Caprock complexity and storage assurance: Advanced findings on CO₂ containment in the Bunter Sandstone Formation

Abstract

The lower Triassic Bunter Sandstone Formation in the Southern North Sea is a promising site for CO₂ storage. This reservoir has a complex structure, particularly in the overlying Haisborough Group caprock, along with fractured shale interlayers, chimney like-structures and fault features. This study highlighted the importance of incorporating all of this complexity into a model of a ‘Bunter-like’ storage site.

Given the limited geological data on Bunter, four scenarios (Cases 1–4) were created to assess the impact of CO₂ storage on the integrity of a complex caprock structure. This study revealed that fracture reactivation within variegated shale interlayers differed when comparing scenarios with a multi-layered caprock or a single caprock. With a multi-layered caprock, the CO₂ plume was able to leak via the chimney region, whereas, with a single caprock, it did not. Furthermore, mineral dissolution/precipitation behaviour, particularly halite within the sub-layers of the shale interlayers, varied between the multi-layered and single caprock scenarios. Additionally, greater fault reactivation was observed in multi-layered caprocks compared to single caprock scenarios. With a multi-layered caprock, fault reactivation resulted in increased fault vertical and horizontal permeability than those observed in the single caprock scenario. The presence of a fault traversing several different layers of a multi-layer caprock led to different mineralogical behaviour than with a single caprock.

This study emphasized the critical importance of intricate feedback interactions in systems with complex seal and reservoir geologies, as these interactions are essential for governing the overall behaviour of plume migration.