Subsurface Characterization of Acid Gas Injection Operations in the Provost Area

Injection of acid gas in the Provost area in southeastern Alberta takes place at six sites into four different stratigraphic intervals. Acid gas mixed with water is or was injected into Lower Mannville hydrocarbon reservoirs at Bellshill Lake and Hansman Lake. At Kelsey, Galahad, Thompson Lake and Provost-Keg River, dry acid gas is injected into aquifers of the Upper Devonian Wabamun and Leduc formations and into the Middle Devonian Keg River Formation, respectively. The oldest acid-gas injection site in the study area is Galahad, where injection commenced in 1994. The Hansman Lake operation was rescinded in 1996 after 15 months of sour water injection. By the end of 2004, approximately 350 kt of acid gas had been injected into deep geological formations in the Provost area.

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 Provost area is basically a safe operation with no potential for acid-gas migration to shallower strata, potable groundwater and the surface.

At Bellshill Lake and Hansman Lake, where sour water is or was injected into currently producing Lower Mannville reservoirs, the injected water will remain within the respective pool outlines and will partly be recycled in the injection/production cycle. In the cases of dry acid-gas injection, the acid-gas plume will likely be reduced by dissolution, dispersion, residual gas saturation and trapping along the migration pathway and, therefore, not reach the overlying aquifers. In the unlikely event of further migration, it would take 5,000 years before acid gas could be detected in the overlying Lower Mannville aquifer at Galahad and Thompson Lake, where acid gas is injected into the Leduc Formation.

Based on available data, it is unlikely there is 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 due to chemical reactions with formation brine and/or acid gas.