Storing carbon dioxide (CO2) in deep geological formations is one part of the carbon capture and storage (CCS) process that is defined as geological CO2 sequestration or CO2 geo-sequestration. Injecting CO2 into a reservoir does not guarantee safe storage because CO2 could leak back to the surface and/or may contaminate specific strata where other energy, mineral and/or groundwater resources are present. Two mechanisms control assurance of storage integrity, which are geological leakage mechanism and wellbore leakage mechanism (Espie, 2005).

Wellbore element was presented as a basic unit for developing a model of an entire wellbore system. This paper is the second to assess the performance of a wellbore system for geological storage purposes. The analytical model adopts concept of sequence scenarios of possible leakage paths, concept of wellbore efficiency for storage purposes and concept of the reference state.

A new analytical model for modeling leakage through a wellbore system has been proposed. The analytical model represents the wellbore system as serially connected wellbore elements. This provides an advantage that each element can differ from the subsequent one and will have its own characteristics. Effective wellbore permeability also can change from element to another. The effect of site formation arrangement between general system and repeated aquitard-aquifers is investigated and compared. The analytical model is validated with the results of Nordbotten et al. (2004).

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