Capturing heat from ocean ridges and transporting it to floating platforms to produce electricity, hydrogen or fertilizer in a virtually carbon-free way: this is the ambitious concept proposed by Viridien, an advanced technology and Earth sciences company.
The mid-ocean ridges at the bottom of the seas hold immense, as yet untapped thermal potential.
These zones of magmatic activity, along which the tectonic plates are moving apart as they expand, have a high thermal flux and gradient and shallow access to reservoirs. They represent a remarkable geothermal potential.
In a white paper published earlier this year, Viridien – formerly CGG – highlighted the potential of offshore geothermal energy as a green energy resource of global importance, and defined a framework for its responsible development. For the past four years, the company has been studying the feasibility of exploiting offshore geothermal systems to harness the heat contained in these ridges.
His thermal models predict that the potential will be higher where systems are isolated from ocean waters by impermeable geological layers, rather than at active hot springs on the seafloor that actually let heat escape.
The aim is to guarantee an affordable, reliable and sustainable supply of basic energy.
A true “blue economy”
Worldwide, mid-ocean ridges – also known as oceanic rifts – cover an area of 65,000 km2. They are present in all major oceans. But these are not the only submarine zones targeted by offshore geothermal energy. This type of heat source also exists along flooded rifts – where these ridges extend landward – and in certain seas formed in areas where the earth’s crust stretches.
Schematic diagram showing the exploration and development of offshore geothermal resources near seafloor spreading centers, producing baseload energy, freshH2O, greenH2 and NH3, with the possibility of storingCO2 and fertilizing the oceans in a controlled way (Image ©Viridien).
Well-identified geological contexts
The heat from the ridges could be used to turn turbines to produce green electricity, but also as a basis for developing an entire production chain.
For example, the large volume of fresh water produced by steam condensate could be used for agriculture in drought areas, and the electricity generated could be used to electrolyze this water and produce green hydrogen. This hydrogen, combined with green electricity, could in turn be used to produce other green fuels or green fertilizers (ammonia).
Some of the offshore geothermal sites may be in mid-ocean “deserts” in terms of plankton productivity, due to a lack of nutrients. The possibility of using geothermal brines to provide nutrients that could enable the development of aquaculture sites is therefore worth investigating, especially as these sites could serve as both a food source and aCO2 sink.
In addition, geothermal fluids brought to the surface could in some cases be used as sources of chemicals and minerals (such as copper), and offshore geothermal projects could be associated with undergroundCO2 storage.
Virtuous impacts
This “blue economy” could have virtuous effects on numerous sectors. It should have a positive impact on the development of onshore infrastructures for the distribution and marketing of products from offshore geothermal exploitation, while supporting the training of engineers and technicians and strengthening the economies of countries located near exploitation zones.
In addition, offshore geothermal energy helps to limit land use by exploiting ocean resources, thus preserving valuable landscapes and terrestrial ecosystems. As a regular and reliable source of base-load energy, it enhances energy security, even in regions prone to climatic extremes and natural hazards. Thanks to rapid advances in drilling technology, the potential for future regulatory support and the technology’s learning curve, the associated costs should fall, reinforcing the viability of this sustainable energy source.
Reusing what the oil industry has learned
Another advantage of offshore geothermal energy described in Viridien’s white paper is that it can draw on technologies, materials and expertise already developed by the oil and gas industry. These include advanced deepwater drilling technology and seismic imaging in complex environments. These technologies would enable precise drilling and optimized management of difficult underground conditions, which is essential to the success of offshore geothermal energy projects.
Other elements from the oil industry could also be used. These include components such as wellheads, risers and flowlines, which withstand the high pressures and corrosive effects of geothermal fluids. Another area of synergy is floating platforms and fluid pumping and treatment systems, which are also resistant to high pressures and temperatures. Finally, all oil exploration techniques (seismic and multiphysics studies, reservoir modeling, the ability to interpret data, particularly seismic data, and to understand geological formations, etc.) will be of great help in locating subsea zones to be exploited.
What is a Viridien patent application?
The innovation of the Viridien patent lies in its ability to combine offshore geothermal energy extraction with :
- A floating platform serving as a base for the wellheads,
- Geothermal energy production,
- Options for green fuel production facilities.
The configuration of the riser connecting the geothermal reservoir to the floating platform – the main feature of this system – enables the pump inside to efficiently draw the geothermal fluid from the reservoir to the surface, where it can be converted to steam and used to generate electricity on the platform itself. At the base of the riser, the geothermal fluid remains in a single-phase state, allowing it to be pumped efficiently and ensuring that it is delivered smoothly to the platform.
