Many different techniques are available for the CO2Carbon dioxide leakage(in CO2 storage) The escape of injected fluid from the storage formation to the atmosphere or water column monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions both within the wellboreThe physical hole that makes up the well, it can be cased, open, or a combination of both; open means open for fluid migration laterally between the wellbore and surrounding formations; cased means closing of the wellbore to avoid such migration and at the surface, around the wellhead (Clyne et al., 20112011 - H. Clyne, I. Phillips, A. JamesCO2 Monitoring: Taking the pulse of the CO2 storage reservoirs.see more, Plasynski et al., 20112011 - Sean I Plasynski, John T Litynski, Howard G McIlvried, Derek M Vikara, Rameshwar D SrivastavaThe critical role of monitoring, verification, and accounting for geologic carbon dioxide storage projectssee more). The monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions methods applicable to abandoned wells are basically limited to surface measurements because the abandoned wells commonly have their casings cut below the ground level and a cap welded on top of the cut casings. Therefore, monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions technologies involving wireline equipment and downhole tools are not applicable to abandoned wells unless these wells are re-entered and the mechanical plugs and cement plugs are drilled to provide access to the wellboreThe physical hole that makes up the well, it can be cased, open, or a combination of both; open means open for fluid migration laterally between the wellbore and surrounding formations; cased means closing of the wellbore to avoid such migration. Since the leakage(in CO2 storage) The escape of injected fluid from the storage formation to the atmosphere or water column of CO2Carbon dioxide from the abandoned wells can be confined to the wellboreThe physical hole that makes up the well, it can be cased, open, or a combination of both; open means open for fluid migration laterally between the wellbore and surrounding formations; cased means closing of the wellbore to avoid such migration or the gas can migrate outside the casingA pipe which is inserted to stabilise the borehole of a well after it is drilled, surfacing around the wellManmade hole drilled into the earth to produce liquids or gases, or to allow the injection of fluids, the known presence of wells leads to a special areal or spatial focus of layouts of surface monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions focussing on the vicinity of wells. Applicable monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions techniques for abandoned wells use direct and indirect methods for monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions CO2Carbon dioxide leakage(in CO2 storage) The escape of injected fluid from the storage formation to the atmosphere or water column. Direct methods measure CO2Carbon dioxide or tracerA chemical compound or isotope added in small quantities to trace flow patterns concentrations or flow rates to detect leakage(in CO2 storage) The escape of injected fluid from the storage formation to the atmosphere or water column, such as analysis of CO2Carbon dioxide concentrations in air or in soil/sediment gas. Indirect methods use measurements which indicate CO2Carbon dioxide leakage(in CO2 storage) The escape of injected fluid from the storage formation to the atmosphere or water column like ecosystem stress monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions, groundwater monitoringMeasurement and surveillance activities necessary for ensuring safe and reliable operation of a CGS project (storage integrity), and for estimating emission reductions etc. These techniques are described in more detail in Section 2.8.