Rapid Large-scale Trapping of CO2 via Dissolution in US Natural CO2 Reservoirs

Naturally occurring CO2 reservoirs across the USA are critical natural analogues of long-term CO2 storage in the subsurface over geological timescales and provide valuable insights into the fate of CO2 in the subsurface (Fig. 1). Previous measurements of CO2 to 3He ratios within gas samples obtained from six natural CO2 reservoirs, located in the United States, show that the CO2 originated from magmatic degassing[1]. Variation in CO2/3He across each reservoir suggests that significant amounts of CO2, equivalent to hundreds of megatonnes, have been stored by solubility trapping[2]. However, the key question of whether CO2 dissolution occurred during emplacement, or by diffusion and convection over geological time remains unanswered.Here we present the results of integrating geochemical measurements with reservoir modelling to quantify both the mass of CO2 emplaced and the proportion dissolved within each of the six natural CO2 reservoirs. Given the magmatic origin of the CO2, we use the known age dates of associated igneous rocks to estimate the timing of CO2 emplacement in each reservoir. Using these emplacement ages, we show there is no relationship between the duration of CO2 storage and the proportion of solubility trapping that has occurred. This shows that the proportion of dissolved CO2 does not significantly increase over geological timescales. Further, we find that the original mass of CO2 does not influence the proportion of CO2 that is solubility trapped. We also find that rock properties, the present-day pressure, temperature and salinity of the reservoirs do not control the fraction of CO2 dissolved, suggesting that the circumstances of CO2 migration and filling are more critical. Our conclusion is supported by reservoir simulation at our exemplar site, Sheep Mountain in Colorado (Fig. 1), where we show that CO2 dissolution after structural trapping is a minor contribution to amount of CO2 residually trapped.Our findings support a model where the majority of solubility trapping occurs on CO2 injection and during the migration of the CO2 plume. This indicates that the proportion of solubility trapping after the CO2 has become structurally trapped is comparatively minor. Therefore, in engineered CO2 stores, considerable amounts of injected CO2 can be solubility trapped within CO2 injection and post-injection monitoring timescales.References[1] Gilfillan et al., 2008, Geochimica et Cosmochimica Acta, 72 (4), 1174-1198[2] Gilfillan et al., 2009, Nature, 458 (7238), 614-618Fig. 1 (a) Location map of the west and central United States, showing the location of the natural CO2 reservoirs investigated in this study. (b) Map of Four Corners area USA, showing the location of significant natural CO2 reservoirs, topographic elevation and Cenozoic-age igneous rocks. The CO2/3He ratio of samples from Bravo Dome, St Johns Dome, McElmo Dome, Sheep Mountain and McCallum Dome show a magmatic CO2 source.