Subsurface fluid injection and induced seismicity in southeast Saskatchewan
Abstract In order to mitigate CO2 emissions while continuing to use fossil fuels as an energy source, CO2 emissions from large point sources such as power stations can be captured and stored in suitable subsurface sedimentary formations. However, concerns have been raised that the injection of pressurized CO2 may alter the subsurface stress state, leading to the re-activation of faults and generating induced seismic activity. Southeast Saskatchewan has seen extensive oil and gas activity since the 1950s. This activity includes, in recent years, a boom in shale oil production entailing hydraulic fracturing. It is also home to two world-leading CCS projects, the Weyburn-Midale CO2 Monitoring and Storage Project, and the Boundary Dam/Aquistore Project. The aim of this paper is to assess whether any of the conventional oilfield operations, shale oil activity or CCS has caused induced seismicity in southeast Saskatchewan. We find that the region has a very low rate of natural seismicity, and that there is no evidence to suggest that any kind of oilfield activity has caused induced events. However, seismicity has been associated with potash mining activities in the region. It is not clear whether the potash mining-induced events are triggered by subsidence above the mined zones, or by re-injection of waste brines. It is of interest to compare the situation in southeast Saskatchewan with other areas that have seen substantial increases in the amount of injection-induced seismic activity. It is notable that in many areas that have seen injection-induced seismicity, fluid injection is into basal aquifers that are hydraulically connected to the crystalline Precambrian basement. In contrast, most oilfield activities in southeast Saskatchewan are in Carboniferous formations, while the only units to have experienced a net volume increase are of Cretaceous age. It is tentatively suggested that the lack of induced seismic activity is due to the fact that injection is hydraulically isolated from the basement rocks, although it is also possible that stress conditions in the region are less conducive to induced seismicity.