The paper describes a method for monitoring CO

2

leakage in geological carbon dioxide sequestration. A real time monitoring parameter

apparent leakage flux (ALF)

is presented to monitor abnormal CO

2

leakage

which can be calculated by atmospheric CO

2

and O

2

data. The computation shows that all ALF values are close to zero-line without the leakage. With a step change or linear perturbation of concentration to the initial CO

2

concentration data with no leakage

ALF will deviate from background line. Perturbation tests prove that ALF method is sensitive to linear perturbation but insensitive to step change of concentration. An improved method is proposed based on real time analysis of surplus CO

2

concentration in least square regression process

called apparent leakage flux from surplus analysis (ALFs)

which is sensitive to both step perturbation and linear perturbations of concentration. ALF is capable of detecting concentration increase when the leakage occurs while ALFs is useful in all periods of leakage. Both ALF and ALFs are potential approaches to monitor CO

2

leakage in geosequestration project.