CO₂ storage resource estimates in unconventional reservoirs: insights from a pilot-sized storage site in Svalbard, Arctic Norway

Storage capacity is a key aspect when validating potential CO₂ sequestration sites. Most CO₂ storage projects, for obvious reasons, target conventional aquifers (e.g., saline aquifers, depleted hydrocarbon fields) with good reservoir properties and ample subsurface data. However, non-geological factors, such as proximity to the CO₂ source, may require storing CO₂ in geologically “less-than-ideal” sites. We here present a first-order CO₂ storage resource estimate of such an unconventional storage unit, a naturally fractured, compartmentalized and underpressured siliciclastic aquifer located at 670–1,000 m below Longyearbyen, Arctic Norway. Water injection tests confirm the injectivity of the reservoir. Capacity calculations, based on the US DOE guidelines for CO₂ storage resource estimation, were implemented in a stochastic volumetric workflow. All available data were used to specify input parameters and their probability distributions. The areal extent of the compartmentalized reservoir is poorly constrained, encouraging a scenario-based approach. Other high-impact parameters influencing storage resource estimates include CO₂ saturation, CO₂ density and the storage efficiency factor. The hydrodynamic effects of storing CO₂ in a compartmentalized aquifer are accounted for by calculating probable storage efficiency factors (0.04–0.79 %) in a fully closed system. The results are ultimately linked to the chosen scenario, with two orders of magnitude difference between scenarios. The fracture network contributes with up to 2 % to the final volumes. The derived workflow validates CO₂ storage sites based on initial feasibility assessments, and may be applied to aid decision making at other unconventional CO₂ storage sites with significant data uncertainty.