Hydrogen vs. Energy Storage in Offshore Deep Depths Salt Caverns Drilled at Vadu, Constanta County, Romania

1. Introduction

The continental shelf of the Black Sea represents the seabed, and the subsoil of the adjacent submarine regions located beyond the territorial sea to a depth of 200 m or over this limit, up to the point where the depth of the waters allows the exploitation of the natural resources of these regions. The platform has the appearance of a plane with a very low slope that descends towards the sea, reaching depths of 200 m [6], see also Figure 1, (a). The intermediate depths (50-100 m) follow the shoreline and include the territorial sea (22 km from onshore) and the exclusive economic zone, further on it being the free zone. The map from Figure 1, (b) shows a research work of a romanian-bulgarian team for determining simultaneously the bathymetry, gravimetry and total field magnetometry, with the R/V Mare Nigrum GeoEcoMar’s ship (Romania) [7, 8], that covered a total shelf and slope area of 47,000 km², see in Figure 2, (b), the marked area [9]. The degree of knowledge of the geology of the aquatic area, near the shore, encourages the elaboration of works that capitalize the information already obtained; the geological exploration of the Romanian continental platform of the Black Sea has a long history, marking almost 55 years since the recording of the first seismic sections and 45 years since the digging of the first well. From a seismological, gravity and magnetic point of view it is evaluated an area of approx. 33,160 km², and in the same time 120 wells were dug, that share the research in three subunits, North Dobrogea, Central Dobrogea and Southern Dobrogea, see Figure 1, (c, d) whose differences are manifested especially at the foundation level, 57 of them being between two geological faults, Capidava-Ovidiu and Peceneaga-Camena, that mark the Central Dobrogea area [9].

In Figure 2, a detail of Figure 1, (d) was made where it has been suggested that in the Vadu area there is a gravity negative anomaly which can be interpreted by various authors as salt deposits. Gravimetry is usually used for salt deposits, and it is not used for deposits that contain iron [10, 11, 12].

As a whole, the geological structure of the Dobrogean continental platform includes the same major units as the adjacent land: the North-Dobrogean Orogen, the Babadag Basin and the Moesic Platform with its subdivisions, Central Dobrogea and Southern Dobrogea, see also Figure 3 [13].

The paper is studying the Vadu structure that is before the Histria Depression, and is situated in Central Dobrogea, see the Figure 2 and Figure 3. The Vadu structure represented by the evaporitic facies from the Upper Jurassic and Lower Cretaceous periods, a unique case in the Romanian sector of the Black Sea continental platform, is mainly composed of salt alternating with limestone. These facies were opened by the 19 Vadu well, at a depth of about 1,500 m, in the interval 620 m ÷ 2,110 m. Towards the east of the basin, these facies are replaced by carbonates [14]. The Callovian-Oxfordian, 2,036 m ÷ 2,110 m is predominantly sandstone with clay intercalations, the Oxfordian – Kimmeridgian, 2,036 m ÷1,965 m is predominantly calcareous with marl intercalations, the Kimmeridgian – Purbekian, 1,818 m ÷ 1,965 m is represented by alternations of argillaceous limestone and limestone dolomitic. The Purbekian-Wealdian follows, 1,820 m ÷ 620 m represented by rock salt, alternating with calcareous - dolomitic or bituminous shale and anhydrite. The evaporitic series passes into the Neocomian, 620 m ÷ 458 m to gypsums with anhydrite. In succession, the Neocominan passes to limestone and conglomerates. In Apţian, 389 m ÷ 458 m the series continues also with conglomerates/pebbles and limestone. The Upper Cretaceous, 311 m ÷ 389 m, is calcareous and sandstone-clay with marl intercalations. The Upper Paleogene, 389 m ÷ 250 m is transgressive, being made up of sandstones and marly sandstones or with clayey intercalations. The Vadu structure was investigated in 1988 – 1989, when the 19 Vadu borehole was dug see Figure 4 [15]. The saline formation that is the object of this study was attributed to the Purbekian-Wealdian. The character of the deposits and the great thickness of the saline formation seem to represent the indicators of a sabkha-type sedimentary environment developed on a platform with very active subsidence [15].

2. The Evaluation Methodology for the Deep Salt Deposit in the Continental Shelf of the Black Sea and Calculus Results for Energy Storage Potential

The section methodology and evaluation of caverns for hydrogen storage summarizes the offshore salt cavern site selection process starting with the characterization of salt deposits, then passing the choice of conditions considered suitable for cavern development by analyzing the proximity of existing infrastructure and finally, on cavern design, placement and storage capacity estimates.

