Overburden Analysis and Seal Integrity Study for CO2 Sequestration in the North Sea (OASIS)

The Norwegian government recently launched a full-scale CCS project (Longship) in the North Sea that will store CO2 captured from Norway and other European countries. One of the proposed CO2 storage sites is “Smeaheia,” located east of the Troll oil and gas field, northern North Sea. Another neighboring prospective storage site, “Aurora” is located south of the Troll field. With a history of oil and gas exploration and production, Norway possesses all the necessary skills and technology to establish and run sub-surface CO2 storage. The geologic evaluation of an area for CO2 storage warrants detailed characterization of the reservoir, seal and overburden rocks. In the case of a hydrocarbon trap, the oil/gas accumulation manifests the caprock and the overburden integrity. However, in geological CO2 storage, a careful investigation of the reservoir, seal, and overburden rocks can avoid leakage risk. The Smeaheia area is bounded by a fault array separating the Troll field in the west and the Basement Complex in the east. The upper Jurassic Sognefjord Formation sandstone is the target CO2 storage reservoir in the Smeaheia area capped by Heather and Draupne Formations shales. In the Aurora prospect, the Johansen and Cook Formation sandstones of Early Jurassic age around the Troll field are potential storage units. The reservoir is enveloped by Amundsen shale, whereas in the southeast, where the Amundsen shale pinches out, the Drake Formation shale directly overlies the Johansen and Cook Formations. The OASIS project deals with various aspects of the seal and overburden rocks above a potential CO2 reservoir. This study aims to evaluate the viability of the seal and overburden rocks using seismic, well-log, and geological data to make sure the injected CO2 remains safely in the underground forever. The work investigated the seals overlying the potential CO2 reservoirs in the area using various techniques. This information will be used at the later stage of the OASIS project to generate and calibrate a field-scale 3D geomechanical model. To assess the integrity of seal rocks, the brittleness and ductility of shales are essential. If a rock is brittle, there is a risk of developing fractures once the pressure increases while injecting CO2. Conversely, a ductile rock preserves its integrity under high pressure. In the Aurora area, the Amundsen and Drake Formation shales’ brittleness relatively increases with depth and temperature; however, our analysis showed a low possibility of fracturing. The Johansen Formation reservoir sandstone quality is improving towards the south at shallower depths within the Aurora area. According to the rock physics analysis, some calcium-rich, thin baffles are found. In the Johansen Formation reservoir sandstone, at deeper levels, the material binding the sand grains is quartz cement, which decreases where the clay/shale percentage is relatively high. The presence of cement might reduce the void spaces between the sand grains, resulting in a reduction of space for CO2 storage. Using a 3D seismic GN1101 in the Smeaheia area, we decomposed the full frequency spectrum to 20 Hz, 40Hz, and 60 Hz frequency cubes. Draping horizons from the Lower Jurassic Drake Formation up to the seafloor on the RGB blend of the three frequencies enabled identifying various sedimentary features within the window below the respective surface. We also generated a dip-steered “Similarity” cube to identify faults intersecting a formation surface and detect associated fractured areas. Based on the seismic data, formations below Quaternary dip towards the west to south-west with several faults associated with the north-south running Vette fault system close to the western edge of the area. The attributes indicate a sedimentary environment ranging from coastal, shallow marine to open marine within the analyzed interval. Weak fault traces are identifiable on the base Quaternary surface; furthermore, we identified several pockmarks on the Draupne Formation sealing layer and the seafloor. The discontinuities/ faults do not extend from the reservoir to the seafloor. However, one linear impression was observed on the seafloor on the low-frequency spectrum, indicating possible palaeo-leakage of hydrocarbon gas from the underlying strata. The risk of CO2 leakage through the pockmarks and the said fault needs further investigation, though these features are not present above the potential storage structures. The Amplitude Variation with Angle (AVA) technique yields valuable information about the reservoir fluids. The AVA models confirmed that the changes in CO2 saturation and conversion to gaseous state within the reservoir are possible to monitor remotely using a time-lapse seismic or 4D survey. We also extracted mechanical properties from the Smeaheia 3D seismic using pre-stack Inversion. These property cubes will be used for making a geomechanical model that will be calibrated using the information obtained by the methods mentioned above.