Geomechanical and petrographic assessment of a CO2 storage site: Application to the Acorn CO2 Storage Site, offshore United Kingdom

Extraction or injection of fluids within the subsurface causes fluctuations of fluid pressures and thus stress conditions. It is paramount to have knowledge of the geomechanical strength of a system's lithologies, and the factors that control it, in order to maintain optimal conditions during extraction/injection. If the yield strengths of the reservoir or caprock are overcome, particularly in the near-wellbore region where stress is amplified, these fluctuations could potentially compromise the system, through compactional or dilatational failure. Here we have used a novel combination of methods to determine the geomechanical and petrographic properties of the reservoir and caprock lithologies to assess suitability of the proposed Acorn CO2 Storage Site, offshore north-east Scotland, for long-term injection and storage of CO2. The Acorn CO2 Storage Site has a highly porous and transmissible sandstone reservoir, with bulk mineralogy that will be stable under CO2-rich conditions, making it ideal for receiving at least 152 MT CO2 injected over similar to 20 years and storage of > 1000 years post-injection, as part of the ACT-Acorn Development Plan. However, due to the high porosity and low cementation of the sandstone reservoir, it has low yield strength and is vulnerable to disaggregation and porosity-reduction if injection rates are too high and stress/pressure conditions exceed their yield strength. The results presented here provide quantitative constraints on the porosity reduction expected should yield occur and place limits on CO2 injection rates. The shale caprock, with a high swelling clay content and very low permeability, present ideal Carbon Capture and Storage seal properties.