Determining the development potential of salt caverns requires the use of multiple techniques and datasets to form a robust analysis of the geology and geophysics of salt formations and associated geomechanics to ensure cavern stability and subsequent energy security. Therefore, characterizing the identified salt formations and evaluating them for their suitability for hydrogen storage are the first steps in the screening process. The selection process for the depth and height of the location of the caverns was based on the lithological heterogeneity, thickness and mode of formation: sedimentary or diapiric. The cavern placement was based on an optimal depth range dependent on the convergence rate of the caverns, hydrogen storage pressure, floor and safety pillar thickness, and recommended cavern shape criteria established from the literature and gas storage experience underground see Figure 5 (a) [16], Figure 5 (b) [16] and Table 1 [17].

The classical method consists in injecting fresh water into the saline layer; the fresh water circulates and becomes salty, forming the brine, which is then extracted through extraction pipes and directed for processing and production, especially of sodium products. The equipment consists of three concentric columns: a sealing column (anchoring column, cemented, which will have the length to where it is estimated that the ceiling of the future dissolution gap should be), an exploitation column (cassing) and a column extraction (tubing). Three concentric spaces are created, which ensure that the circuits of the cell are three technological fluids: industrial water, insulating fluid and brine [27]. In our case at Vadu well, the induistrial water can be marine water, and, also, the brine can be discharged back into the sea, as no facilities for salt processing are implemented yet. The construction of the well begins with the drilling of the well for the anchor column, continues with its tubing, cementing, tightness check and finishing with the drilling and tubing of the exploitation hole see Figure 6 (a) [28]. In Figure 6 (b) the cavern was simplified to a cylinder with conical top and bottom with the values of: Ød~300mm – the diameter of the cemented column, ØD – the diameter of the underground cavern, h₁ = 1/3D, h₃ = 1/6D, h₂ = H-(h₁ + h₃), where H is the total hight of the cavern.

The volume of the simplified cavern from Figure 7 (b) is given by the relation:

$$V={\displaystyle \frac{\pi }{12}}·D^2·\left(3·H-D\right) [\text{m}]$$

(1)

The conditions for carrying out marine drilling are more special, considering the lack of fresh water. However, the Black Sea has a medium to low salinity around the Vadu structure. For the area near the coast, the values for temperature and salinity measured in June are in the range of 20.68°C - 26.2°C, respectively 9.8 PSU (practical salinity unit – 9.8 g salt/1000g water) in the Danube mouth area and 17.98 PSU in the Vadu - Constanta - Eforie areas, see Figure 7 [29].

Another condition, according to the Romanian experience in the exploitation of salt through wells, is imposed by the depth: it is estimated that up to 1250 m there are no special technical problems [15]. On the world level, with the taking of additional measures, salt was also exploited from depths up to 3000 m. For example, since 1972, the company KemOne has been extracting the salt from deep caverns (1500–3000 m) in the form of brine [30]. In the Vadu structure, the exploitation of salt in order to create natural gas storage deposits is leased on the intervals of 705 - 905 m and 1542 - 1800 m. In the laboratories of the University of Petroşani, Petroşani city Romania, research was carried out on the samples of salt extracted from the 19 Vadu well to determine the geomechanical characteristics of rock salt, namely: physical-mechanical, deformation and rheological. The average values of these characteristics are shown in Table 2 [15]. In Table 3 [15], compressive strength measurements performed on salt samples from different salt mines in Romania are presented comparatively.

The diameter of the underground cavern is calculated with the relation (2) [31] (p. 58):

$$R=\sqrt{{\displaystyle \frac{Q\cdot C_{sat}}{220·\pi \cdot w}}} [\text{m}]$$

(2)

where:

R – the radius of the dissolution cylinder;

Q – the flow of water introduced into the cavern, Q = 30 m³/h;

C_sat – extracted brine concentration (saturated), in Kappeller degree (1 Kappeller degree = 10 grams / liter of brine), C_sat = 31.5 g/dm³;

w – the lifting speed of the operating column, w = 0.00287 m/h, given by the relation (3) [31], (p. 59):

$$w={\displaystyle \frac{0.0167}{1000}}·{C_{sol}}^2 [\text{m}/\text{h}]$$

(3)

where: C_sol – solvent concentration in Kappeller degree = 13.1 g/dm³;

For these parameters, resulted in a diameter of the underground reservoir of ~40 m. The dissolution massic speed of rock salt in the horizontal direction in [kg/(m²h)] can be evaluated with the following empirical formula established by Külle (URSS) [32] (p. 59), [32] (p. 63), which also considers the salinity of the water introduced, in our case sea water:

$$V_{mass}=\left(3.25·{\Psi }^{\mathrm{0,5}}+1.8\right)·\left(1-{\displaystyle \frac{C_{sol}·{\gamma }_{sat}}{C_{sat}\cdot {\gamma }_{sol}}}\right)\left(1+{\displaystyle \frac{t}{22.4}}\right)=5.82 [\text{kg}/{\text{m}}^2·\text{h}]$$

