Improving characterisation of the Bunter Sandstone through history-matching at the depleted Esmond gas field (Flexible Funding 2024)

Dr Hayley Vosper, British Geological Survey, was awarded funding in the UKCCSRC’s Flexible Funding 2024 call to look at “Improving characterisation of the Bunter Sandstone through history-matching at the depleted Esmond gas field”.

The Bunter Sandstone Formation in the Southern North Sea (SNS) has been the subject of extensive study by the British Geological Survey (BGS) and the UK carbon capture and storage (CCS) community over many years.  Its importance for CCS on the UK continental shelf is demonstrated by the seven CO₂ storage licenses that have been issued by the NSTA. Abel at al. (2025) studied the structural boundaries of the formation and concluded that the Silverpit Basin (Figure 1) is surrounded by geological structures which are likely to be sealing. In support of this work they complied pressure data, sourced from the National Data Repository (NDR)[1] from across the Bunter Sandstone Formation in the SNS. It was apparent that these data have the potential for additional lines of enquiry, providing further insights into the behaviour of gases in the Bunter Sandstone Formation.

Figure 1. Location and depth of the top Bunter Sandstone model used in this study extending to a 30 km radius centred on the Esmond field (Bridger et al., 2021). The black outline shows the extent of the connected aquifer within in the Silverpit Basin, Bunter Sandstone Formation. Esmond (containing wells 43/13a-C3 and 43/13a-5), Forbes (containing well 43/08a-A3) and Gordon fields are shown with dashed lines. Contains information provided by the North Sea Transition Authority and/or other third parties

Gas was produced at the Esmond field (within the Silverpit basin, Bunter Sandstone) from 1985 to 1995. An additional well was drilled in 2008 to assess the potential of the field for gas storage. This provides a valuable insight into how much the pressure in the field has recovered since it was depleted. Back in 2017 Bentham et al. (2017) performed a history matching exercise to see what could be said about the properties of the Bunter Sandstone Formation, based on the gas production, pressure during production, and pressure after production. They concluded that the permeability was in the range 35-56 mD and the connected aquifer extended at least to an equivalent radius of 18.5 km.

The aim of this Flexible Funding 2024 project was to build upon the work of Bentham et al (2017) in light of the additional pressure data that has become available. A model was built based on seismic interpretation by Bridger et al (2021), populated primarily based on Noy et al. (2012) and simulations run using ECLIPSE 100 (Schlumberger).

Exploration well 43/12-1 was drilled in 1984, to test a structure 10 km south of the Esmond field. This, together with Esmond exploration well 43/13a-C1 at Esmond, provides a comprehensive set of initial conditions. From this we derived that the brine density at Esmond is around 1196 kg/m³ (Figure 2).

Figure 2. Pressure data corresponding to initial conditions pre-production at Esmond (43/13a-C3) and 10 km south (43/12-1). Pressure data from wells is shown as triangles (Source: NDR1). Model equilibrated initial pressure is shown as circles, density values are given in kg/m3. Contains information provided by the North Sea Transition Authority and/or other third parties.

Pressure data from the Esmond field, including the well from 2008 showed that a range of permeability values could be used, together with a range of volumes of connected aquifer, depending on whether the pressure at the Esmond field is still recovering or has already reached an equilibrium (Figure 3). The additional data which really helped to constrain the permeability was from 43/12-2, drilled 12.5 km east of Esmond in 1988. History-matching with this well showed that the permeability is likely to be in the range 25-40 mD, with the most likely being 30 mD. Given this permeability, pressure matching at Esmond suggests that the best solution is found when incorporating the entire pore volume of the Silverpit basin.

Going forwards, this study has provided some support for the hypothesis that the Silverpit basin acts as a single reservoir compartment. Also, confidence has been added to parameter values in this region of the Bunter Sandstone. A brine density value of 1196 kg/m³ is similar to ones that have been used for some time, e.g. by Noy et al. (2012). Permeability values of 25-40 mD were found to give the best fit to the available pressure data.  Previous modelling in the ALIGN-CCUS project shows that even with these lower permeability values, also observed in core in the Silverpit basin (excluding Endurance), significant volumes of CO₂ can be stored within this region Bunter Sandstone Formation due to the large, connected pore volume.

Figure 3. Pressure results at the top reservoir at the Esmond field over time. Black triangles show observed data, lines indicate results fitted in this study. K indicates horizontal permeability, given in mD and r indicates the radius of connected aquifer, given in km and 61 km corresponds to the equivalent pore volume of the entire Silverpit basin. Contains data from the NDR.

[1] https://www.nstauthority.co.uk/data-and-insights/data/uk-national-data-repository/

References

L. E. Abel, J. D. O. Williams, K. A. Wright, T. A. Randles, P. Bridger, H. A. Morris, T. J. H. Dodd, G. E. Plenderleith, and J. C. White. 2025. Structural controls on pressure communication across the Bunter Sandstone Formation, UK Southern North Sea.” Geoenergy

Bridger, P., White, J., Williams, J., Williams, G., & Hannis, S. (2021, October). Geological Characterization of the Hydraulically-Connected Bunter Sandstone Formation Saline Aquifer in the Southern North Sea. In 82nd EAGE Annual Conference & Exhibition (Vol. 2021, No. 1, pp. 1-5). European Association of Geoscientists & Engineers.

Bentham, M., Williams, G., Vosper, H., Chadwick, A., Williams, J., & Kirk, K. (2017). Using pressure recovery at a depleted gas field to understand saline aquifer connectivity. Energy Procedia, 114, 2906-2920.

Noy, D. J., Holloway, S., Chadwick, R. A., Williams, J. D. O., Hannis, S. A., & Lahann, R. W. (2012). Modelling large-scale carbon dioxide injection into the Bunter Sandstone in the UK Southern North Sea. International Journal of Greenhouse Gas Control, 9, 220-233.

Read more on Hayley’s Flexible Funding 2024 project page.