothermal systems [25].

The presence of seamounts serves as further evidence of substantial subsurface heat flow and geothermal potential. Preliminary assessments of these offshore resources employ a range of geophysical and geological methods, including bathymetric mapping to understand seafloor structures, gravity measurements to detect subsurface density anomalies, magnetic surveys to identify variations in rock magnetism, and broader regional geological studies. Findings from these surveys have revealed distinct features such as volcanic arc alignments, outer-arc ridge structures, and the occurrence of hot springs, surface manifestations indicative of underlying geothermal systems. By integrating the results of these varied analytical approaches, researchers have been able to pinpoint likely zones of geothermal activity, often marked by combinations of significant elevation, high gravity values suggesting dense, hot rocks beneath the surface, and low geomagnetic readings, which may indicate hydrothermal alteration [25]. Collectively, these early investigative efforts highlight Indonesia’s promising potential for harnessing offshore geothermal energy in tectonically active, volcanically influenced regions like the Sangihe Archipelago [25,26]. North Tech Energy is partnering with Indonesian developers and turbine producers to establish small offshore geothermal power stations. This collaboration aims to harness Indonesia’s abundant geothermal resources by deploying compact, efficient power generation units at sea [26,80].

Other countries and initiatives have considered electricity generated from offshore geothermal plants, which include India, Portugal, Italy, the Philippines, Japan, and Russia, along with Central America and the Caribbean [10]. The analysis of various configurations for power production from offshore geothermal resources in the geothermal field of Reykjanes, Iceland, has been developed looking increment geothermal potential [10]. Alternatives investigations are underway in the near-shore Pacific region to explore geothermal energy as a reliable and renewable base-load power source for U.S. Naval operations [81].

4.3. Petroleum and Offshore Geothermal Exploration Subsurface Technology Synergy

Exploring offshore geothermal resources relies heavily on the expertise and technology developed for offshore oil and gas exploration and production. Decades of investment and developments in maritime subsurface research have equipped the oil industry with advanced tools and methods for subsurface prospections. As a result, efforts to identify and characterize offshore geothermal resources will inevitably draw on the established technologies, operational experience, and service providers of oil and gas companies. For instance, geophysical techniques such as seismic and potential field methods, which are widely used in oil and gas exploration, play a crucial role in the effective characterization of geothermal resources beneath the seafloor. This transfer of technology and experience is expected to accelerate the development of offshore geothermal energy by reducing technical risks and exploration costs. This synergy and technology transfer have been documented in several reports and pointed out [18,82]. Here, we show some examples of geophysical and geological studies to characterize offshore geothermal fields, integrating with petroleum datasets and processes.

Seismic data serves as an essential tool for offshore geothermal characterization, like their application in petroleum exploration. In the Gulf of Candarli, Turkey, seismic reflection data combined with petrophysical measurements has been utilized to develop a three-dimensional pore-pressure temperature model, also named a static temperature cube [83]. This 3D petrophysical modeling approach has been chosen as the primary method for seismic interpretation correlation and then for comprehensive subsurface integration data. Interval seismic velocities obtained at shot points were converted into point data and imported into specialized software to create detailed velocity models. These models were then validated against expected lithological formations in the area to ensure geological accuracy. Subsequently, the velocity data was transformed into static temperature estimates using the formula proposed by Ryan et al. [84], whose method has been shown to match to temperature profiles observed in geothermal fields such as those in the West Indies [83].

This methodology offers a reliable framework to correlate seismic velocity with petrothermal properties, thereby improving the precision of subsurface temperature predictions. The results from this study suggest that the Gulf of Candarli exhibits low-to-medium geothermal gradients (Figure 9), with subsurface temperatures potentially reaching around 100 °C, indicating moderate geothermal potential in the region [83].