(4)

where: C_sat – the saturation concentration of the brine = 0.315 g/cm³; C_sol – solvent concentration ~ 0.18 g/cm³ (marine water in Vadu area, see above); Ψ – 1.57 rad (corresponds to the 90° angle made by the vertical wall of the dissolution chamber with the horizontal); γ_sat – the specific density of the saturated brine = 1.24 g/cm³; γ_sol – the specific density of the solvent = 1.13 g/cm³. To express the dissolving speed in [m/h] in radial direction, should be divided the V_mass obtained by Kulle's relation to the specific density of the salt γ_salt = 2160 kg/m³ and in this case

$$V={\displaystyle \frac{V_{mass}\left(fromrelation\left(4\right)\right)}{{\gamma }_{salt}}}=0.0027 \text{m}/\text{h}$$

(5)

The management of the dissolution is done through a rigorous control of the quantities of seawater introduced into the well, the quantities of brine extracted and its concentration, considering the initial salinity of the seawater. The completion time of a step can be calculated with the relation (6) [32]:

$$T={\displaystyle \frac{0.6\cdot R}{V}}=203 [\text{days}]$$

(6)

where: R – radius of the underground reservoir, R = 20 m; V – mean radial velocity of dissolution = 0.0027 ^. 24 = 0.0648 m/day. Results the completion time of T = 200 days. For a radius of the underground reservoir, R = 30 m, the completion time is T = 300 days. The optimal height of the dissolution step was determined by the relation (7) [15]:

$${h_{stage}=K}_1·K_2·{\displaystyle \frac{C_m\cdot Q}{\pi \cdot R\cdot V\cdot {\gamma }_{salt}}} [\text{m}]$$

(7)

where: K₁ – correction coefficient = 1.13; K₂ – coefficient that depends on the speed of salt dissolution = 14.4; γ_salt – the specific density of the salt (from above); V – mean radial velocity of dissolution = 0.065 m/day; C_m – medium concentration of brine = 166.5 g/dm³.

In conclusion, for these parameters, the height of the dissolution step will be 8.7 m for a radius of the underground reservoir, R = 20 m, and 5.9 m for a radius of the underground reservoir, R = 30 m, in completion time of T = 200 days for the first case and T = 300 days for the second case. For the geomechanical conditions in the Vadu structure the parameters are: the thickness of the package of covering rocks and sea water, h₁ = h_r + h_w = 700 m + 100 m = 800 m; volume density of the covering rocks, γ = 2.0 · 10³ kg/m³; the diameter of the dissolution cylinder, D = 40 m; tensile strength of the salt, σ_rt = 1.4 N/mm²; density of salt, g = 9.8 m/s² (standard gravity).

The thickness of the salt floor h_f see the relation below [15]:

$$h_f\ge {\displaystyle \frac{n\cdot \gamma \cdot {·g·D}^2}{2\cdot k_f\cdot k_s\cdot {\sigma }_{rt}}}\cdot \left[1\pm \sqrt{{\displaystyle \frac{4\cdot k_f\cdot k_s\cdot {\sigma }_{rt}}{n\cdot D^2\cdot {\gamma }^2}}q}\right];h_f\ge 35 \mathrm{m} [\text{m}]$$

(8)

where: D – the diameter of the reservoir; q – the load given by the covering rocks, sea water, and the contrapressure of the brine, k_f = 4.85 (the correction coefficient), k_s = 1.3 (safety coefficient) and n = 1. For q see Appendix A.

The calculation results are given in the tables below:

The time is almoust double as given in literature [15] due to the salinity of the see water used for cavern creation and that is lowering the proces.

In Table 6 below calculations are given over the hydrogen storage capacity of each type of cavern at the given depth of 700 m respectivelly 1,400 m. Also, the termal gradient with depths of about 30°/1,000m have been taken into consideration, see Table 5. Heve been taken into consideration the netto volume of hydrogen, the netto mass stored and the netto energy potential storage.

For a request of energy of 10 MWh at a given time, the energy stored in cavern A ensures a total functioning time of 2,344.3 hours ~ 97.7 days ~ 3 months and 1 week. Cavern B ensures a total functioning time of 9,393.9 hours ~ 391.4 days ~ 13 months (1 year and 1 month). Energy is a netto energy without taking into consideration the captived energy, that represents the minimum energy stored required for the correct functioning of such a storage system.

