Compressed Air Energy and Hydrogen Storage Potential in Salt Structures in the UK Sector of the Southern North Sea

Decarbonisation of energy grids as part of the accelerating zero-carbon energy transition requires the massive deployment of renewable energy sources like wind and solar power. Because of their intermittent nature, grid-scale energy storage solutions are required on all scales ranging from battery storage, pumped-storage hydroelectric power and subsurface energy storage solutions.

Energy storage in geological formations, for example, depleted hydrocarbon reservoirs, saline aquifers and man-made rock and salt caverns, has been employed for decades and is ideal because of the potential to safely store large volumes of energy with negligible effects on the surface environment.

Man-made salt caverns have been in widespread use for natural gas storage. Salt formations are ideal due to the inert, impermeable and self-healing nature of salt. Gas can therefore safely be stored in large geometrical volumes at high storage pressures, making it optimal for not only natural gas storage but also electrical energy storage in the form of compressed air (CAES) or hydrogen (HES).

The study is in the Southern North Sea basin (SNS). The SNS is characterised by the accumulation of massive cyclical evaporitic sequences, which were extensively deformed by post-Permian salt movement of Zechstein salt sediments resulting in thicknesses ranging from less than 50m to greater than 2500m.

The North Sea oil and gas sector has been at the heart of the UK energy security. The SNS has undergone decades of extensive exploration yielding large volumes of geological and geophysical data available for renewable energy and energy storage research.

By means of 3D seismic reflection and well data interpretation, this work employs an established screening method used in the hydrocarbon industry, Play Fairway Analysis, to identify potential exploration targets on a regional scale. This approach involves determining the presence and efficiency of a source, migration pathway, a reservoir, a seal and assigning a risk factor to each based on those criteria.

The source and migration pathway equivalents are the location of current and future offshore infrastructure, the reservoir equivalent is defined by the characteristics and distribution of the salt structures and the seal equivalent is defined by the operational constraints within which energy storage can safely occur. The design and placement of salt caverns is governed by the characteristics of the salt deposit and the thickness of the salt layers above and below the cavern. The operational depth range required for sustainable and safe operation is between 400-1500m. The resultant common risk segment map highlights areas with highest energy storage potential.

Initial results indicate that numerous prospective salt structures in the UK sector of the SNS are readily located near existing and future planned offshore wind parks. Future work will build on this through geological characterisation of the target salt structures, shallow hazard assessment, geomechanical assessment to determine cavern placement and storage capacity. The key outcomes of this study include a regional overview of the number and distribution of salt structures, their respective storage potential and the overall feasibility of CAES and HES implementation as part of the UK energy transition strategy.