Top seal assessment of Drake Formation shales for CO2 storage in the Horda Platform area, offshore Norway

Abstract

Evaluating top seal integrity is crucial for successful subsurface CO2 storage. Caprock shale geomechanical properties are complex and influenced by various parameters and processes. It is challenging to understand the role of various factors affecting the geomechanical properties; therefore, an integrated approach is required to evaluate top seal shales. In this study, we investigated the caprock properties of the Early Jurassic shaly Drake Formation overlying the reservoir sandstones of Early Jurassic Cook and Johansen formations. The study area is the potential CO2 storage site Aurora (the Longship CCS project), located in the Horda Platform area, offshore Norway. Based on lithological variations, the Drake Formation is subdivided into upper and lower Drake units. Variations of the geomechanical properties are investigated using wireline logs from 50 exploration wells, two 3D seismic cubes, and several 2D seismic lines. Elastic property-based brittleness indices of the Drake Formation caprock shales are evaluated to identify possible top seal quality. Moreover, seismic attributes and gas leakage scenarios are investigated qualitatively to assess the possibility of injected CO2 escaping from the reservoir. Low brittleness indices value of the Drake Formation shale near the Aurora injection site indicated that the seal rock might diffuse the injection-related stress change and act as an effective top seal. Based on the integrated qualitative assessment, it is likely that the Drake caprock shale will be acted as an effective top seal in and around the Aurora storage site. However, due to the complex nature of caprock shales, we recommend field-scale numerical simulation to evaluate the injection-induced stress-strain effect.