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Abstract
Geological storage of CO2 requires that policy makers, regulatory agencies, and industry ensure that these operations are safe for the public and that sequestered CO2 is permanently removed from the atmosphere. One possible risk identified with storing CO2 into the subsurface is the potential for leakage through existing wells penetrating the cap rock. This study evaluated the well integrity for existing wells in the Wabamun Area as part of a University of Calgary lead study which examined the feasibility of storing 20 Mt-CO2/year for 50 years within 60 km of Wabamun (WASP- Wabamun area CO2 Sequestration Project). A number of large stationary CO2 emitters are located in central Alberta with cumulative annual emissions in the order of 30 Mt CO2. A discrete approach was taken where each individual well was investigated and the need for any additional workovers were on the basis of a decision matrix developed based on current knowledge of well integrity for CO2 injection schemes. When analyzing the existing well population only 4 out of 27 wells were identified as work over candidates. This result demonstrates that well leakage from existing wells is less of a mitigation problem than was first anticipated for this area. For the existing wells only a few had production casing installed through the Nisku formation, which is a situation more prone towards leakage. The other wells had cement plugs through the cap rock with a cement type that will prevent leakage through the Calmar shale. For existing wells that do require work overs, it is recommended they be performed prior to pressurizing the reservoir area. The cost and complexity of properly abandoning these wells will be higher at higher pressures or when CO2 and possibly H2S are present.

Introduction
To successfully inject CO2 into the subsurface to mitigate green house gases CO2 has to be trapped in the subsurface and not leak to surface or to potable water sources above the injection horizon. Potential leakage can occur through several different mechanisms such as natural occurrences or along wells. To avoid leakage from injection wells the integrity of the well has to be maintained both during the injection period and after, as long as free (mobile or movable) CO2 exists in the injection horizon. In addition to injection wells, monitoring wells will most likely be required to monitor the plume movement and possible leakage. For instance the Environmental Protection Agency (EPA) in the United States has stated that their goal is to be able to account for 99% of the injected CO2 (NETL, 2009).

In addition to the new injection and monitoring wells saline aquifers, seen as attractive storage sites for CO2, are often located in areas with concurrent or past oil production and there is a large number of existing wells. In the province of Alberta there are already more than 350,000 wells and and annual drilling rate of approximately 15,000 wells (ERCB, 2009). The integrity of existing wells penetrating the capping formation to avoid leakage also needs to be addressed.