9.2. CO2 monitoring
The key feature for geological storage is that CO2 will be retained for extremely long periods, of the order of magnitude of 103 year, without any appreciable seepage back to the surface. Models show that a leakage rate above 0.1% per year will invalidate the effectiveness of CCS in global warming control [185]. Moreover, migration of the injected CO2 inside the storage volume should be monitored to assess that it will not interfere with the surrounding environment and in particular with the groundwater.
The monitoring strategy includes pre-injection, during injection and post-injection phases utilizing a suite of techniques aimed to assure the integrity of the reservoir, the absence of leakages, the quantification of the volumes of the stored CO2 and the identification of the geometry of the injected plume of CO2. Monitoring is also a key to verify the project׳s aims, including its predicted performance and long term containment.
The variety of monitoring techniques can be grouped into several families, each one having its range of application in function of the data to be acquired and of the environmental condition of the storage area, as shown in Table 9.
Seismic monitoring: Both active and passive systems can be employed. For active seismic, an energy source is used to generate acoustic waves, which will be detected and interpreted to gain information about the underground geology of the storage area; while in passive seismic, the tremors and micro-earthquakes generated by the movement of fluids or by the formation of fractures are recorded by geophones. When used during the pre-injection phase these methods are aimed to identify the characteristics of the storage area and its structural integrity. During the injection and post-injection, seismic is applied to the monitoring of the evolution of the CO2 plume. 3D seismic generates a tri-dimensional image of the underground structures including the dimension of the injected plume of CO2; time lapse or 4D monitoring is used to track the evolution through the time of the CO2 plume [186] and [187]. High quality 3D is able to identify CO2bodies of mass above 106 kg at depths of 1–2 km with optimal results in off-shore monitoring where the presence of water as medium enhances the penetration of the seismic waves [187].
Geoelectrical methods: These are based on the variation of resistivity caused by the presence of CO2. When CO2 displaces fluids with higher conductivity, i.e. brines, the induced variation in resistivity can be measured giving information about the grade of CO2 saturation of the reservoir and the spatial distribution of the injected plume. The bigger the difference in conductivity between CO2 and displaced fluids, the stronger is the signal. Once CO2 is dissolved in water the difference in resistivity will drop below appreciable values, and therefore, this method is only valid for monitoring free CO2 before dissolution[188].
Temperature logs: A range of thermal processes are involved in CO2 injection (i.e. Joule–Thomson cooling, advective heat transfer, heat transport) causing changes in temperature around the CO2 plume. Monitoring the variation in temperature can help in identifying the flow paths inside the reservoir. For more reliable results mathematical models can be developed based on the geology of the storage area, the volumes of injected CO2 and its interaction with the surrounding fluids [189].
Gravimetry methods: Changes in underground density due to the injection of CO2 can be detected by small perturbation in the local gravitational field; a loss in density is observed when CO2 displaces denser brine inside the reservoir. Monitoring these changes gives information on the diffusion rate of CO2. Limits are due to the distance between the gravimetric meters and the plume. The shape of the plume also affects the results, with vertically elongated plumes generating a stronger signal than flat spread ones [187].
Remote sensing: The injection of large volumes of fluids in the reservoir, mostly when the hydraulic conductivity is not very high, can generate a certain degree of overpressure leading to deformation of the surface that can be detected by Interferometric Synthetic Aperture Radar (InSAR) airborne or satellite monitoring. This method is based on the use of synthetic aperture radar to map the surface of the storage area through the time identifying the displacements. The injection of 3 Mt CO2 in the In Salah Gas Field (Algeria) caused a lifting of 5 mm/y which was detected by InSAR [190].
Geochemical sampling: It is possible to collect samples of fluids from boreholes inside the storage area and observe the chemical variation induced by the injection of CO2. The most evident effect is a drop in pH and changes in the concentration of minerals, such as carbonates and some silicates, due to the acidification. Measuring the pH drop in groundwater allows the identification of CO2 leakages of the order of 103 t/year [191]. Dissolved gas analysis is also a reliable tool for the quantification of the presence of CO2in the formation fluids and to track the migration of the CO2 plume [192].
Atmospheric monitoring: CO2 could seep from the reservoir and reach the surface, leaking into the atmosphere. Monitoring the atmospheric concentration of CO2 in the storage area can be used to identify anomalies above the natural baseline. Large natural variation in CO2 values due to soil respiration, organic matter decomposition or peculiar climatic condition may affect the reliability of these techniques [144].
Tracers: Co-injection of specific compounds together with CO2 can generate a specific “fingerprint” of the stored CO2. These tracers can be detected even in very small concentration (ppm) allowing an identification of any seepage from the reservoir. SF6 and CH4 have been used as tracers in the storage of CO2 inside a depleted natural gas field and their presence was identified in samples collected from a monitoring well 700 m from the injection point about 150 days after the beginning of the injection, thus giving an estimate of the diffusivity of the CO2 inside the reservoir [191].
Soil gas: Monitoring the composition of the soil gas, and in particular the concentration of CO2, before the injection defines the baseline. Time lapse monitoring can be used during the injection and post-injection phases to assure the absence of CO2 seepage [192].
Microbiology: Samples of fluids and sediments can be collected before the injection for a baseline on biocenosis to be compared with the modification induced by the presence of CO2. Biological analysis is useful to identify biogeochemical processes which can affect the diffusion of CO2 within the reservoir [193].