Another advanced seismic use for geothermal purposes focuses on the use of energy from earthquakes as a seismic source. In the regional study, Mulumulu et al. [85] demonstrates the application of passive seismology to create geothermal 3D models that connect onshore and offshore thermal fields in the Aegean region of Turkey. Ambient noise tomography (ANT), a passive and non-destructive seismological imaging technique, is employed to explore the crustal structure at a relatively low cost. By cross-correlating ambient noise signals recorded at various seismic stations across the region, the shear wave velocity of the crust is measured to depths of up to 18 km. Turkey, known for its intense seismic activity, provides abundant data through which these measurements are derived. The resulting velocity models help identify low-velocity zones that are potentially favorable for geothermal resources, thereby guiding future geothermal exploration campaigns. Enhancing the understanding of crustal structure is critical for developing offshore geothermal energy by revealing the geological relationships that control the distribution of geothermal reservoirs.

A 3D conceptual model shown in Figure 10 was constructed from shear wave velocity (Vs) computations combined with previously reported P-wave velocity (Vp) and Vp/Vs anomalies, effectively linking onshore geothermal fields with offshore areas and supporting planning for upcoming geothermal projects [85].

The Sciacca basin, situated in the southern region of western Sicily, is the home of geothermal resources known as the Sciacca Geothermal Field, which is closely associated with the Sciacca Fault System. To better understand the characteristics of this geothermal field towards offshore in this area, Civile et al. [86] conducted a high-resolution seismic reflection interpretation. The results of this study led to the hypothesis that active fault zones play a crucial role in enabling the upward movement of geothermal fluids from deeper underground. These faults maintain open fractures and dynamically preserve effective permeability within the geothermal reservoir, thereby facilitating the continuous flow and accessibility of thermal fluids (Figure 11). This insight is important for assessing geothermal potential and for future resource management in the region [86].

5. Discussion and Challenges

Offshore geothermal energy showed potential low-carbon energy sources. Despite its potential, the technology remains largely in the concept phase, with limited progress made in terms of demonstration or commercial production. The exploration of geothermal energy in offshore environments faces challenges, resulting in a relatively slow advancement compared to onshore geothermal developments or other renewable energy sources.

Analyzing the number of patents published over the last 25 years revealed a low count, especially compared to those related to renewable energies from intermittent sources. This highlights the need for increased investments. Although there are more published articles, many remain focused on conceptual studies. A combined increase in research and patent activities could significantly benefit the development of offshore geothermal resources.

The technology for exploring and exploiting offshore geothermal fields is well established. Most of the technologies originate from the petroleum industry and onshore traditional geothermal harness. However, some improvement and transfer technology shall be required.

The primary barrier limiting the expansion of offshore geothermal energy production is related to economic challenges. High upfront costs, uncertain resource assessments, and the financial risks associated with deepwater drilling make these projects less attractive compared to more established renewable energy sources. Consequently, economic challenges continue to hinder the large-scale development of offshore geothermal energy, even though the technical capabilities exist to support [10]. Additionally, the protection of the marine environment during exploration activities is essential and should be carried out carefully. Many near-shore areas are designated as protected zones to preserve biodiversity and prevent damage from human activities and economic exploration is not permitted. Another limitation in implementing co-production of electrical energy on petroleum platforms is the space and weight of the required equipment. Offshore platforms have strict constraints on available space and load capacity, making it challenging to retrofit existing installations with additional machinery.

To explore offshore geothermal resources, the use of multidisciplinary data is essential in accurately estimating geothermal potentials. An integrated approach that combines various types of data— including, but not limited to, bathymetry, elevation, residual gravity, seismic and magnetic measurements—provides a comprehensive understanding of the subsurface conditions and helps identify promising geothermal sites [63].

High enthalpy areas associated with volcanic activity and hot spots are widely distributed throughout the globe, making them a significant and promising source of geothermal energy. This broad geographic distribution offers an advantage for countries aiming to develop clean energy solutions, particularly in offshore environments where such high-temperature resources can be harnessed. The availability of geothermal resources with high temperatures enables efficient conversion, which could potentially lead to competitive production costs compared to other energy sources in the future.

Hydrocarbon reservoirs store substantial thermal energy mainly due to their large size and amount of hot water production. Nonetheless, most have temperatures below 150 °C. At these relatively low temperatures, the thermal energy quality is low, limiting the efficiency of energy recovery and the range of extraction methods requires advanced technology. Using organic fluids with lower boiling points than water can be a significant breakthrough for low-enthalpy electric conversion.