3. FEA Analysis of Underground Hydrogen Storage Caverns: Structural Integrity Under Pressure

To assess the structural integrity of underground hydrogen storage, finite element analysis (FEA) simulations were conducted on caverns located at two different depths: 700 m and 1400 m. The 3D models are at real scale. These caverns are subjected to varying hydrogen pressures, as shown in Table 6 above, the shallower cavern at 700 m operating within a range of 30–120 bar, while the deeper cavern at 1400 m experiences pressures between 60–230 bar.

The simulations aimed to evaluate the mechanical response of the surrounding salt deposit under these pressure conditions, identifying potential stress concentrations and failure points. The results provide insight into the feasibility of sustained hydrogen storage at different depths and pressure levels.

SolidWorks Simulation add-on was used to create and run the FEA analysis, due to its capability of defining custom materials. The caverns must withstand the lithostatic pressure exerted by the overlying rock strata and sea water. To account for this, an equivalent mass was determined based on the average density of each stratum and the mass of sea water above (shown in Figure 8. In order to be able to drill the 60 m caverns, they were interspersed between the 40 m ones. Also, in order to validate this result, the mass was also calculated based on geostatic pressure. Since SolidWorks does not support direct input of such high distributed mass values, an additional domain with an adjusted density was introduced to replicate the correct weight. Series of simulations were done for both salt deposit and minimum and maximum cavern pressure. The matrix was simplified to 3x3 for 40 diameter caverns and 2x2 for 60 diameter caverns.

In Figure 9 von Mises stress distribution across the first set of salt caverns is depicted (40m diameter, at 700m depth). The scale was limited to the lower value of compressive strength (22 MPa as of Table 2) showing that there is no risk of failure. It is shown that in the center of the matrix, the compressive strength is greater than at the margins.

In Figure 10 the scale was limited to the dilatation threshold (13 MPa as of Table 2), a point where the caverns will suffer plastic deformations.

For the minimum pressure of 30 bar, higher stress is seen at the extremities of the cylindrical part of the caverns, where the upper limit of the compressive strength is reached (36 MPa as of Table 2), which can be avoided by increasing the minimum pressure of the vessels, see Figure 11 and Figure 12. The hidrogen withdrawal is available between 120 to 30 bar, and the cushion gas in conclusion should have 30 bar pressure minimum.

At 1400m depth, since the geostatic pressure is much higher, this results in higher operating conditions of the caverns in order to avoid failure due to the exceeding of the compressive strength. At maximum operating pressure, at the extremities of the cylindrical part, some spots that are close to the compressive strength limit can be seen in Figure 13 and Figure 14.

At the minimum pressure of 60 bar, a maximum stress of around 65 MPa is reached, that would result in a critical failure of the salt caverns, in the zone of the top and bottom cones. The conclusion is that it is not possible to withdraw the hidrogen under the pressure of 60 bar in the cavern at 1400 m depth. The hidrogen withdrawal is available between 230 to 60 bar, and the cushion gas in conclusion should have 60 bar pressure.

In Table 7 data have been collected for cavern storage sites indicated for drilling salt storage caverns [33].

4. Short Comparison Between H₂ Salt Cavern Storage and H₂ Hydrate Storage

The conventional H₂ storage options include salt caverns, saline aquifers, and depleted gas reservoirs. Generally, storing hydrogen in salt caverns is preferable as the cushion gas is very low comparativelly with the other solutions, and it can be implemented on a large scale once these caverns exist. If these do not exist, the salt caverns are created by solution mining in salt-rich formations and the costs with mining are about $25.5/m³ [49]. For a cavern of 150,000 m³ at 700m depth, the cost of salt cavern creation is about $3,825,000, without other costs that are about $5,800,000 [49] taking into consideration the values from Table 6. As a first approximation, the cost of a new salt cavern at 700m depth is about $10,000,000. For a cavern of 320,000 m³ at 1400m depth, as a first approximation, the cost of a new salt cavern is about $30,000,000 taking into consideration the values from Table 6, about 3 times greater than a cavern at 700 m depth, but the storage mass is about 4 times greater than the storage mass in the cavern located at 700m depth.

A new technology for hydrogen storage is hydrate-based H₂ storage, based on thermodynamic hydrate promoter. A pure H₂ hydrate can achieve 5.3 wt% H₂ storage capacity at extremely high pressures of 300 MPa at 249 K (-24°C). The addition of thermodynamic hydrate promoters (THPs), such as tetrahydrofuran (THF), 1,3-dioxolane (DIOX), cyclopentane (CP) in the 5.56 mol%, have shown promotion effect on shifting H₂ hydrate equilibrium to moderate pressure and temperature ranging from 8.3 MPa to 18.3 MPa and 277…284K (4…11°C) [50].

