Measurements of 222 Rn, 220 Rn, and CO 2 Emissions in Natural CO 2 Fields in Wyoming: MVA Techniques for Determining Gas Transport and Caprock Integrity
An integrated field-laboratory program evaluated the use of radon and CO 2 flux measurements to constrain source and timescale of CO 2 fluxes in environments proximate to CO 2 storage reservoirs. By understanding the type and depth of the gas source, the integrity of a CO 2 storage reservoir can be assessed and monitored. The concept is based on correlations of radon and CO 2 fluxes observed in volcanic systems. This fundamental research is designed to advance the science of Monitoring, Verification, and Accounting (MVA) and to address the Carbon Storage Program goal of developing and validating technologies to ensure 99 percent storage performance. Graduate and undergraduate students conducted the research under the guidance of the Principal Investigators; in doing so they were provided with training opportunities in skills required for implementing and deploying CCS technologies. Although a final method or “tool” was not developed, significant progress was made. The field program identified issues with measuring radon in environments rich in CO 2 . Laboratory experiments determined a correction factor to apply to radon measurements made in CO 2 -bearing environments. The field program also identified issues with radon and CO 2 -flux measurements in soil gases at a natural CO 2 analog. A systematic survey of radon and CO 2 flux in soil gases at the LaBarge CO 2 Field in Southwest Wyoming indicates that measurements of 222 Rn (radon), 220 Rn (thoron), and CO 2 flux may not be a robust method for monitoring the integrity of a CO 2 storage reservoir. The field program was also not able to correlate radon and CO 2 flux in the CO 2 -charged springs of the Thermopolis hydrothermal system. However, this part of the program helped to motivate the aforementioned laboratory experiments that determined correction factors for measuring radon in CO 2 -rich environments. A graduate student earned a Master of Science degree for this part of the field program; she is currently employed with a geologic consulting company. Measurement of radon in springs has improved significantly since the field program first began; however, in situ measurement of 222 Rn and particularly 220 Rn in springs is problematic. Future refinements include simultaneous salinity measurements and systematic corrections, or adjustments to the partition coefficient as needed for more accurate radon concentration determination. A graduate student earned a Master of Science degree for this part of the field program; he is currently employed with a geologic consulting company. Both graduate students are poised to begin work in a CCS technology area. Laboratory experiments evaluated important process-level fundamentals that effect measurements of radon and CO 2 . Laboratory tests established that fine-grained source minerals yield higher radon emissivity compared to coarser-sized source minerals; subtleties in the dataset suggest that grain size alone is not fully representative of all the processes controlling the ability of radon to escape its mineral host. Emissivity for both 222 Rn and 220Rn increases linearly with temperature due to reaction of rocks with water, consistent with faster diffusion and enhanced mineral dissolution at higher temperatures. The presence of CO 2 changes the relative importance of the factors that control release of radon. Emissivity for both 222 Rn and 220 Rn in CO 2 -bearing experiments is greater at all temperatures compared to the experiments without CO 2 , but emissivity does not increase as a simple function of temperature. Governing processes may include a balance between enhanced dissolution versus carbonate mineral formation in CO 2 -rich waters.