Challenges and opportunities of CO2 storage in depleted shallow gas reservoirs in Alberta Oilsands area, Western Canada Sedimentary Basin, Canada

Shallow (<900 m) depleted natural gas reservoirs in northeastern Alberta are not generally considered suitable CO₂ storage due to concerns of high leak risks and inefficient storage in low-density gas phase. Re-examination of the depleted gas reservoirs revealed that a) initial reservoir pressure is lower than regional hydrostatic pressure for most gas reservoirs in the region. A non-equilibrium state against inward pressure gradient over geological time is self indicative of effective containment of the natural gas in physical traps; b) under the same reservoir condition, the buoyancy of CO₂ is about 85 % of that from methane dominated natural gas, and the top seal is adequate for CO₂ storage in those gas reservoirs. If pore pressure is higher than the initial reservoir pressure, the buoyancy from further compressed CO₂ column becomes even less. Unless reaching fracture closure pressure or greater than breakthrough pressure, the leak risk of the top seal is low; c) our model suggests that the probability of leaking through injection induced fracture is low if we take 0.6 of the fracture closure pressure (FCP) as the maximum injection and optimal safe storage pressures; d) If the post-injection storage pressure is maintained at 0.6 of FCP, storage capacity in pore spaces from depleted and residual gas intervals alone reach 3036.5 million tonnes (Mt), about five times of the previously estimated 610 Mt. The storage capacity can be even greater if additional pore spaces from the associated sub-economic gas-bearing intervals are considered. The results from this study provide insights into the potential storage of CO₂ within shallow depleted gas reservoirs in the vicinity of Alberta oil sands operations.