Fig. 4.

Monte Carlo runs of the three scenarios. The graphs show cumulative leakage of CO₂ as a percentage of the total injected. Each scenario shows the P50 output as a blue, green, or magenta line, with shading above showing the P05 limit, and below for the P95 limit, defining occurrence envelopes for the three scenarios. These are calculated on 12 Gt CO₂ injected between 2020–2050, with subsequent storage and leakage over 10,000 years. Black dotted lines show comparisons based on time averaged yearly constant leak rates for 0.1, 0.01, 0.001 and 0.0001% per year (the latter represented by the unlabelled dotted line at the base of the diagram). Offshore Well-Regulated storage (blue) is found to be the most reliable Scenario. Onshore Well-Regulated storage (green) exhibits higher leakage due to a higher density of abandoned wells acting as potential leakage pathways. Poorly-Regulated Onshore storage (magenta) exhibits the highest leakage rates in the short term due to the prevalence of unidentified abandoned wells that are unplugged or degraded