Methodology Systems Analysis Framework

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The Systems Analysis framework that was applied to the long-term assessment for the IEA Weyburn Project is based on the well-established methodology that has been, and still is, used widely and successfully in the field of radioactive waste management and disposal (Cranwell et al., 1982; Goodwin, 1991; Stenhouse et al., 2001). The basis of this approach is to develop a transparent and systematic treatment of the 'system' being assessed, as well as generating documentation that is auditable, should the need arise.

One of the first steps in the methodology is to define what is meant by the 'System' to be assessed. Figure 2 provides a schematic diagram of the basic components of a CO2 storage system, viz:

• The CO2 storage reservoir situated at depth, in this case the Weyburn field, within the

• Geosphere, which consists of a number of geological and hydrogeological units above and below the reservoir (units are not shown explicitly in Figure 2); and the

• Biosphere, the name given to the surface, or near-surface environment, where potential environmental impacts are normally evaluated.

The arrows shown in Figure 2 represent potential CO2 migration pathways away from the reservoir. Two abandoned wells are also shown in this diagram, representing wellbores as potential (fast) pathways for reservoir CO2 to migrate to the surface or near-surface. One of the major tasks of the IEA Weyburn Project was the detailed geological and hydrogeological characterization of the region around the Weyburn field, so that the main features of the geosphere, principally those representing potential pathways or sinks for CO2, could be identified and incorporated in the migration modeling (see Section 3).

Having defined the 'System', the next stage of the process involved the identification of scenarios. Scenarios are defined as plausible and credible ways in which the Weyburn CO2 storage system might evolve over decades to hundreds to thousands of years. One scenario was central to the modeling effort in Phase 1 - the Base Scenario, defined as "the expected evolution of the Weyburn CO2 storage system". Table 1 provides a description of the elements that constitute the Base Scenario. Other (alternative) scenarios were also identified, though not discussed here. The documentation of scenarios occurs primarily through the listing of relevant features, events and processes (FEPs) - those factors that can affect the CO2 storage system.

BIOSPHERE

Potable Water Aquifei

Surface / Near-surfac« Environment

GEOSPHERE

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Figure 2. Schematic diagram of CO2 storage system (cf. the more realistic representation shown in Figure 3).

Table 1. Key elements of the Base Scenario for the Weyburn CO2 storage system._

Base Scenario . Abandoned wells: All wells within the

• Modeling domain: the Weyburn field (75

patterns) and the surrounding 10-km zone.

• Time frame: From the end of CO2 injection to

5,000 years, or at which time 50% of the CO2 in-place has migrated from the reservoir.

• Caprock: Based on existing information, the caprock integrity is not impaired; any fractures or discontinuities that exist are all

isolated or sealed (see also abandoned wells).

• Geosphere: A series of aquitards/aquifers exist above and below the reservoir horizon; these formations may contain fractures and fissures.

• The System Model takes into account physical trapping features that have contained the oil/gas in the reservoir.

• Geochemical processes - e.g., water chemistry changes, CO2 removal (by solubility and ionic trapping), precipitation of carbonate minerals - occur in the aqueous phase of all aquifers.

• The Biosphere exists from the deepest potable aquifer, i.e., extends to ~ 300 m below surface. It includes soil, surface water and the atmosphere as well as the flora and fauna that are found within these areas.

modeling domain are included and are assumed to have been abandoned according to current field abandonment procedures. This assumption includes wells that may have been abandoned previously according to different abandonment practices.

Well seals will slowly degrade after abandonment. These seals consist primarily of the cement used to fill the annulus between casing and borehole, cement and metal plugs used to fill the casing bore, and the cap welded onto the casing ~ 4 m below ground surface.

The metal casing will also degrade with time.

FEPs that address storage and migration of CO2 include hydrodynamics, buoyancy, geochemistry (precipitation / dissolution of minerals) and density-driven flow, dissolution of CO2 in water and residual oil, and pressure-temperature changes occurring in the geological formations.

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