Geologic Sequestration Training and Research (GSTR) -
Recovery Act: Measurements of 222Rn, 220Rn, and CO2 Emissions in Natural CO2 Fields in Wyoming
Performer: University of Wyoming
Project No: FE0002112
NETL is partnering with the University of Wyoming (UW) to conduct a systematic survey of discrete radon (Rn) and CO2 flux measurements in soil gases at field sites in Wyoming where previous work has demonstrated geologic correspondence of moderate to high gamma background radiation (i.e., potential Rn degassing) with naturally-occurring CO2 (Figure 1). Natural CO2 analogues provide a means of understanding and predicting behavior in geologic storage reservoirs, particularly as test beds for investigating and improving technologies and protocols aimed at assessing the integrity of caprock formations. Radon is a noble gas and the only naturally occurring radioactive gas. It has two isotopes, 222Rn and 220Rn, both of which are relevant to this project. 222Rn is a short-lived decay product derived from the 238U (Uranium) decay series, with a half-life of 3.82 days. 220Rn is a decay product derived from the 232Th (Thorium) decay series and has an even shorter half-life 56 seconds) that makes it useful in identifying areas of very fast soil-gas transport. Elevated Rn emissions are strongly correlated with high CO2 emissions in volcanic systems; thus, 222Rn provides a means to identify deep CO2 flow in geologic storage projects, and map active, high porosity regions prone to CO2 movement. This proposed Rn-CO2 relationship will be tested to determine if discrete radon and CO2 fluxes could be used to indicate potential leakage in CCS projects.
Since chemical reactions affect the distribution of nuclides within the 238U and 232Th decay series (i.e., U-series), UW will replicate field conditions in a laboratory setting in order to evaluate the effects of mixed phase CO2-H2O-rock reactions on subsequent Rn degassing. Measuring CO2 and both isotopes of Rn from the same samples will constrain the relationship between Rn degassing and CO2 flux. The different half-lives provide important constraints on the source/depth of the Rn since 220Rn—because of its extremely short half-life (56 seconds)—will decay over long transport times. Because of its short half-life, 222Rn possesses the unique advantage of being used to estimate the timescale of CO2 migration. However, because the main source of the measured Rn (shallow soil degassing, deep reservoir degassing, or both) is undetermined, the nature and relevance of the temporal constraints from 222Rn remain uncertain.