Subsurface Characterization of the Edmonton-Area Acid-Gas Injection Operations

Injection of acid gas in the Edmonton region takes place at five sites into three different stratigraphic intervals. Acid gas dissolved in water in the Redwater oil field, and the resulting weak acidic solution ("sour" water) is injected into the depleted Leduc Formation Redwater reef trough 47 alternating wells. Dry acid gas is injected at four other sites. At Golden Spike and Watelet, the acid gas is injected into deep Devonian carbonate aquifers (Beaverhill Lake Group and Cooking Lake Formation, respectively). At Acheson and Strathfield, the acid gas is injected into depleted gas reservoirs in the Cretaceous Lower Mannville Group. Acheson, operating since 1990, is actually the oldest acid-gas injection operation in Canada and the world. By the end of 2003, more than 200 kt of acid gas have been injected into deep geological formations in the Edmonton region.

If only the natural setting is considered, including geology and flow of formation waters, the basin and local-scale hydrogeological analyses indicate that injecting acid gas into these deep geological units in the Edmonton region is basically a safe operation with no potential for acid-gas migration to shallower strata, potable groundwater and the surface. At Redwater, the acid gas is already dissolved in water and it will dissolve further in the formation water, with no possibility for migration or leakage, being contained in the Redwater reef. At Golden Spike, the acid gas injected into an isolated, confined reefal carbonate in the Beaverhill Lake Group is contained by the surrounding and overlying low-permeability argillaceous limestone at the top of the Beaverhill Lake Group. At Watelet, upward migration is impeded by the overlying thick and tight shales of the Ireton Formation. While the acid-gas plume may migrate updip, it will dissolve in formation water long before it may reach the sub-Cretaceous unconformity. Even there, thick overlying Cretaceous shales will impede any upward migration. At Strathfield and Acheson, the acid gas will be contained within the gas reservoir that is the respective injection target. Upward migration is not possible, as a result of thick and tight overlying shales of the Clearwater Formation and its equivalents. Lateral migration within the gas reservoir has been recorded in 2003 at Acheson, where, after 13 years of injection, CO2 has been detected at an offset producing well at 3625 m distance in the same gas pool. However, migration within the same unit, particularly in a gas reservoir, is expected and its occurrence should not come as a surprise.

The entire stratigraphic interval from the Beaverhill Lake Group to the Lower Mannville Group is overlain by thick shales of the Colorado Group and Lea Park Formation. There are many barriers to acid-gas migration from an injection zone into other strata, and the flow process, if it will ever happen, would take an extremely long time on a geological time scale. Any acid-gas plume would disperse and dissolve in formation water during flow on such large time and spatial scales.

There is no potential for acid-gas leakage through fractures. However, the possibility for upward leakage of acid gas exists along wells that were improperly completed and/or abandoned, or along wells whose cement and/or tubing have degraded or may degrade in the future as a result of chemical reactions with formation brine and/or acid gas. A review of the status and age of wells that penetrate the respective injection unit at each site shows that most wells were drilled in the 1950s and 1960s, and that the majority of wells are abandoned. Although no leakage has been detected and reported, the potential for this occurring in the future should be considered.