HYDRODYNAMIC TRAPPING OF INJECTED ACID GAS IN THE ALBERTA BASIN, WESTERN CANADA

摘要

Geological sequestration of CO_2 is an option for significantly reducing emissions into the atmosphere. Various operators in Alberta are currently injecting CO_2 and/or CO_2-H_2S-mixtures (acid gas) into deep formations, for acid gas disposal. In the currently 42 active acid gas injection operations in western Canada gas is injected either into depleted oil and gas reservoirs, or into deep sandstone or carbonate aquifers. The long-term containment of acid gas appears to be relatively certain in depleted reservoirs as indicated by the previous accumulation and trapping of hydrocarbons in these structures. Also, the reservoir capacity is well known from the volumes of the produced oil or gas. Conversely, aquifers represent migration pathways for fluids, and potential travel times of injected acid gas and injection capacity depend on the flow regime and aquifer permeability. Examples from acid-gas injection into a depleted reservoir and a regional aquifer in Devonian carbonates show that the assessment of both the local and regional hydrogeology is essential for predicting the long-term fate of injected acid gas in geological formations. In the Alberta Basin there are many hydrostratigraphic (thick, regionally extensive aquitards) and hydrodynamic barriers (low flow velocities of formation water) to acid gas migration, resulting in a long migration pathway from the injection zone into shallower strata and ultimately to the ground surface. Migration times in the order of thousands to millions of years would lead to the dissolution of acid gas in formation water and dispersion of any acid gas plume along the flow path. The experience with acid-gas injection in Alberta has shown that the process of CO_2-injection into deep geological media is technically feasible. However, the theoretical analysis of the long-term containment of the injected gas still needs to be confirmed by actually monitoring the movement of acid gas in the subsurface.