Monitoring CO2 gas-phase migration in a shallow sand aquifer using cross-borehole ground penetrating radar

摘要

Understanding potential pathways of gaseous CO2 into and through the shallow subsurface from deep geological storage is one of many requirements related to risk assessment of a carbon capture and storage (CCS) site. In this study, a series of field experiments were carried out at a site located in Vrogum in western Denmark. Up to 45 kg of gaseous CO2 was injected into a shallow aquifer approximately 8 m below the groundwater table. In the upper 6 m, the aquifer consisted of fine Aeolian sand underlain by coarser glacial sand. The migration of the gaseous CO2 was tracked using cross-borehole ground penetrating radar (GPR). A total of six GPR-boreholes were installed around the injection well and in the dominant flow direction of the groundwater. The GPR measurements were collected before, during, and after the CO2-injection. The GPR method proved to be very sensitive to desaturation of the aquifer when gaseous CO2 evolved and the method was thus useful for mapping the migration of the CO2 gas plume. The experimental results demonstrated that the migration of the gas plume was highly irregular. Initially, the gaseous CO2 migrated upwards due to buoyancy effects and subsequently it moved laterally and transversely to the groundwater flow direction. As the injection continued, the main flow direction of the gaseous CO2 shifted and CO2 gas pockets with a gas saturation of up to 0.3 formed below lower-permeable sand layers. CO2 gas was detected in a GPR-panel 5 m away from the injection point after 21 h. The GPR measurements showed that CO2 gas never penetrated the fine Aeolian sand at 6 m depth and that the gas saturation appeared to become constant in the survey area after less than 24 h of CO2 injection. The results of the experiments have emphasized that lateral spreading is of significance in case of leakage from a CCS site, and that even small changes in the formation texture can create barriers for the CO2 migration. (C) 2015 Elsevier Ltd. All rights reserved.