In the final blog in this series about Compressed Air Energy Storage, we look at the potential offshore application of the technology. The most notable example of a successfully deployed, grid-scale Compressed Air Energy Storage plant is Huntorf, onshore Germany. It has a 312 MW capacity and has been successfully operating for over 40 years. Huntorf features two large salt caverns, meaning that it can remain operational during maintenance for emergency power. Can the engineering design and lessons learnt from project such as this be applied for future offshore Compressed Air Energy Storage developments?
One of salt’s (halite) greatest strengths with respect to secure storage is the fact that it is tight with respect to gas, with high entry pressures and low permeability (Warren, 2006). It deforms ‘viscoplastically’, similar to the ‘rheid’ characteristics of the mantle under pressure (Carey, 2008). This often has the added benefit of sealing faults which may have developed in the surrounding strata. However, this same viscoplasticity under pressure raises concerns when excavating salt caverns for Compressed Air Energy Storage, particularly in deep offshore settings.
Southern North Sea Compressed Air Energy Storage Evaluation
One recent research project aimed to map the volumetric and geomechanical properties of several subsurface salt structures in the UK sector of the Southern North Sea, with a view to evaluating their potential for Compressed Air Energy Storage. Merlin’s Junior Geologist, David Whitworth, looked at potential offshore locations for Compressed Air Energy Storage at the kind of scale which can have an impact on the grid over the Sole Pit High and Silver Pit Basin (Whitworth, 2020). The objectives of the study were to:
- Assess the structural framework of the Zechstein salt interval and the overburden.
- Characterise the structures using a traditional hydrocarbon play-fairway model.
- Quantify the volumetric and geomechanical attributes of the most promising ‘leads’.
To achieve this, a regional seismic investigation was conducted using data supplied by the OGA as part of their 3D Seismic MegaSurvey. Interval horizons were carefully mapped for the Top and Base Salt, the seafloor, Top Cretaceous, Base Cretaceous, Top Triassic and Top Rot Halite. These horizons enabled a simple velocity model to be developed to define the depths of the salt interval.
The identified depth range for salt caverns within a safe and workable pressure distribution is around 400-1400 m (McEwen-Read, 2019). Twenty-six leads were identified from the mapping within this depth interval and assigned a letter from A-Z, Figure 1. A more detailed depth-pressure relationship was established within the regional area of interest which indicates that maximum potential cavern operating pressures of 18 MPa (Wang et al. 2019) were reached in the area of interest at a depth of 1880 m. This indicated a larger depth range for cavern volume estimation could be used in future studies.
What did we learn from this study?
The 26 structures covered a range of structural styles from salt walls to anticlines and diapirs, and were ranked using a proprietary risking system based on traditional hydrocarbon risking methodologies. The two most promising, lowest risked leads were Lead A – a classic salt diapir; and Lead C – part of an elongate ‘salt wall’ structure, Figure 2.
From analysing the structures with a pseudo-play fairway approach, it was concluded that almost all leads in this stratigraphy were likely to contain ‘rafts’ of Platten Dolomite, a result of extensional thinning of the evaporite sequence during diapirism. These rafts are represented in the seismic by segments of high-reflectivity reflectors seemingly floating within the low-reflectivity halite bodies. They pose a risk to any drilling operation; however, good quality seismic imaging means they can be easily picked and avoided. Combining this with the moderate volume of viable homogeneous salt they contain they are interpreted to represent a ‘moderate risk, moderate reward’ case.
Another risk, more often associated with the salt wall structures, is low internal reflectivity. As the slopes of the salt walls are much steeper than those of the gentle diapirs, seismic imaging is often less clear on the internal geometries of the bodies resulting in higher drilling risks. Combining this risk with the huge volumes of viable salt within a salt wall, they are interpreted to represent a ‘high risk, high reward’ case.
The total salt volume of the high-ranked structures was calculated in Petrel, and a 3D mock-up of the structures concluded that Lead A (diapir) could accommodate a total of 543 caverns in a hexagonal pattern over two levels, Figure 3. Based on this number of caverns, the volume of compressed air that could be stored was calculated at 0.2 km3 of compressed air, allowing for factors such as insoluble sump, cavern irregularity, and cushion gas ratio. Comparatively, Lead C was found to be capable of accommodating 240 caverns on a single level, corresponding to 0.1 km3 of compressed air. Both caverns showed operating pressures in the 8-12 MPa range, well below the 18 MPa cut-off.