Oil industry companies and mostly those with substantial offshore experience possess notable competitive advantages. Their expertise in subsurface characterization, sea drilling, subsea technologies, and complex project management provides them with the skills required to develop and operate offshore geothermal facilities. Additionally, these companies already have established infrastructure and supply chains, allowing for a more efficient transition into geothermal energy compared to traditional land geothermal firms that are less familiar with offshore environments.

Although oil and gas reservoirs offer significant potential for energy development, current technologies for generating electricity from this heat are mostly in the pilot stage and have not yet been widely applied. The main technical challenge is the low efficiency of heat exchange processes, which restricts the scalability and practical use of these methods [52].

Advances in both practical projects and theoretical knowledge are essential to fully realize the potential of unconventional geothermal energy within the oil and gas sector. This emerging method of geothermal energy recovery not only enhances the production of low-carbon power, thereby reducing the sector’s carbon footprint, but can also offer a sustainable approach to extend the operational lifespan of existing infrastructure. By integrating geothermal solutions, petroleum operators and service companies can minimize the costly and complex process of decommissioning mature fields. Additionally, these innovations help optimize resource utilization and enable the delay of field closures, ultimately improving the economic and environmental outcomes for the industry.

Environmental concerns related to potential offshore geothermal plant operations require further detailed and comprehensive studies. The oil industry, with its extensive experience in offshore monitoring, prevention, and safety, offers valuable lessons that could be applied to geothermal development. By drawing on the successes and challenges of numerous offshore platforms, the geothermal sector can improve safety measures and minimize environmental risks in this emerging field.

The topic of geothermal energy with the use of oil and gas infrastructure and offshore harness is still a major challenge with no clear commercial economic examples. Generally, the examples are demonstration (potential) and case studies. Few publications and studies present aspects related to economics issues, since it is a frontline topic in academic and business research.

For the offshore geothermal projects, the TRL—Technology Readiness Level—could be considered as 7–8 for retrofitted petroleum infrastructure (technology has been proven to work and operate a pre-commercial scale) and, for volcanic areas, as 5 (a large-scale prototype development unit, has been qualified through testing in intended environment, simulated or actual).

In sum, there are different challenges to be overcome in order to develop offshore geothermal energy worldwide (Figure 12). These challenges have different features depending on the barriers that are associated with them. In terms of exploration or better assessment of resources, challenges refer to developing a systematic approach for resource exploration benefiting from the expertise from the oil and gas upstream activities. In the development phase, there are several challenges mostly related to well integrity and the supply chain. Although there are synergies with the oil and gas industry, supply factor costs can also be affected and constrained by the oil and gas industry demand. This would also affect the economic viability of the offshore facilities. To overcome economic barriers, the upfront cost is the major issue to be addressed through financing, demonstration plants, and subsidies. Finally, regulatory barriers arise from the fact that in most countries, there are no rules in place (including licensing procedures) to implement offshore geothermal facilities and deal with their possible impacts.

6. Final Remarks

This review has analyzed the increasing development interest in both the academic and industrial sectors for advancements in offshore geothermal energy as a low-carbon and reliable energy resource. Numerous patents and research papers have highlighted the potential of this energy and the challenges that must be overcome to successfully install geothermal plants offshore.

Offshore areas characterized by high geothermal gradients, particularly those near volcanic formations, and retrofitting decommissioned petroleum fields represent potential pathways for advancing geothermal energy beyond traditional onshore applications. Achieving progress in this domain will hold strong benefits through synergy between conventional petroleum technologies, which can facilitate the transition and adaptation of offshore geothermal systems.

However, this energy option also faces significant challenges, which include the lack of comprehensive regulation for marine geothermal exploration. Currently, there is no operational project and the planned pilot studies are mostly in early stages of development.

Continued research and development are crucial to address the current technical, economic, and regulatory challenges associated with offshore geothermal energy resources. Future studies should not only focus on technical developments but also emphasize economic evaluations and regulatory assessments. A comprehensive understanding of the resource potential alongside cost effectiveness is essential to determine the overall feasibility and sustainability of these emerging technologies.