Physicochemical behavior and impact of CO2 and CH4 plumes during gas-rich water leakage in a shallow carbonate freshwater aquifer

Abstract

Carbon capture and storage (CCS) is a promising technology for reducing CO₂ emissions. Significant concerns have emerged about the potential leakage of CO₂ into shallow aquifers, highlighting the risk to water quality and environmental safety. This underscores the importance of finding monitoring tools suitable for different geological scenarios. If leakage occurs in the context of depleted reservoirs being used for CO₂ storage, residual CH₄ from the storage complex will likely be entrained together with CO₂. However, few studies have addressed the implications of CH₄ presence and its potential as a monitoring parameter during CO₂ leakage.

To address this gap, we simulated a leakage event by injecting water enriched with CO₂ and CH₄ into a shallow limestone aquifer. The impact of the injection was monitored using a combination of laboratory measurements on water samples and in-situ sensors located downstream from the injection well.

All parameters were affected by the simulated leakage. Some monitoring tools allowed us to differentiate the leakage event from natural variations. A key finding of this study was that at 7 m from the injection well, the CH₄ breakthrough occurred roughly one day before the CO₂ breakthrough, highlighting the potential of CH₄ as an early indicator of CO₂ leakage and suggesting interesting prospectives for industrial-scale sites. However, further research is needed to confirm the potential of CH₄ as a CO₂ leakage indicator at industrial scales, due to potential methane oxidation and loss of the signal with longer times and distances.

This study contributes to a better understanding of the potential risks and effective monitoring strategies associated with CO₂-CH₄ leakage in carbonate aquifers.