Because of the climate-warming effect of CO2 released to the atmosphere, measures should be taken to reduce, utilize or dispose of industrial CO2 emissions. For landlocked large sources of CO2, such as thermal power plants located in the interior of continents, one solution for reducing CO2 emissions into the atmosphere is its utilization is enhanced oil recovery processes (EOR), and disposal into deep sedimentary aquifers or depleted oil and gas reservoirs. Previous laboratory work by Gunter et al. (1993) has shown that it is possible to trap CO2 in sedimentary formations through geochemical reactions. Conceptual work by Bachu et al. (1994) has shown that CO2 can be hydrodynamically trapped in deep open aquifer systems for extremely long periods of time (up to millions of years) because of the slow velocity and long path of CO2 movement in the subsurface. This creates an alternative to using depleted oil or gas reservoirs which have limited capacity, or utilizing CO2 in EOR operations which only delay the release of CO2 into the atmosphere.
A number of coal-based thermal power plants with a total capacity of more than 4,000 MW are located near Lake Wabamun in central Alberta, Canada. A hydrogeological study of the sedimentary succession at this site in the Alberta Sedimentary Basin was undertaken in order to identify aquifers which meet various requirements for CO2 disposal, particularly with regard to depth and confinement. Two aquifers, the relatively thick carbonate Nisku aquifer (60 m thick) and the thin siliciclastic Glauconitic Sandstone aquifer (13 m thick), were selected (based on their properties and groundwater flow characteristics) for further investigation. Numerical modelling was used to study the capacity of these aquifers to accept large quantities of CO2 injected in the supercritical state and to retain this CO2 for long periods of time. The multi-phase, multi-component numerical model STARS. developed by the Computer Modelling Group (CMG) of Calgary, Alberta, Canada, was used to simulate the isothermal flow of injected CO2 and aquifer water in rocks of variable properties. The capacity of the system to receive and retain CO2 was examined for a whole series of parameters including aquifer depth and thickness, properties of host rock and water (I.e. porosity, permeability, salinity and temperature), and injection characteristics (e.g. injection pressure).
Even generally low-permeability (6 md) aquifers, which are common in the Alberta Basin, can accept and retain large quantities of injected CO2 for long periods of time, provided that near-well zones of high permeability (100-400 md) (termed "sweet" zones) are found, in order to attain high injection rates without reaching pressure limits imposed by rock-fracturing thresholds. The numerical simulations indicated that disposal of 2,000 t/d/well and 12,000 t/d/well of CO2 was possible for the thin siliciclastic Glauconitic Sandstone aquifer and the thick carbonate Nisku aquifer, respectively, under optimum conditions. The overall results show that injection of CO2 in the supercritical state into deep aquifers in sedimentary basins is viable and offers a short-to-medium term solution for reducing the emission of CO2 into the atmosphere.
Based on these simulations and a steady state, radial outflow well model, a correlation was developed for predicting CO2 injectivity into homogenous aquifers taking into account aquifer thickness, depth, permeability anisotropy and the physical properties of CO2. A chart was also prepared for prediction of CO2 injection rates into deep aquifers in locally high permeability "sweet" zones. The effect of a high permeability near-well zone or regional permeability and on the size and permeability of the sweet zone. Both these tools can be used to target aquifers for detailed evaluation for CO2 disposal and long term storage in other parts of the Alberta Sedimentary Basin or in other sedimentary basins of the world.