Cross-well multilevel pumping tests – A novel approach for characterizing the changes of hydraulic properties during gas storage in shallow aquifers

Gas leakage may occur in pipelines or subsurface storage sites. The appearance of a new gas phase in the geological subsurface can impede groundwater flow and increase total media compressibility, with both effects increasing with larger gas phase volumes. In near-surface aquifers, these changes can be assessed by determining changes in hydraulic conductivity and specific storage using conventional pumping tests. These allow an observer to detect and delineate the occurrence of a new gas phase. In this study, this idea was tested using methane gas and hydrogen gas injection experiments, which were conducted at the TestUM field site in Germany. A series of multi-level cross-well and depth-oriented pumping tests were conducted before and after injection of the hydrogen and methane gas phases. The results show a significant decrease of hydraulic conductivity and a significant increase of specific storage estimated from the drawdown data due to the presence of the gas phases. Hydraulic conductivity and specific storage were found to vary by up to 86% and 65,900% respectively after gas phase injection, while repeated baseline tests yielded a variation of less than 28% and 340% prior to gas phase injection. These changes are correlated, and their occurrence and magnitude can be used to roughly locate the gas phases. As only standard hydrogeological field equipment and evaluation techniques are required, this approach represents a new monitoring option to detect an intruding gas phase in near-surface aquifers.