**4. Conclusion**

Myriads of techniques are currently in existence to detect and monitor CO2 interactions with water/brine, rock/soil and other gases as well as its migration through complex pore networks. These techniques utilise the physico-chemical and electromagnetic properties of the CO2 -water/brine and rock/soil system as well as the induced events such as micro-seismicity. However, prior to the full-scale deployment of the monitoring technologies, it is necessary to understand the principles of operations and limitations of the adopted technologies as well as obtain experimental and practical information from them. Some of them are suitable for deep geological layer while many are appropriate at the shallow aquifers.

Among the monitoring technologies, geophysical tools have gained more grounds in monitoring pilot sequestration projects across the globe. Techniques like seismic method, electrical resistivity tomography (ERT) offered good promise, especially at deeper levels in the scale of hundred metres to kilometres, while the likes of direct current geoelectric and ground penetration radar (GPR) are only good for monitoring at near-surface or shallow storage reservoirs. However, in order to effectively assess the potential effects of CO<sup>2</sup> leakage for any of the methods, a pre-injection baseline is critical followed by critical assessments during the storage process and post-injection period.

Among the challenges in the majority of the field applications are the accuracy in leakage quantification and the myriads of factors that can influence the outputs of the measurement techniques, making them non-unique. Accuracy in leakage quantification is often due to the offset in background natural variability and the detection limits of the techniques currently available. Factors that can influence the results of these technologies include pressure, temperature, initial salinity level, initial pH level, porosity, fluid properties, porosity, pore geometry and tortuosity, pore size distribution, wettability, reservoir mineralogy, geochemistry and surface chemistry.

Finally, it is encouraging that important instruments and tools for laboratory and shallow aquifer monitoring techniques are readily available and may be affordable by intended users. However, the cost of the deployment of full-scale monitoring technique for deep geological layer sequestration remains a challenge. Thus, focus should be on bringing down the cost by encouraging price competition among potential manufactures while governments should also make necessary fund available.
