Work Package # 4

The objective of this work package is to investigate the migration of CO₂ in shallow aquifers, and its detection, in a field experiment. Liquid CO₂ will be injected into a sandy aquifer. After decompression a CO₂ gas phase will form and migrate upwards through the saturated zone as an analogue to a CO₂ gas bubble resulting from depressurization of escaped supercritical CO₂ fluid. The detection of the gaseous phase in the saturated zone will be assessed by using geophysical methods. Based on borehole information and geophysical mapping a detailed geological model will be established to characterize the subsurface and the hydraulic parameters at the injection site. This information will be shared with WP3. The "regional" 3D structures will be mapped using transient electromagnetic while a 3D resistivity monitoring array will be permanently installed for time lapse monitoring of resistivity changes in the subsurface associated with the injection of CO₂. The magneto resonance sounding technique will be applied to map the water content before the experiment. To enable a detailed 3D reconstruction of changes in resistivity and water saturation, a scheme facilitating joint time lapse inversion is implemented. The TOUGH code (WP2) will be used to establish a 3D numerical multi-phase flow model for the field site where the geological model and parameterization is incorporated which also will assist in the design of the CO₂ injection experiment. Simulations on stochastic realizations (Lee et al., 2007) of the hydraulic parameter distribution will be generated to quantify the uncertainty of the CO₂-migration pattern. Model simulations will subsequently be compared to the field data (geophysical mapping of the spatial and temporal variation of CO₂ saturations and measurements of CO₂ fluxes at the soil surface) in order to understand the CO₂ movement. The possibility of up-scaling will be examined with the purpose of extending the modelling approach to larger depths, e.g., near the CO₂ storage reservoir, where the density of data points will be low. Based on the experiments and the model simulations, general recommendations for monitoring the shallow groundwater systems for CO₂ leakage from CCS storages will be worked out.