Evaluation of the CO2 Sequestration Capacity in Alberta's Oil and Gas Reservoirs at Depletion and the Effect of Underlying Aquifers

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Abstract Geological sequestration of CO2 is an immediately available means of reducing CO2 emissions into the atmosphere from major point sources, such as thermal power plants and the petrochemical industry, and is particularly suited to landlocked Alberta. Trapping CO2 in depleted hydrocarbon reservoirs and through enhanced oil recovery (EOR) will likely be implemented first because the geological conditions are already well known and the infrastructure is partially in place. Assuming that the volume occupied by the produced oil and gas can be backfilled with CO2, the ultimate theoretical CO2 sequestration capacity in Alberta's gas reservoirs not associated with oil pools is estimated to be 11.35 Gt. The sequestration capacity in the gas cap of oil reservoirs is 865 Mt of CO2, but this additional capacity will become available sometime in the more distant future after both the oil and gas have been produced from these reservoirs. The theoretical ultimate sequestration capacity at depletion in oil pools in single drive and primary production is only 615 Mt of CO2. Depending on the strength of the underlying aquifer, water invasion has the effect of reducing the theoretical CO2 sequestration capacity of depleted reservoirs by 60% on average for oil pools and 28% on average for gas pools, if the reservoir is only allowed to be repressurized back to its initial pressure. Weak aquifers have no effect on reservoir CO2 sequestration capacity. If other factors are taken into account, such as reservoir heterogeneity and CO2 mobility and buoyancy, then the effective ultimate CO2 sequestration capacity at depletion in hydrocarbon reservoirs in Alberta is estimated to be 9,860 Mt for nonassociated gas pools and 242 Mt for oil reservoirs currently in single drive and primary production. However, most reservoirs have a relatively small CO2 sequestration capacity, rendering them largely uneconomic. In addition, shallow reservoirs are inefficient because of low CO2 density, while very deep reservoirs may be too costly because of the high cost of CO2 compression, and also inefficient in terms of the net CO2 sequestered. If only the largest reservoirs in the depth range of approximately 900 m to 3,500 m are considered, each with an ndividual capacity greater than 1 Mt CO2, then the number of reservoirs in Alberta suitable for CO2 sequestration in the shortto- medium term drops to 565 non-associated gas reservoirs and 22 oil reservoirs in single drive or primary production, with a practical CO2 sequestration capacity of 2,660 and 115 Mt of CO2, respectively. This practical capacity of Alberta's oil and gas reservoirs for CO2 sequestration may provide a sink for CO2 captured from major point sources that is estimated to last for a few decades. Introduction As a result of anthropogenic CO2 emissions, atmospheric concentrations of CO2, a greenhouse gas, have risen from pre-industrial levels of 280 ppm to the current level of more than 360 ppm, primarily as a consequence of fossil-fuel combustion for energy production. This has led to climate warming and weather changes.