Experimental and numerical investigations of subsurface transport of gaseous CO₂

CO2 is the primary greenhouse gas generated by human activity. In response to this, the geoengineering method Carbon Capture and Storage, CCS, is being investigated as a mitigation method to reduce the release of CO2 to the atmosphere. The actual storage of CO2 will take place at more than 800 m depth in suitable reservoirs overlain by low permeable high entry pressure caprock, which is anticipated to prevent leakage. However, leakage is still a concern as leaking CO2 would migrate upward and end up in aquifers used for drinking water supply and/or return to the surface. Here the CO2-gas could constitute a threat to the environment and be a potential hazard to human health. The potential leakage pathways through the caprock will most likely occur through abandoned/poorly sealed wells or higher permeable faults zones. Present research indicates that leakage is unlikely when proper care is taken in site selection using detailed geological characterization and detailed design of the storage facility. Despite the low probability of leakage, there is still a public demand for exploring all the processes occurring as a result of leakage from a
storage site.
The focus of this PhD study has mainly been on investigating the migration of CO2 in shallow heterogeneous aquifers caused by a potential leakage from a CO2 storage site. To help enlighten the dominant processes and mechanisms controlling CO2 migration in the shallow subsurface, a combination of laboratory and field experiments were conducted. The results from these experiments were compared to modelling results obtained using a multiphase numerical code.
In the laboratory, complimentary one-dimensional column and two-dimensional tank experiments were performed by injecting gaseous CO2 at three different rates (0.01, 0.1, and 1.0 l/min) into constructed heterogeneous porous media. Soil moisture sensors were installed in the porous media to monitor the
movement of the gaseous phase. The results confirmed that larger-scale heterogeneity controls overall gaseous CO2 migration in porous media while processes at pore-scale control gas saturations. Monitoring the movement and concentrations of the gas-phase is difficult even in constructed heterogeneous media, and point measurements do not always capture the full dynamics of the system. The experimental data were analyzed using the numerical multiphase modelling code T2VOC. The numerical model provided good estimates of the general flow of the gaseous phase around larger heterogeneous features and satisfactorily estimated the total amount of gaseous CO2 in the test system. The numerical model was not able to adequately describe the processes at the pore scale such as dissolution of CO2 and unstable gaseous movement at low flow rates.
In the field, experiments were carried out to observe the migration of gaseous CO2 in a shallow aquifer at the Vrøgum site in Denmark. The field experiments involved injecting up to 45 kg of gaseous CO2 over two days into a relatively well-described geology. The injection well was located approximately 8 m below the groundwater table, and the studied aquifer consisted of finer Aeolian sand in the upper 6 m underlain by coarser glacial sand. The migration of the gaseous CO2 was tracked using cross-borehole ground penetrating radar, GPR. In total six GPR boreholes were installed around the injection well and downstream
of the dominant groundwater flow direction. The GPR measurements were collected before, during, and after the CO2-injection. The geophysical method proved to be very sensitive to detecting the gaseous CO2 in the saturated zone. The collected data documented that the distribution of the gas phase was highly
iv irregular and heterogeneous. Initially, the gaseous CO2 migrated slightly upward due to the buoyancy effect. Thereafter the gas moved laterally east of the injection well. However, as the injection continued the main flow direction of the gaseous CO2 shifted towards south and CO2 gas pockets with up to 0.3 gas
saturation formed below low permeable layers south of the injection point. Furthermore, the GPR measurements showed that the gaseous CO2 never penetrated the finer Aeolian sand at 6 m depth, and that gas saturation reached a steady-state condition within the survey area after less than 24 hours of CO2-
injection. The results of these experiments documented that even small variation in material properties can induce significant lateral spreading and are therefore of paramount importance to consider when attempting to predict leakage pathways from a CCS site.
We developed a simple model in a radial setup with a few controlling layers in the T2VOC code to simulate the overall behavior of the gas migration in the field experiments. Based on the available and fairly detailed geological information of the field site, we would expect the simulation to capture the vertical distribution
and the impact of the controlling layers. However, the initial simulations showed that based on the best estimates of the hydraulic parameters the model did not capture the observed migration. Thus calibration of the permeability and/or the entry pressure was necessary to produce simulations that were comparable
to the field observations. Unfortunately, in most cases it is impractical to characterize the geological settings at a CCS site in the details required for an accurate gas flow prediction. Furthermore, the exact location of the leakage from a storage site is most commonly unknown. Nevertheless, numerical models
can be used for scenario simulations and hereby help in risk assessments.