Offshore geothermal resources (June 2022)

We are continuing the discussion on alternative markets to the oil and gas industry that may initiate new opportunities for diving and ROV companies with “offshore geothermal resources” for power production. The name “Geothermal” comes from the Greek gêo (earth) and thermos (hot). Geothermal energy designates both the science that studies the internal thermal phenomena of the globe and the industrial processes that aim to exploit it. The advantage of this energy is that it is independent of external climatic events and is available 24/7. The phenomena of convection and internal conduction of the globe associated with the degradation of the natural radioactive elements contained in the subsoil produce this heat flow. It is commonly admitted that the temperature of the subsoil is no longer influenced by daily climatic events (day-night changes, rain, winds, etc.) at approximately 20 m depth, corresponds to the average temperature (hot & cold periods) at the surface at 100 m, and increases with an approximate average of 3°C every 100 m beyond this level. However, this phenomenon, called "geothermal gradient", varies according to the geological and structural context, the circulation of underground waters, and the altitude. Thus, a rise of 10 °C every 100 m is noted in some areas that can fall to only 2 °C in some others. Note that the thickness of the earth's crust ranks from approximately 30 km to 60 km, depending on the place. Also, its temperature can be up to 1000 °C near the upper mantle. The temperature of the mantle is estimated from 1000 (upper mantle) to 3000 °C (Mantle near the outer core).

Specialists usually classify the geothermal resources as follows: • A “very low geothermal resource” provides a temperature below 30°C. It is the energy contained in the ground or shallow groundwater. It can be exploited using closed exchangers to maintain the temperature of individual houses and buildings and produce hot water. A closed exchanger system consists of water treated against freezing and circulated in a closed circuit by a pump. The pipes are placed at depths varying between 2 and 100 m underneath or near the house or the building they are planned to heat during the cold season and cool during the hot season (see below). Such geothermal energy is, of course, not usable offshore. • “low geothermal resource” corresponds to a resource whose temperature is between 30 and 90°C. It is directly usable for thermal applications through closed-circuit systems similar to those above. Most of these resources are located in sedimentary basins and employed to heat towns and individual habitations. • A “medium geothermal resource” corresponds to a resource whose temperature is between 90 and 150°C. It generally uses the hot water from aquifers and is indirectly recoverable for electricity production by using low vaporization temperature fluids in closed circuits and large heat exchangers, or be directly used for the applications already described above. That can be done by capturing water from hot water springs at the surface or by drilling a well and installing pumps that circulate water to and from a heat exchanger that captures the calories from the aquifer. A second borehole returns this water to the aquifer. Note that this water cannot be sent as it is to a heating network because it is usually loaded with mineral salts which crystallization would quickly clog all pipes and valves. For this reason, this water transfers its heat to an exchanger before being reinjected into the aquifer. This reinjection is also necessary because the salts from these deep aquifers cannot be released into surface ecosystems. Moreover, if the water is pumped without reinjecting it into the groundwater, the pressure quickly decreases, and the pumping well dries up. The quantity of calories captured depends on the depth and the presence of a thermal anomaly. When there is no groundwater, another process consists of injecting water from the surface through a borehole and recovering the heated water at the surface to extract the calories before injecting it back to depth. Note that pumping water cannot be done without energy, so the power necessary for this process has to be considered. • A “high geothermal resource” corresponds to a resource whose temperature is above 150°C. It is directly exploitable for electricity production and is generally located near large volcanic arcs. Exchangers (see a model from Trianon below), and steam turbines (see a model built by Alstom below) are commonly used to exploit it.

Unlike what many people promoting this type of energy production pretend, these installations exploit non-renewable heat. That is linked to the fact that they gradually cool the rocks and the aquifer between the pumping and injection wells. As a result, the heat transfer is gradually less efficient, and the wells are considered dead after a duration estimated between 30 and 100 years, depending on the initial temperature and the size of the aquifer. Also, it is estimated that the earth loses approximately three times the power currently produced and consumed by humanity. As this energy is continuously dissipated, many consider it inexhaustible on a human scale. However, some specialists say that since humankind's energy consumption doubles every 30 years, this energy reserve might not be sufficient from approximately 2050 if this consumption trend is continued. Nevertheless, despite the fact it is not renewable and potentially limited, geothermal energy does not produce waste. Thus, there is no reason not to exploit it. It must be noted that geothermal energy remains marginally employed, even though several production plants are in service and despite the high potential of efficiency in many regions of the world. Also, offshore geothermal energy has not been considered a feasible option for many years, mainly when the oil and gas prices were low. Nevertheless, the continuously increasing hydrocarbon prices and new approaches to exploiting this resource, such as electricity and hydrogen production, make its utilization attractive. It also presents the advantage that it can be implemented on oilfields that are no longer productive and thus replace hydrocarbons production without too expensive an investment. Also, the technology to exploit this energy offshore is already there. As an example, deep drilling processes that were problematic several decades in the past are today well controlled, and water injection is a process commonly used in oilfieds. The advantage of offshore production plants compared to facilities installed on land is the same as those of offshore wind farms. Thus, they do not need a detailed assessment of conflicting activities with neighbors, which is mandatory in many countries, and no significant land space acquisition is required. That avoids direct conflicts with most local populations and is a substantial factor for easy exploitation. Of course, ecological evaluations of their impact are to be undertaken. Nevertheless, it has already been said that the environmental impact of these production units is limited and has already been done in the case of recycled oilfields. Some conflicts may arise with fishermen if the fields are too close to their fishing areas. That has already happened with wind farms and tide turbines, even though the absence of hydrocarbons should have made such production plants more admitted and have contributed to considering them underwater life reserves. Regarding this point, it must be noted that recycled oilfields would provide an additional advantage as their exclusion zones are already delimited. Also, as said in our discussion on wind farms, far offshore sites offer the advantage that they are less visited by anglers and thus, are more admitted by this category of people whose interests often conflict with those arising from such production plants. Like almost all sustainable energy systems available on the market, the economic side is the main problem regarding the feasibility of these production units. As said above, this disadvantage diminishes as the prices of hydrocarbons rise, and recent conflicts show that energy independence is crucial for developing countries. Also, these costs are said to be widely cut for projects that consist of recycling oilfields at the end of life. Multiple configurations are considered by specialists. Their final selection will depend on their feasibility and their productivity. Note that it is evident that a significant challenge is to keep the water extracted as hot as possible to obtain maximum efficiency. Among the solutions commonly discussed, the one that consists of using jackets or other offshore facilities to install equipment that transforms the energy from the hot water into electricity above the surface of the sea on the extraction site is often considered (see below). In this case, the scalding water is pumped from the seabed to the facility and reinjected after usage, as described previously.