CONSTRAINTS ON THE MAGNITUDE AND RATE OF CO2 DISSOLUTION AND BRAVO DOME NATURAL GAS FIELD

The Bravo Dome natural CO₂ reservoir in northeastern New Mexico has been studied as a natural analog for the long-term fate of gaseous CO₂ in saline aquifer settings. The site contained 1.3±0.6Gt of gaseous CO₂ at the start of commercial extraction in 1981. Using variations in the CO₂/³He ratios of the gas measured in the reservoir, petrophysical measurements of gas saturation and porosity, and borehole logs for reservoir volume, we compute that only 22±7% of the CO₂ has dissolved since the gas entered the reservoir. 40% of this dissolution occurred in the residual brine in the CO₂ plume, with the rest of the CO₂ dissolved into the underlying brine or trapped in a mineral phase. To date the entry of the CO₂ , we use Apatite (U-Th)/He thermochronology. Previous noble gas isotope measurements show that the gas came from a magmatic source, so the heated gas may have caused 4 Apatite crystals located 17km from the gas source to be raised above the Apatite closure temperature of 75°C. Apatites from a location 34km were not reset by this event, and were measured to have thermal ages between 12Ma and 17Ma. Using permeability and porosity measurements from 3456 core samples, we compute that there is little potential for convective dissolution of the CO₂ gas due to the low permeability of the reservoir rock, an aeolian sand with fluvial channels. The low permeability layers in the reservoir are the likely cause of the long term persistence of the gas phase CO₂. Data from other saline aquifers shows that the potential for convection at Bravo Dome is typical for other US saline aquifers, limiting solubility trapping as a mechanism for CO₂ storage in these settings.