Helium enrichment during convective carbon dioxide dissolution

Motivated by observed variations of the CO₂/He ratios in natural carbon dioxide (CO₂) reservoirs, such as the Bravo Dome field in northeastern New Mexico, we have performed laboratory experiments equilibrating gas mixtures containing Helium (He) and CO₂ with water, at close to ambient conditions in a closed system. The experimental design allows for continuous measurement of headspace pressure as well as timed interval measurements of the CO₂/He ratios and the δ¹³C value of CO₂ in the headspace. Results from three dissolution experiments are reported: 1) pure Helium system, 2) 98% CO₂ + 2% Nitrogen system, and 3) 97% CO₂ and 3% Helium. Final equilibrated experimental results are compared to theoretical results obtained using Henry's Law relationships. The evolution of the amount of dissolved CO₂ computed from gas pressure and gas compositions are in good agreement with Henry's Law relationships. For example, the CO₂ + N₂ system was initially pressurized with pure CO₂ to 1323 mbar and after six days it equilibrated to a measured headspace pressure of 596 mbar. This compares very well with a calculated equilibrium headspace pressure of 592 mbar for this system. The CO₂ + He system was pressurized to 1398 mbar CO₂ and after six days equilibrated to a measured headspace pressure of 397 mbar. This measured pressure is slightly higher than the predicted equilibrated headspace pressure of 341 mbar, indicating a possible leak in the system during this particular experiment. In both experiments the initial pH of the water was 9.3 and the final equilibrated pH was 5.4. The δ¹³C value of equilibrated headspace CO₂ was within 0.25‰ of its starting δ¹³C value, demonstrating insignificant carbon isotope fractionation at low pH. Measured Helium/ CO₂ ratios throughout the CO₂+Helium experiment preserve a non-linear trend of increasing He/ CO₂ ratios through time that correlate very well with the measured pressure drop from CO₂ dissolution. This indicates that gas composition, in particular the He/ CO₂-ratio, can be used to infer the amount of dissolved CO₂ in the field where pressure evolution is not available. Our experiments show that the rate of dissolution is determined by convective mass transfer in the brine. Convective transport is driven by the increase of water density with increasing CO₂ saturation. However, unlike previous experiments with analog systems we do not observe a constant dissolution rate. This is due to the continued drop in gas pressure that continuously reduces the equilibrium aqueous CO₂ concentration and with it the driving force for convection. This feed back may significantly reduce the magnitude of solubility trapping that can be expected during geological CO₂ storage.