The technology of hydrate formation is promising as long as for 1000 kg of water (1 m³ of water) there is 56 kg captured hydrogen, or 1.9MWh/ tone water captured energy in conformity with Table 6, taking into consideration the pure water. Also, for pure water hydrogen hydrate formation it is necessary to have a pressure of over 300 Mpa at low temperatures. Using 5.56 mol% thermodynamic hydrate promoters (THPs), the amount of hydrogen stored decreased at ~4.3wt% see Appendix B, but the pressures also decrease dramatically, with positive slightly above 0°C temperatures.

From [51] for an amount of stored H₂ = 5.4 milion Nm³, the initial cost of implementing the hydrate technology is about 15.2 billion JPY or 99 milion USD. From the above the initial cost for implementing 2 caverns, one at 700m and the other one at 1400m, is about 40 milion USD, for an amount of 39.5 million Nm³ H₂.

In conclusion from a rough estimation, hydrogen hydrate technology is a very promising one with a ratio of 1:6 costs compared to salt cavern storage technology up to date.

The advantage of salt cavern tehnology in offshore areas is that the technology does not require fresh water for dissolving and the technologies are quite well-known. The greatest desadvantage is that technology takes a very long time to create a new cavern.

The offshore hydrogen hubs are a necessity not just for Romania but also for other countries especially those with access to the sea and ocean for large capacity container ships transporting to reduce and eventually eliminate greenhouse gas (GHG) emissions from international shipping [52].

5. Conclusions

The storage of hydrogen in Romania, as it was said in the summary of the work, is included in the Romanian Energy Strategy 2016-2030 [53] regarding the development of production, transport and storage infrastructure in the development of reliable projects to reduce the carbon footprint, and not only, but especially in the creation of a clean and intelligent planet. There are laws implemented at the European level for the gradual reduction of the carbon footprint, such as the reduction of at least 40% in 2030 and 70% in 2050 compared to the levels recorded in 2008 of CO₂ emissions for transport units, especially naval which brings a huge contribution to marine and environmental pollution. Romania had in April 2024 about 3,000 MW (16.5%) represented by wind farms [54], and in March 2024 about 3,000 MW (16.5%) represented by photovoltaic farms (including the prosumers) [55], which means 33% of the power installed in electricity production units in Romania. The energy given by regenerable sourses at maximum production is in summer. In the winter the production of the regenarable sourses decrease dramatically. The hydrogen storage strategy plays an important role in maintaining stability and uniformity of energy production.

For Romania, the implementing of the hydrogen storage in the Romanian Energy Strategy, is of capital importance in terms of the further development of hydrogen production and transport capacities, in the context in which renewable energy is rapidly developing and the need for decarbonization comes into force. There are also projects to introduce hydrogen into natural gas networks, as well as projects to introduce hydrogen into electrical and thermal energy production capacities like fuel plants, with new generation engines, see Siemens Energy Strategy [56]. The opening of storage capacities offshore to the Black Sea, also opens up the opportunity to use stored hydrogen in order to feed maritime and fluvial, commercial and military ships in order to mix conventional fuels with percentages of hydrogen to reduce de carbon footprint [57]. The total number of wells that could be opened in the area of the trapezoid shown in Figure 2 and Figure 3 of about 595 km², taking into consideration the margins shown in table 1, considering a diapiric deposit, and both levels, is aproximativelly 20,600 (10,300 in the 700 m first level and 10,300 at the 1,400 m second level), considerring a diameter of 60 m for the cavern and a distance between them of 240m (4 ^. D), see also the trapezoidal area from Figure 2 and Figure 3, divided into 18+5.35 sectors of 441 wells each shown in Figure B1 from Appendix C.

The total capacity in tonnes is given (from table 6) by the relation:

$${\mathcal{N}}_{mass\_stored}=\mathrm{10,300}·704+\mathrm{10,300}·\mathrm{2,821}=\mathrm{36,307,500}tonnes{H}_2$$

(9)

over 36 million tonnes H₂. The total amount of energy stored is given by relation (9):

$${\mathcal{E}}_{total\_stored}=\mathrm{10,300}·23.44+\mathrm{10,300}·93.939~1209TWh\left(1.209PWh\right)$$

(10)

over 1.2 PWh energy stored. The conclusion is that Romania is at the beginning of the road in terms of hydrogen storage capacities but has a huge potential in developing them and in increasing the production and transport capacities of hydrogen, ensuring a sustainable future in the Green Energy Sector.