Estimating carbon dioxide residence time scales through noble gas and stable isotope diffusion profiles

The study of natural carbon dioxide reservoirs provides fundamental insight into processes involved in carbon capture and storage. However, the calculations of process rates such as dissolution of CO2 into formation water remain uncertain due to indirectly determined ages of the CO2 influx. The proposed ages for the Bravo Dome gas field in New Mexico, USA, vary from 56 ka to 1.5 Ma. Here we demonstrate that residence times can be estimated from simple modeling of noble gas and stable isotope diffusion profiles from the gas-water contact through the gas column. The Bravo Dome gas field shows a gradient in noble gas concentrations and isotopic ratios from east to west across the 70-km-wide field. A mantle-like end member with a He-3/He-4 (R/R-A) ratio of up to 4.7 is found in the west in contrast to a groundwater end member with high concentrations of air-and crustal-derived noble gases in the east. The air- and crustal-derived noble gases decrease gradually toward the west. Stable isotope compositions (C and O) also vary across the field. Diffusion modeling of He, Ne, Ar, Kr, Xe, and delta C-13 data yield residence times for the CO2 between 14.1 +/- 0.2 ka and 16.9 + 1.1/-0.5 ka. This is far less than the previous estimates of 1.2-1.5 Ma based on apatite ( U-Th)/He thermochronology, leading to a dissolution rate of 29,900 + 11,800/-10,700 t/a to 35,900 +/- 12,300 t/a, implying that 28% of the total emplaced CO2 dissolved. This new method can be applied to a wide variety of gas fields with variation in the concentration of groundwater-derived noble gases and allow a better assessment of the time scale of other diffusive fluid-fluid interactions.