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The residence time of CO2 in some site of injection (aquifer, for example) depends mainly of the quality or impermeability of the rocks or even of the technical solutions that avoid the escape of this gas. Certainly vertical and lateral movements of CO2 will occur immediately after the injection. To avoid the leakage, i.e., transfer of CO2 to the atmosphere or ocean is fundamental the monitoring of underground CO2.
The efficacy of the CO2 storage is linked to the quantity of CO2 injected in some site. The control of the great quantity seems to be very difficult because the complexity and heterogeneity of the geological sites.
After the injection the CO2 will migrate laterally through porous and permeable formations or vertically, through fracture pathways. Evidently the situation expected is a mixed situation which the migrations will arrive lateral and vertically. Initially the CO2 will be conducted by floatability and by hydrodynamic from the injection site until to find a vertical pathway. In this case certain quantity of CO2 will continue its way toward to surface as a result of some progressive decompression and diminution of the temperature. Some other quantity of CO2 dissolved can leave of this vertical pathway in function of decreasing of temperature and pressure. The vertical migration of CO2 will arrive in short time varying from some years to ten years or less, contrary to lateral migration which the migration will arrive in more than thousand years (Bachu, 2002) ; (Celia & Bachu, 2002).
This work shows a summary of some techniques that can be used for monitoring of CO2 considering shallow site of injection (hundreds of meters). These are geophysical, gravimetric, sonar bathymetry, electrical, electromagnetic or even chemical. Some experiments about the new techniques are been realized in several parties of the world. Majority of them will be confined to laboratory scale. The application of these in basin or aquifer scale can be difficult and expensive. Some of the techniques to monitoring CO2 had been used by oil industry in the process of enhanced oil recovery. We will mainly focalize on seismic techniques.
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