In this study, a new monitoring approach for detecting CO2 leakage is proposed that utilizes hydrogen gas as a tracer for CO2 geological storage. The gas leakage from a shallow formation is studied using 20-100-m deep boreholes at the Ito Natural Analog Site field testing facility in Fukuoka, Japan. Direct measurements of CO2 concentrations may yield unreliable results, particularly in summer when high levels of CO2 flux are produced from soil respiration. A gas mixture (CO2:H2/99:1) was injected through a well to the subsurface. The concentrations of gas emitted from the soil were measured in pipes where the gas was trapped; hydrogen was detected at 15 ppm immediately in less than 30 min after the mixed gas was released to the water-saturated zone. Repeated measurements were conducted in the area and elevated H2 levels of 15-65 ppm were recorded for 2 weeks. CO2 measurements in the pipes showed elevated levels at one monitoring point a day after the mixed gas was released. The field result was confirmed by laboratory experiments of the mixed gas, verifying that H2 is detected earlier than CO2. The elapsed time between H2 and CO2 after the mixed gas released was observed in this study suggesting that H2 has a potential as signal precursor for a future of CO2 arrival. However, further experiment should be conducted to demonstrate the applicability of H2 as a monitoring tool in CCS.
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