Studies into the migration of gas injected at shallow depths: implications for CO2 geological sequestration

Some shallow gas-injection tests were carried out in different Italian geological scenarios. Tests sites are characterised by fractured volcano-sedimentary cover (Latera, Latium), fractured clay cover (Siena basin, Tuscany) and non-fractured clay cover (Dunarobba basin, Umbria). A gas mixture (helium, carbon dioxide and, in the Latera test, also argon) was injected about 20 m below the ground surface through an inclined borehole intercepting the fault plane, into the shallow aquifer. The gas-injection pressure was above hydrostatic plus capillary pressure, but below lithostatic pressure. Soil-gas, soil-exhalation flux and groundwater analyses were carried out to monitor gas breakthrough and behaviour. The experiment simulated gas rising naturally up a fault, where most of the geological boundary conditions were known. Results were different for the different gases, in terms of breakthrough time and location, depending upon the unique physical – chemical properties of each injected gas and the permeability of the sedimentary cover. The gas velocity and migration pattern (namely gas channelling along the fault) were consistent with theoretical models of gas migration. Gas–water partitioning and the evolution of soil gas and groundwater anomalies clearly reflected the conservative behaviour of He and the partial dissolution of CO2 into the aquifer. A general model of gas migration and behaviour is suggested which may have implications in the geological storage of CO2.