Surface controls on the characteristics of natural CO 2 seeps: implications for engineered CO 2 stores

CO 2 injected into rock formations for deep geological storage must not leak to surface, since this would be economically and environmentally unfavourable, and could present a human health hazard. In I taly natural CO 2 degassing to the surface via seeps is widespread, providing an insight into the various styles of subsurface ‘plumbing’ as well as surface expression of CO 2 fluids. Here we investigate surface controls on the distribution of CO 2 seep characteristics (type, flux and temperature) using a large geographical and historical data set. When the locations of documented seeps are compared to a synthetic statistically random data set, we find that the nature of the CO 2 seeps is most strongly governed by the flow properties of the outcropping rocks, and local topography. Where low‐permeability rocks outcrop, numerous dry seeps occur and have a range of fluxes. Aqueous fluid flow will be limited in these low‐permeability rocks, and so relative permeability effects may enable preferential CO 2 flow. CO 2 vents typically occur along faults in rocks that are located above the water table or are low permeability. Diffuse seeps develop where CO 2 (laterally supplied by these faults) emerges from the vadose zone and where CO 2 degassing from groundwater follows a different flow path due to flow differences for water and CO 2 gas. Bubbling water seeps (characterized by water bubbling with CO 2 ) arise where CO 2 supply enters the phreatic zone or an aquifer. CO 2 ‐rich springs often emerge where valleys erode into CO 2 aquifers, and these are typically high flux seeps. Seep type is known to influence human health risk at CO 2 seeps in Italy, as well as the topography surrounding the seep which affects the rate of gas dispersion by wind. Identifying the physical controls on potential seep locations and seep type above engineered CO 2 storage operations is therefore crucial to targeted site monitoring strategy and risk assessment. The surface geology and topography above a CO 2 store must therefore be characterized in order to design the most effective monitoring strategy.