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Monitoring Stored CO2 to Document Permanence


 Injection of CO2 into porous rocks deep in the subsurface intrinsically has a low risk of leakage for three reasons: (1) Although CO2 is buoyant compared to brine, layered rocks limit vertical migration. Injection zones are chosen with at least one well-qualified confining system that should isolate CO2 within the selected interval. (2) Capillary trapping of CO2 in the pore system, dissolution of CO2 into formation fluids, and sorption and reaction of CO2 with organics and minerals will further attenuate CO2 lateral or vertical migration. (3) Wells are designed to isolate fluids in the deep subsurface from the overlying water and other resources; even a poorly constructed well will greatly retard migration of stored CO2.
 To add certainty to the planned storage, a monitoring program can be designed to document that the plume is migrating as predicted (and therefore that the planned-storage permanence is likely) and to assess any remaining uncertainties, for example, to further reduce well leakage or off-site migration risk.
 In an offshore setting, time-lapse seismic is the most preferred tool. A survey collected prior to injection provides the baseline survey. Injected CO2 displaces water and also increases pressure. During repeat surveys, the resulting changes in seismic velocity are measurable. Subtracting the repeat surveys from the pre-injection survey provides a high-quality image of the areas where CO2 has migrated.
 Other tools of high value are downhole-pressure measurements and wireline logs used to assess the zones at which CO2 is leaving the injection well(s). Gravity surveys are also used to assess the change in fluid density as CO2 is emplaced, and seismic sensors may be of value to assess microseismic response to injection. An available suite of environmental tools can be deployed to determine if any unexpected change occurs in the overburden, sediment, and water column. Advanced deployments such as fiberoptic cables and remotely operated sensor packages may have high value.
 Complexities of and limitations to monitoring must be critically assessed. The material impact of loss of CO2 from storage is forward modeled to determine if tools and techniques selected are sensitive to the changes. For example, if natural gas charge is present in the subsurface, seismic change from injection of CO2 is likely to be small and may be less than noise. Only by rigorous documentation that leakage is detectable can the expected outcome, high-quality storage, be documented. New work is being undertaken to develop optimized and commercializable monitoring for storage projects in the Gulf of Mexico.
 This work is related to a new multiyear project assessing the suitability of nearshore Gulf of Mexico for CO2 storage.