CHEMICAL AND ISOTOPIC EVIDENCE FOR CO2 CHARGE AND MIGRATION WITHIN BRAVO DOME AND POTENTIAL CO2 LEAKAGE TO THE SOUTHWEST

Bravo Dome is a natural CO₂ reservoir in north-eastern New Mexico where CO₂ is produced for enhanced oil recovery. This study involves analyses of gas from ten producing gas wells, as well as one shallow effervescent water well, all of which are west of any previously published data, to help identify the source of the CO₂, the location of the charge into the reservoir, and migration within the reservoir.

Ranges of the data from these gas wells are consistent with published reports: CO₂ concentrations of 98.77 to 99.86 mol% CO₂, δ¹³C values of -6.75 to -3.7, and R/R_A values ranging from a high of 3.75 in the east to 0.67 to the west. These values are similar to reported values but with an opposite geographic trend. The R/R_A values reported here and in published reports appear to be directly correlated to ⁴He concentrations, which are highest along the margins of Bravo Dome.

The published reports of CO₂/³He ratios are highest in the west (4.82), and decrease to the east (2.25), and are used to interpret greater CO₂ dissolution to the east as CO₂ is more soluble than ³He. The CO₂/³He ratio in this study ranges from 0.48 in the west to 3.59 within Bravo Dome, which indicates that CO₂ dissolution within this sample set is instead increasing to the west. These opposite trends may validate a published hypothesis that the CO₂charge is located in the north-western portion of Bravo Dome, essentially between the wells of the two datasets. If that is the case, it is possible that the reservoir is filling both updip to the west and downdip to the east.

The gas from the shallow water well contained gas that was 99.24 mol% CO₂, with a δ¹³C of -4.06, and a R/R_A of 2.28, which indicates a similar source as the gas in Bravo Dome. The surficial sediments around the well are likely from the Santa Rosa Formation, which is productive within Bravo Dome, but that production is over 50 km north east of the effervescing well. The water well data indicate that gas similar to that of Bravo Dome leaks to the near surface. There is no reported evidence of leakage of CO₂ to the surface at Bravo Dome, which has been used as an indication that Bravo Dome is stable and can be used as an analogue for CO₂ storage projects. However, either gas from Bravo Dome is leaking to the southwest or there is a secondary migration pathway from the source that charges shallow strata southwest of Bravo Dome.