Analytical and numerical approaches to understand the correlation between atmospheric pressure changes and fluctuating gas concentration in wells

To understand the natural signal of gas concentration in vadose zone wells, the real-time CO2 monitoring was conducted in the K-COSEM site before performing the artificial CO2 injection test. There are two different types of monitoring wells: I) Nested wells which are bundled pipes with different level depth and short screen bottom and II) a single well (full screened). The meteorological data such as wind speed and direction, precipitation, atmospheric and borehole pressures, temperature, and humidity were also collected near the ground surface. Unexpected large swing pattern of CO2 concentrations was detected and these patterns were sequentially changed depending on the depths (5, 8, 11, and 14 m) as increasing or decreasing CO2 gas concentrations. Because this natural phenomenon could provide a possibility of error in the interpretation of the source origins it is important task to investigate the relevant factors caused confusing the CO2 leaking signal from the storage. Using the atmospheric pressure data, the subsurface pressure (Pa) and the volumetric gas flux (l/m) were calculated analytically and numerically. The output data was comprised with fluctuating CO2 gas concentrations in wells. The results represented that depending on the variation and the magnitude between two different environmental pressures, the concentration of gas in wells could be changed. The additional experiments were also performed to verify the results of models. Based on the results, we proposed daily conceptual diagram in vadose zone.

Key words: soil gas, natural gas flow, CO2 gas monitoring, and vadose zone

Acknowledgement Financial support was provided by the "R&D Project on Environmental Management of Geologic CO2 Storage" from the KEITI (Project Number: 2018001810002)