Using an equation derived from the ideal gas law, theoretical ‘exergy’ output can be calculated from the volume of air stored. Exergy is defined in thermodynamics as ‘maximum useful work done which can be extracted from a system.’ The combined theoretical exergy for both leads was calculated to be 267 GWh, comparable to the outputs of large-footprint windfarms.
Conclusions and Next Steps
In the Southern North Sea, Compressed Air Energy Storage potential of just two leads in the study area show considerable potential. Theoretical storage volumes of up to 267 GWh were calculated which is more than comparable with the outputs of major windfarms, and equivalent to more than 10 hours of the entire UK energy demand.
A single 7 MW turbine can create over 6 million kWh per annum, and Hornsea Project One, located in the northern part of the study area, contains 174 such wind turbines, providing a total annual energy capacity of 1.2 GW. As of 2024, the EU need to install 33 GW of wind energy capacity per year to meet its 2030 target of 425 GW. The European Wind Energy Association estimated only 40 GWh was installed in EU waters by 2020. With 50% of the EU’s energy demand due to be met by wind energy by 2050 (4C Offshore, EWEA 2020), WindEurope anticipates significant increases in infrastructure build-out toward the end of the decade (WindEurope, 2024).
From these figures, it is clear that the capability to store over 200 hours of energy from just two leads in the study area would have a significant impact on the grid-scale reliability of wind power in the future. In addition, there are financial incentives, given that renewables are becoming increasingly affordable, in line with fossil fuel energy costs. In 2017, Hornsea 2 Offshore Windfarm project in the Southern North Sea had costed wind energy production at €65/MWh, compared with €70 for gas and €72 for coal (UK Government, 2018). In 2023 however, the UK government was offering £73/MWh to wind operators, in a bid to increase green investment, while gas was trading down at €37 in the EU due to factors such as a mild winter, and swollen European gas storage facilities in the wake of the switch from Russian suppliers (Reuters, 2024). This reinforces the need for a sustainable, domestic energy supply, whether renewable, hydrocarbon, or a versatile mix, which is less at risk from external factors.
David Whitworth, Junior Geologist
References
4C Offshore, EWEA, 2019. Map of offshore windfarms in the United Kingdom.
Carey, S.W., 1953. The rheid concept in geotectonics. Journal of the Geological Society of Australia 1, 67–117. https://doi.org/10.1080/14400955308527848
McEwen-Read, F., 2019. CAES Potential in Salt Diapirs in the Coastal Areas of the Southern North Sea (Master of Science – Petroleum Geoscience). Royal Holloway, University of London.
Reuters, 2024. Europe’s swollen gas stocks drive prices lower. URL https://www.reuters.com/business/energy/europes-swollen-gas-stocks-drive-prices-lower-kemp-2024-02-13/
UK Government, 2018. Hornsea Project Three – Offshore Wind Farm. Funding Statement Annex 2 – Ørsted Annual Report.
Wang, T., Li, J., Jing, G., Zhang, Q., Yang, C., Daemen, J.J.K., 2019. Determination of the maximum allowable gas pressure for an underground gas storage salt cavern – A case study of Jintan, China. Journal of Rock Mechanics and Geotechnical Engineering 11, 251–262.
Warren, J.K., 2006. Evaporites: Sediments, Resources and Hydrocarbons. Springer-Verlag Berlin Heidelberg, Berlin, Heidelberg.
Whitworth, D., 2020. Compressed Air Energy Storage (CAES) Potential of Salt Anticlines & Diapirs In The Sole Pit High & Silver Pit Basin, UK Sector, Southern North Sea (Master of Science – Petroleum Geoscience). Royal Holloway, University of London. https://www.researchgate.net/publication/352491296_Compressed_Air_Energy_Storage_CAES_potential_of_salt_anticlines_diapirs_in_the_coastal_areas_of_the_Sole_Pit_High_Silver_Pit_Basin_UK_Sector_Southern_North_Sea
WindEurope, 2024. Wind energy in Europe: 2023 Statistics and the outlook for 2024-2030. Windflix. URL https://windeurope.org/intelligence-platform/product/wind-energy-in-europe-2023-statistics-and-the-outlook-for-2024-2030/#
