Environmental and Process Monitoring

Abstract

For enhanced gas recovery (EGR) using CO₂ as well as for CO₂ storage in depleted gas fields it needs to be shown that injection and storage is save and neither population nor environment is exposed to risks during operation or afterwards. This requires the development and application of methods to monitor groundwater, vadose zone and atmosphere. Therefore, extensive investigations of the near-surface aquifers were performed to characterize the geological structure and the geochemical and hydraulic conditions as part of a baseline-monitoring and to specify input parameters for model simulations. If CO₂ leakage should occur and CO₂ migrates upwards from the storage complex, shallow freshwater aquifers are the first protected good that might be affected. Based on the model simulations, parameters that would be affected by leakages were specified and parameter changes as well as spatial extension of the expected changes quantified. A comparison of the model results with measured natural variabilities show that especially pH and TIC (total inorganic carbon), but under certain conditions also electric conductivity and aqueous calcium concentration (Ca) are most suited parameters for the detection of CO₂ leakages based on observation wells in shallow aquifers. It was an important result that the temporal fluctuations of groundwater composition are generally small but spatial variations are large.

The simulation results demonstrate that CO₂ gas and dissolved inorganic carbon should be expected and measured in the upper part of a confined aquifer. They also show that CO₂ gas will follow the steepest gradient of the overlying aquitard and accumulate in anticlinal structures, while the dissolved inorganic carbon migrates in groundwater flow direction.

Since the area that can be monitored by observation wells and also the extension of leakage plumes are limited, it is advised to additionally use further, e.g. geophysical monitoring techniques, which are able to cover wider areas. The tested methods ERT (electrical resistivity tomography) and especially SkyTEM (helicopter borne electromagnetic survey) can provide quite comprehensive information about the aquifer down to a depth of 300 m. This is to image the geological structure, which would have the strongest influence on the distribution of rising CO₂ gas and is therefore important for a site-specific monitoring concept. Repeated aero-electromagnetical measurements are so far the only method that could be used for an area-wide leakage monitoring on the relevant scale. All current laws and regulations demand an “adequate monitoring”, but it will be an iterative discussion in the future between operators, public, authorities and scientists to define what should be measured, how often and where.

In case of a large CO₂ leakage rate the gas penetrates the shallow aquifer structures and migrates into the vadose zone. Here the CO₂ can be measured before it discharges into the atmosphere. The CO₂ concentrations in the soil are temporarily variable due to microbial degradation of organic carbon. Soil gas monitoring can be a suitable and sensitive additional method to detect non-natural CO₂ concentrations in the vadose zone. The preconditions are the possibility to measure the CO₂ concentration significantly below microbiologically active soil zones but above varying ground or tail water levels. Both constraints have to be tested by preliminary site investigations. Because of strong local and seasonal variations individual baselines for at least 2 years are required for interpretation of the CO₂ concentration measurements.

CO₂ might leak through the geological formation into the atmosphere, but it can also be released due to accidents in the surface facilities. Only very few investigations exist how CO₂ spreads in the atmosphere and where hazardous concentrations are reached. Where CO₂ is decanted or handled the potential hazards of CO₂ release at the surface or around the facilities need to be considered. Calculations emphasize that the hazardous areas resulting from unwanted releases of CO₂ cannot simply be estimated. Specific hazards must be determined in an event- and location-specific manner. Nevertheless, in that way site-specific prognoses and thus the introduction of specific monitoring and intervention measures are possible. The simulations demonstrate that simple recommendations can improve safety, like turning the exhaust of the safety valves vertically upwards to enhance mixing in the atmosphere.

It is outlined here that many different aspects with regard to the monitoring of atmosphere, vadose zone and groundwater need to be taken into account with regard to EGR and CO₂ storage. In a future step a coherent monitoring concept needs to be defined now for a whole site on the demonstration scale. An intensive discussion between legal authorities and scientist will be required to harmonize what is technically feasible and what is socially and legally expected.