*4.4. Exergy Storage Assessment—Methodology*

Figure 2 summarises the exergy storage assessment process. From borehole log data and map information in the public domain and held by the British Geological Survey, the tops, bases, and thicknesses of the halite deposits and major faults were mapped within each basin. These data were input to ArcGIS, which was used to obtain potential cavern locations, depths, and basic cavern parameters such as heights, diameters, available volumes, spacing and casing shoe depths based upon criteria applied to the design, development, and construction of gas storage caverns in the same strata [42]. The halite beds were evaluated over the depth ranges under consideration for CAES operations, with casing shoe depths (and thus pressures) in general between 500 and 1300 m [23] as well as up to 1500 m depth as at the proposed CAES plant at Larne, Northern Ireland [43]. Then, these basic cavern data were input to a modelling tool and used to estimate the exergy storage potential of prospective UK onshore and offshore East Irish Sea areas, using pressure and temperature ranges derived from gas storage investigations in these areas. Basic theoretical storage estimates are derived (Figure 3) that are only that and which, for various reasons, are clearly unrealistic totals. Most obviously, not all cavern locations will ultimately be available or suitable for cavern creation due to geological constraints, salt quality across the basin, together with economically and operationally viable cavern sizes. Therefore, a series of filters, based upon likely cavern height ranges, sizes, and differing storage operations, have been applied to derive more realistic CHT storage estimates for each basin. These can be compared to the annual UK electricity demand of 300 TWh [44].

In an attempt to obtain realistic assessments of the potential provided by the bedded halite resource, the total exergy storage estimates from each basin were filtered in a variety of ways:


The figure of 1% is not based on industrial experience or previous studies. However, it provides a first pass understanding of the potential cavern numbers and storage capacities over the differing depth ranges, against which estimates set against the constructed or planned UGS cavern numbers in the two most developed basins can be evaluated (S2, Table S1): Cheshire Basin (73 caverns = 3.5–4%, for 500–1300 and 500–1500 m depths, respectively), Eastern England (37 caverns = 2–4%, respectively). Therefore, the figure of 1% is lower than these percentages and thus appears a reasonable gauge against which exergy storage estimates might be assessed initially (Figures 3 and 4). However, Figure 3 reveals very high storage estimates for the Wessex Basin, which is a potentially large region, but one in which the halite beds are less well characterised; halite beds were unknown in the area until oil and gas exploration began in the 1970s [37]. Consequently, further refinement of the estimates was attempted, reflecting more the degree of exploration and the proven potential and capabilities of the halite beds in each of the main halite basins. The greater numbers of storage caverns in the Cheshire Basin (73) and Eastern England (37) mean that these basins represent the most mature areas in terms of exploration and development of the halite beds. Thus, they potentially provide the more accurate and greater storage estimates when compared to using only the planned or permitted 24 and 14 cavern numbers for the lesser exploited EISB [45] and Wessex [46] basins, respectively. The latter two basins currently represent higher-risk target storage horizons, where in the case of the EIS, remoteness and its offshore location also increase CAPEX and OPEX costs of storage projects [45].

Additional efforts to derive realistic exergy storage estimates were also undertaken through filtering the storage outputs based upon cavern heights, with two sets of caverns assessed based on the experience of the maximum heights of gas storage caverns in the same salt beds, or those of proposed storage caverns in the Wessex Basin and the EIS. Firstly, caverns of 100 m and greater were selected, arising from the general sizes of UGS caverns developed or proposed in the same halite beds (refer S2, Table S1). Caverns smaller than ≈90 m in height are less economic to operate for gas storage purposes and are likely even more so for CAES due to the lower volumetric energy density (≈2.4 kWh/m<sup>3</sup> ) of air in comparison to natural gas (1100 kWh/m<sup>3</sup> ) [23]. Importantly, those caverns in which diameters are much larger than cavern heights of a few tens of metres could be geomechanically less stable, with caverns thus requiring smaller diameters and thereby likely to also result in uneconomically small cavern volumes [47]. Secondly, very large (tall) caverns carry stability issues and operational limits for rapid cycle storage, and therefore, cavern heights were limited to 100–150 m. This is in part based upon the nature of halite beds in the Wessex Basin, where geophysical logs reveal that the insoluble content can comprise significant percentages of the Dorset Halite Member (DHM) [37] and are likely to significantly impact cavern volumes, stability, and location. This will likely limit areas of development to those with suitably clean halite for cavern construction. However, salt exploration boreholes for the construction of 14 gas storage caverns have proved a saliferous sequence 470 m thick in areas of the basin, with the main halite unit (referred to as 'S7') up to 140 m thick with a low insoluble content of ≈16.5% and in which it was

assessed that caverns of 100 m in height could be constructed for the purposes of gas storage [46]. Elsewhere, the Winterborne Kingston Borehole in the NE of the basin proved halite beds to be 190 m thick [37,48]. Thus, constraining cavern heights to between 100 and 150 m was thought to be realistic for the UK in general and the Wessex Basin in particular. As previously, a percentage of potentially available caverns and volumes, based upon filtering for 1% and the percentage of UGS caverns relative to the UGS cavern numbers, was also extracted for the two cavern height ranges.
