Field test study on leakage monitoring at a geological CO₂ storage site using hydrogen as a tracer

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.