Risk assessment of abandoned wells affected by CO₂

Flawless operation during injection and a long-term seal of the storage reservoir after injection are the prerequisites for successful CO₂ storage in the geological subsurface. This requires the sealing of the porous permeable horizons by a tight caprock and long-term integrity of open and abandoned wells which have penetrated these horizons. As a basic requirement for the inception of a CO₂ injection scheme in any sequestration project, the integrity of the existing wells in the field must be proven. This paper reviews the relevant items including the geology, relevant wells and their standards, well integrity, and integrity evaluation methods. At the end, a novel concept is introduced to evaluate well integrity for abandoned wells. It is concluded that the verification of technical well integrity is significantly more difficult for abandoned wells, because measurements and tests to close information gaps are no longer possible. If information is lacking for a direct assessment, the quantification of the risk of leakage is only possible indirectly. For an indirect assessment, a realistic risk assessment should consider the composite system casing-cement-rock as a whole and also include self-healing caused by CO₂ interaction with the composite system and/or the indigenous rock. The proposed new method covers both qualitative and quantitative analysis, and it is more comprehensive, sophisticated and easier to implement.