Subsurface Characterization of Acid-Gas Injection Operations in the Peace River Arch Area, 2007

Injection of acid gas in the Peace River Arch area occurs at 11 operations in 3 major stratigraphic units, the Upper Devonian Leduc Formation and Wabamun Group, the Permo-Mississippian Belloy and Kiskatinaw formations, and the Triassic Halfway Formation. By the end of 2006, approximately 340 million cubic metres of acid gas were injected into deep geological formations in the Peace River Arch area.

At Dunvegan, Eaglesham, Gordondale (Halfway), Mirage, Normandville, Parkland and Puskwaskua injection occurs or took place into depleted hydrocarbon reservoirs. By regulation, downhole pressures will not exceed the respective initial reservoir pressures, and the injected gas will remain within the respective pool outlines. At Mirage and Gordondale-Halfway, acid gas was injected into currently producing Halfway reservoirs, which will be partly recycled in the injection/production cycle. In the cases of acid-gas injection into deep saline aquifers (Gordondale-Belloy, Mulligan, Pouce Coupe, Rycroft and Wembley), the extent and migration of the acid-gas plume will likely be limited by dissolution, dispersion, residual gas saturation and trapping along the migration pathway and, therefore, not reach the overlying aquifers.

The entire stratigraphic interval from the Leduc Formation to the Halfway Formation is overlain by at least one contiguous thick shale sequence, the Smoky Group. Other aquitards (i.e., Wilrich, Fernie, Charlie Lake, Montney, Banff-Exshaw) form additional barriers to acid-gas migration from the various injection zones 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.

Tectonics greatly affected the sedimentary framework, particularly the Paleozoic succession, in the Peace River Arch area. The main tectonic activity occurred during the late Carboniferous, resulting in thick Mississippian to Permian sediment accumulation in the area of the Dawson Creek Graben Complex. Under the present-day stress regime, faults that are present in the various local-scale study areas do not appear to act as fluid conduits through aquitards overlying the respective injection horizons. However, the displacement of strata along faults that are present near injection wells results in a partial lateral confinement of acid-gas within injection intervals. Based on available data, it seems there is no potential for acid gas leakage through faults and 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 has degraded, or may degrade in the future, as a result of chemical reactions with formation brine and/or acid gas. The Peace River Arch area has a high well density and the wells penetrate hydrocarbon-bearing strata down to the Granite Wash overlying the Precambrian basement. Wells in the Peace River Arch area were drilled, and successively abandoned, as early as the late 1950s and, considering the old age, damage to or improper well completion is very likely in some cases. No leakage has been detected and reported to date; however, the potential for this occurring in the future should be considered by both operators and regulatory agencies.