Impact of caprock complexity on carbon dioxide plume behaviour and storage security in the Bunter Sandstone Formation

Alsayah, Ali; Rigby, Sean P.

Abstract

Carbon capture and storage (CCS) is expected to play a vital role in achieving greenhouse gas reduction targets. The lower Triassic Bunter Sandstone Formation in the UK Southern North Sea is considered one of the most promising CO2 storage sites. The Bunter Sandstone reservoir is structurally complex, featuring shale inter-layers, fractures, and, in particular, vertical and horizontal variations in the structure of overlying caprocks within the potential storage zone. This study demonstrated the need to include all of this complexity in a model of a ‘Bunter-like’ storage site.
Due to the lack of detailed geological information on Bunter, four different plausible scenarios (Cases 1–4) were developed to evaluate the impact of CO2 storage on the integrity of the complex caprock structure. The study simulated the injection of 34.2 million tons of supercritical CO2 at a rate of 0.683 million tons per year over 50 years, followed by a 950-year shut-in period (no injection) to monitor the long-term behaviour and migration of the injected CO2.
The study findings indicate that the use of an oversimplified caprock model, which assumed only a single impermeable caprock layer and no CO2 leakage, would give rise to misleading conclusions about CO2 plume migration. When comparing CO2 plume migration between scenarios with either a multi-layered, variegated caprock, versus just a single caprock, it was found that 20 % of the injected CO2 leaked in the former, while no leakage was observed through the latter. Further, the presence of a chimney-like structure, within a multi-layered caprock, facilitated lateral CO2 plume movement due to advection forces, unlike with a single, uniform caprock.
In a scenario with both shale inter-layers in the reservoir and a chimney in the caprock, while, during the post-injection period, fracture re-activation was observed in the upper inter-layer near the chimney zone in both multi-layer and single caprocks, this occurred significantly earlier for the former compared to the latter. The presence of a chimney in the caprock led to significant localised downward CO2-rich brine fingering in the reservoir below, caused by gravitational instability and heterogeneity in petrophysical properties, due to leakage through the chimney.
Calcite minerals significantly influenced caprock porosity across all Cases studied, while halite changes within sub-layers varied between multi-layer and single caprocks. Over the 1000-year simulation period, most of the injected CO2 remained in the supercritical phase, followed by dissolved CO2, hysteresis trapping, and finally, mineralisation. The long-term spreading behaviour of the leaked fraction of the plume is very different for multi-layer, as opposed to single, caprocks.
This study demonstrated the importance of intricate feedback interactions, occurring in systems with complex seal and reservoir geologies, for controlling the overall plume migration behaviour.

Citation

Alsayah, A., & Rigby, S. P. (2025). Impact of caprock complexity on carbon dioxide plume behaviour and storage security in the Bunter Sandstone Formation. Geoenergy Science and Engineering, 254, Article 214011. https://doi.org/10.1016/j.geoen.2025.214011

• Journal Article Type: Article
• Acceptance Date: Jun 3, 2025
• Online Publication Date: Jun 4, 2025
• Publication Date: 2025-11
• Deposit Date: Jun 26, 2025
• Publicly Available Date: Jun 26, 2025
• Journal: Geoenergy Science and Engineering
• Print ISSN: 2949-8929
• Electronic ISSN: 2949-8910
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 254
• Article Number: 214011
• DOI: https://doi.org/10.1016/j.geoen.2025.214011
• Public URL: https://nottingham-repository.worktribe.com/output/50714766
• Publisher URL: https://www.sciencedirect.com/science/article/pii/S2949891025003690