Carbon Sequestration in New Mexico’s Bravo Dome

The Science

The carbon dioxide (CO₂) at the Bravo Dome gas field in New Mexico is volcanic in origin, and its emplacement began more than a million years ago, not 10 thousand years ago, as previously estimated. Averaged across the reservoir, only 20% of the CO₂ has dissolved into the field’s saline brine over 1.2 million years, while the rest remains as a free gas trapped by the cap-rock, suggesting that safe long-term storage in geological sites is viable.

The Impact

This study documents the first field evidence for the safe long-term storage of very large amounts of CO₂ in saline aquifers. This illustrates the feasibility of geological CO₂ storage and informs policy makers on the requirements for carbon capture and storage technologies.

Summary

Carbon capture and geological storage is of critical importance to control the ever-increasing atmospheric CO₂ levels from fossil fuel-based energy production. The strategy is to capture the CO₂ at large point sources and sequester it into deep geological formations where it could dissolve into the saline brine and convert into minerals. Optimally, dissolution rates are studied in the field, because the spatial variability of geological reservoirs has a strong effect on the dissolution rates. It is, however, extremely hard to determine the CO₂ dissolution rate from short-term injection pilot projects. Scientists from the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center led by the University of Texas, Austin, exploited the Bravo Dome field as a natural laboratory to estimate the CO₂ dissolution rates on millennial timescales. This gas field is unique because decades of complementary industry and academic data are available. By combining these two existing data bases and adding their own analyses that determine the heating of the reservoir by volcanic CO₂, the researchers determined that the CO₂ emplacement at Bravo Dome began about 1.2 to 1.5 million years ago. To their surprise, the researchers found that only 20% of the 1.6 gigatons (a typical coal-fired power plant could emit ~10 to 15 megatons of CO₂ per year) of emplaced CO₂ have dissolved into the brine over the last million years. Observations indicate that half of the dissolution occurred during emplacement, a result that suggests that careful design of CO₂ injection can lead to significant dissolution early in the life of a storage project. The study also provides evidence that dissolution continued after emplacement. Numerical studies suggest continued dissolution is due to a process known as convective dissolution, which has not been observed in the field before. These results therefore provide important conceptual support for simulation studies used to predict the long-term evolution of geological CO₂ storage. Simulations generally predict fast convective dissolution of CO₂ within 10,000 years. The slow average dissolution rates inferred at Bravo Dome are likely due to geological heterogeneity and highlight the importance of capturing heterogeneity in the prediction of CO₂ dissolution rates.

Funding

Energy Frontier Research Center program, U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences

Publications

K.J. Sathaye, M.A. Hesse, M. Cassidy, D.F. Stockli, “Constraints on the magnitude and rate of CO₂ dissolution at Bravo Dome natural gas field.” Proceedings of the National Academy of Sciences 111, 15332-15337 (2014). [DOI: 10.1073/pnas.1406076111]