An equitable project
The company has published a patent application (see box above) for a combination of specific technologies for the exploration and development of offshore geothermal resources. The aim of this filing is to ensure the rapid, responsible and equitable development of these resources.
With such a wide range of offshore geothermal resources available and such a multiplicity of potential economic applications, there is a real and urgent need for developed and developing countries to work together, sharing knowledge, technologies and expertise, to ensure equitable distribution of benefits and better protection of waters internationally.
Geothermal energy, which currently accounts for just 1% of the world’s energy, is set to play a bigger role in the future energy mix. The opportunity represented by offshore geothermal energy could be a real game-changer. Not to mention the fact that it fits perfectly with the United Nations Development Programme’s (UNDP) sustainable development objectives: clean energy, climate action and partnerships for sustainable development.
Source: Offshore geothermal energy: a globally important green energy resource and its responsible development, Rebecca Bolton, Rob Crossley, Alex Fowler, Ulrich Schwarz-Schampera and Lucy Njue. Viridien, February 2024.
3 questions for Rebecca Bolton, Offshore Geothermal Team Leader at Viridien
What stage has the offshore geothermal project reached since you wrote your white paper in February 2024?
The publication of the offshore geothermal white paper was a pivotal moment for Viridien (then known as CGG before our recent rebranding). It served as a catalyst to position our team for open dialogue with key stakeholders, including policy-makers, civil servants, engineers, geologists and end-users.
It also provided us with the platform we needed to raise awareness of the potential of offshore geothermal energy, particularly in rift zones, and to engage in serious conversations about its future development.
We took advantage of an invitation from the International Seabed Authority (ISA) to a workshop dedicated to offshore mining to present offshore geothermal energy as a viable solution for power generation, but also as a “responsible” way of extracting minerals from geothermal fluids.
At the same time, we explored different methods of resource exploration and tested our techno-commercial model to ensure its viability. We also focused on establishing partnerships and consortia in high-potential areas to accelerate project development.
Are mid-ocean ridges the only focus of your project, or are you interested in other tectonic contexts?
Although oceanic ridges are an attractive option due to their high heat flux and volcanic activity, other tectonic settings can also be considered. The geological characteristics of the site determine whether it is suitable for geothermal development, but other factors, such as local energy needs or the economic model, also play an important role in whether a project is pursued.
We’ve focused much of our communication on ridges, particularly ocean ridges, because it’s important to maximize the chances of success for a first-of-its-kind offshore geothermal project.
That said, other sites, such as subduction zones and transform faults, active volcanic islands and seamounts, may also host viable geothermal systems. These areas can experience significant heat flows, although the geological heterogeneity of these environments means they can be more complex to explore and develop. However, advances in exploration methods, reservoir modeling and technology could make these environments increasingly attractive in the future.
How to locate deposits?
To identify and locate offshore geothermal deposits, we plan to use a range of geophysical, geological and geochemical exploration techniques. While the overall strategy is similar to that of onshore geothermal exploration, offshore environments can benefit from enhanced seismic imaging and controlled-source electromagnetic (CSEM) methods. Although seismic methods can be useful for characterizing certain types of onshore areas, their high cost and imaging difficulties often limit their frequent use. By taking advantage of advanced offshore seismic techniques, we aim to reduce resource uncertainty and improve the chances of geological success, which can have a substantial impact on project economics.
Seismic surveys are essential for mapping underground structures, such as fault systems and heat sources, helping us to visualize potential geothermal reservoirs. In addition, CSEM could play a role in identifying variations in subsurface resistivity, which can signal the presence of geothermal fluids. This method could be particularly useful offshore, where it would enable us to differentiate conductive geothermal fluids from the surrounding rock, thereby increasing the chances of successful exploration efforts.
We could also integrate magnetotelluric (MT) surveys, gravity data and temperature gradient measurements to refine our understanding of the subsurface. These techniques, combined with geochemical fluid sampling, could provide vital information on the temperature, fluid composition and overall potential of a geothermal reservoir.
Although seismic imaging remains the primary offshore objective due to the absence of surface indicators and its proven success rate in imaging key components of the zone, the combination of seismic, CSEM and other methods enables us to obtain a complete picture of potential geothermal zones. This integrated approach enables us to effectively target the most promising areas for further exploration and development.
By Véronique Molénat.