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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.
Wellbore integrity is essential to ensuring long-term isolation of buoyant supercritical CO2 during geologic sequestration of CO2. In this report, we summarize recent progress in numerical simulations of cement-brine-CO2 interactions with respect to migration of CO2 outside of casing. Using typical values for the hydrologic properties of cement, caprock (shale) and reservoir materials, we show that the capillary properties of good quality cement will prevent flow of CO2 into and through cement. Rather, CO2, if present, is likely to be confined to the casing-cement or cement-formation interfaces. CO2 does react with the cement by diffusion from the interface into the cement, in which case it produces distinct carbonation fronts within the cement. This is consistent with observations of cement performance at the CO2-enhanced oil recovery SACROC Unit in West Texas (Carey et al. 2007). For poor quality cement, flow through cement may occur and would produce a pattern of uniform carbonation without reaction fronts. We also consider an alternative explanation for cement carbonation reactions as due to CO2 derived from caprock. We show that carbonation reactions in cement are limited to surficial reactions when CO2 pressure is low (< 10 bars) as might be expected in many caprock environments. For the case of caprock overlying natural CO2 reservoirs for millions of years, we consider Scherer and Huet's (2009) hypothesis of diffusive steady-state between CO2 in the reservoir and in the caprock. We find that in this case, the aqueous CO2 concentration would differ little from the reservoir and would be expected to produce carbonation reaction fronts in cements that are relatively uniform as a function of depth.