Loss of CO2 Gas into Formation Water at the Natural CO2 Deposit of Bravo Dome, New Mexico, USA

Cassidy, Martin M.; Ballentine, Chris; Hesse, Marc

Bravo Dome field, in the NE corner of New Mexico, contains approximately 10 Trillion cubic feet of 98% pure CO2 in the Permian age Tubb sandstone formation on top of granite basement beneath a regional seal of the Cimmaron anhydrite formation at the Eastern edge of the Quaternary Raton-Clayton Volcanic field. The trap for the deposit is a lateral facies change to the NW of the Tubb sandstone draped over the SW plunging Sierra Grande uplift. The field is approximately 49 miles NW-SE and 50 miles NE-SW.

Evidence from distribution of noble gases in 14 wells spread across the field shows that within eight miles of the down dip gas water contact at the east of the field, CO2 has left the reservoir by dissolution in the underlying formation water. Noble gases have partitioned into the CO2 from the water. In the west the noble gases and their isotopes are in low concentrations and distinctive of the mantle. In wells to the east the noble gases increase in concentrations because they have entered the CO2 from the water. The gradient allowed calculation of mantle concentration of the gases, Ballentine, et al. 2005.

However the 3He increases because it is left behind as the CO2 dissolves in the water.

Maps of variation of each noble gas (except xenon, radon) will be presented, each showing an increase in concentration from west to east across the field. A surprise was that 3He doubles in concentration at the gas/water contact to the east, yet it is not carried in any quantity by the water below the field. The accumulation of 3He near the gas-water contact is therefore due to depletion of CO2 in the gas due to dissolution in the underlying water.

From the doubling of 3He concentration at the gas-water contact one can surmise half of the volume of the CO2 has been lost at the gas-water contact. More detailed calculations will be shown. The continued dissolution of the CO2 into the brine may be favored by the sinking of the dense CO2 saturated water away from the gas-water contact. The mass transport of CO2 away from the gas-water contact is due to natural convection and much larger than diffusive mass transfer and may explain the observed large 3He build-up.

In summary, it is highly probable that CO2 has been lost down dip in formation water, and this may well be a general situation in nature (Gilfillan et al., 2009) and in CO2 sequestration projects.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013