Experimental study addressing fault slip Implications for derisking of the Smeaheia potential CO2 storage site

Concerns about the impact of high levels of anthropogenic CO2 emissions in the atmosphere have led to the establishment of carbon emission limits of the 2015 Paris Agreement. Carbon capture and storage (CCS) has been considered an integral part of the lowest cost path to meet these climate change targets. The development of the large-scale Aurora CO2 storage site (North Sea), overseen by the Northern Lights project, has been greenlighted as of March 2021. The neighboring fault-bounded Smeaheia prospect offers excellent reservoir quality, scale-up potential and proximity to Aurora’s infrastructures. Injection of CO2 in Smeaheia is planned to occur in the sandstones of the Sognefjord formation, the main reservoir rock. The overlying shales of the Draupne formation will function as top seal, preventing the escape of the buoyant CO2. The regional Vette Fault Zone (VFZ) bounds the injection site to the East and displaces both reservoir and caprock formations. For Smeaheia to become a viable storage site, it is necessary to understand the VFZ sealing potential and reduce uncertainties concerning fault stability. Simplified models like the shale gouge ratio (SGR) are used to quantify the clay content within a fault and evaluate its sealing potential. Fault stability is often evaluated using the Mohr-Coulomb failure criteria, including friction and cohesion. Due to the inaccessibility of real-fault conditions in depth, these parameters can be estimated through experimental laboratory studies performed on analogue materials. Most of the current friction and cohesion data has been derived from triaxial tests which are adequate for intact samples from reservoir or caprock. However, there is limited data for slip along complex fault zones structures with a mixture of clay and sand, for which the direct shear test (DST) is a more appropriate methodology. In this thesis, two experimental direct shear methods, based on the SGR model, have been designed to address slip within the Vette Fault Zone, namely (1) within fault gouge and (2) along the interface between gouge and sandstone (sandpaper). The synthetic gouge tested consisted of binary homogenous mixes with varying ratio of crushed Draupne shale from the Ling Depression and Cuxhaven sand from offshore Germany, used as analogues to the Draupne and the Sognefjord formation respectively. The drainage results obtained from the consolidation of the gouge mixes showed a correlation to the SGR critical thresholds (15-20% and 40-50%), with very high permeability for SGR under 20%, transitional properties for 40%, and significantly lower permeability for SGR above 60%. The results of the direct shear tests performed on the synthetic gouge showed a systematic decrease in shear strength and friction angle with increase of crushed Draupne gouge content, with an average of 30º for pure sand and 20º for pure Draupne gouge. The interface tests revealed a dominant influence of the sandpaper on the shear strength and friction, yielding friction angles around 27º-30º. The friction of pure Draupne gouge was higher than the friction of fractures in the same type of shale, obtained in previous studies. In sum, this experimental study offers insights into the properties of slip along different elements of a fault core, contributing to the derisking of the Vette Fault Zone, a key structure of the Smeaheia potential CO2 storage site.