More clearly than ever, scientists with the Center for Frontiers of Subsurface Energy Security (CFSES) have demonstrated that carbon dioxide can be contained deep underground for more than a million years. Geologic sequestration of carbon dioxide is one of the most promising options for controlling its concentration in the atmosphere, and therefore, its effect on global climate. This technology can only be effective if there is no leakage of carbon dioxide from the subsurface. Marc Hesse, a principal investigator at the center, and his colleagues turned to one of the few carbon storage sites that has been "in business" for more than a million years. Bravo Dome is a gas field in northeastern New Mexico and is an ideal natural analog for studying long-term behavior of carbon dioxide in the subsurface, with an added bonus of numerous academic and industrial data sets available to constrain the behavior of carbon dioxide.
Bravo Dome natural gas field covers an area of 3,600 km2, the size of about 672,000 football fields, and was used for the commercial production of carbon dioxide since the 1980s.
The original gas reserves were estimated at 1.3 gigatons before the beginning of production. The gas is exceptionally pure, 99.8 percent carbon dioxide, with just a small amount of impurity gases. The isotopic signature of one of the impurity gases—helium (3He isotope)—indicates the volcanic origin of this gas reservoir. Compared to carbon dioxide, helium has a lower solubility in groundwater and therefore the ratio of carbon dioxide/3He can be used to estimate the amount of carbon dioxide lost because of its dissolution into the groundwater. The researchers used the changes in concentration of this isotope together with a large petrophysical data set to determine that 366 megatons of carbon dioxide have been lost from the gas phase since its emplacement.
After finding out “how much” gas has dissolved, another important question arises: “How long did it take?”
Various estimates in the literature placed the age of the Bravo Dome at only 10,000 years, based on a very old radiocarbon date of an early human campfire that was thought to be indicative of the age of volcanism in the area. More recent direct dating of the volcanic rocks in the area shows that volcanism occurred from 56 thousand to 1.7 million years ago. That’s a huge range if one is trying to estimate a dissolution rate!
To estimate how fast carbon dioxide dissolves into groundwater, researchers need to know precisely at which point in time this large carbon dioxide plume entered the reservoir. The authors found an elegant way to estimate this age. They analyzed the buildup of radioactive decay products in apatite, an abundant mineral in the reservoir. Although these minerals are in the 250-million-year-old rock and should be at least as old as the rock itself, they contained only 1.2 to 1.5 million years of decay products. This indicates a thermal perturbation of the reservoir related to the entrance of hot volcanic carbon dioxide that released previously accumulated decay products and hence re-set the radiometric clock, which dates the age of the reservoir.
Therefore, the researchers were able to discern that significant dissolution into the groundwater took place during the emplacement, when the gas plume was actively moving through the subsurface, and the rate of dissolution declined as carbon dioxide formed a stagnant subsurface pool.
A huge volume of carbon dioxide—1.3 gigatons—has been contained in the subsurface for more than 1 million years. This is three orders of magnitude larger than the longest-running geologic carbon dioxide storage site, the Sleipner field in the North Sea, where about 17 megatons of carbon dioxide have been stored since 1996.
Further research will continue to lead to a better understanding of the physical and chemical phenomena associated with the geologic carbon storage.
