| Summary: | The increasing concentration of carbon dioxide (CO2) continues to be a global environmental issue, and climate models reveal that negative emissions are necessary to achieve optimal CO2 mitigation goals. Carbon capture and storage (CCS) is one way to achieve negative emissions, and therefore contribute to net sequestration of CO2. Snøhvit, a CO2 storage complex off the coast of Norway, has been sequestering carbon in geological saline aquifers under the seabed since 2008. The potential for leakage is one risk that must be considered in CCS. Simple climate models show that as little as 1% net leakage per year can produce net temperature increases within a century. Seismic chimneys, which are seismically detected vertical leakage pathways, contribute to leakage from storage reservoirs, and can develop due to overpressure of fluids in the storage complex. Sensitivity analyses were conducted to determine effects of select parameters on three metrics: increase in pressure, average CO2 saturation, and cumulative leakage as a percentage of the CO2 injected into a hypothetical storage complex. While increased well-chimney separation and increased CO2 density led to larger increases in pressure, increased chimney radius, aquifer thickness and aquifer permeability led to smaller increases in pressure in the injection aquifer. Cumulative leakage increased with increased chimney radius and CO2 density, but decreased with increased chimney-well separation. The same three metrics, when applied to the model representing the Snøhvit storage complex, provided a better understanding of potential leakage in the realistic system by the end of the century. While pressure returned to near pre-injection levels, or decreased, most of the CO2 remained in injection aquifer and the top aquifer at the end of the century, and leakage was found to be approximately 8.5%, which could therefore induce significant net temperature increases within the century.
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