1. Aquifers are Important Candidates for CO2 Disposal
Currently there is considerable interest in using geological structures (salt domes, brines, coal beds, oil and gas reservoirs, or aquifers), the deep ocean or enhanced oil recovery as sinks for disposal of CO2. In the short term, enhanced recovery projects can utilize CO2 if these projects are near the CO2 source. However in the longer term, deep aquifers appear to offer the best promise for accepting large quantities of waste gases. Their global distribution and their large disposal volume are appealing for disposal of waste fluids or gases, particularly from point emission sources. Carbon dioxide is an ideal candidate for aquifer disposal because of its high density and high solubility in water at the relatively high pressures which may be imposed in aquifers.
2. Aquifer Disposal Pipelines and Wells Small Part of Total CO2 Disposal Cost
Aquifer disposal of CO2 from a site in the Alberta (geologic) Basin containing over two-thirds of Alberta's coal-based electricity generation (i.e. Wabamun, Sundance, Keephills and Genesee power plants) was examined. A coal based power plant with 500 MW of net electricity output and a 30 year life (used as a base case) will generate close to 15,000 tons of CO2 a day. The CO2 would be injected as a liquid using a distribution system of 15 disposal wells, with each well accepting 1,000 tons per day. The analysis indicates that capture and compression facilities are the major disposal costs. The CO2 transfer pipelines and disposal and monitoring well system will have a capital cost in the order of $2,200 per tonne per day of disposal capacity. However, the aquifer disposal system represents only a small portion of the overall CO2 capture, purification and compression costs - capital and operating.
3. The Alberta Basin Aquifers Can Accept CO2 Generated by 500 MW Plant
Over a 30 year period, 0.16 Gt (i.e. gigatons) of CO2 would be produced from a 500 MW coal-fired power plant. Carbonate (i.e. Wabamun) and siliciclastic (Lower Mannville) aquifers were identified in this study area in the Alberta Basin capable of accepting well above this quantity of CO2. Modelling of injection into these aquifers concluded that disposal of 1000 tons/well of CO2 was obtainable.
4. The Best CO2 Traps are Mineral Traps
Concern about the stability of the long term capture of CO2 has been addressed. Conventional wisdom has restricted the disposal of CO2 to the subsurface where there are stratigraphic traps. In what we think is a novel approach, we advocate the consideration of mineral traps in aquifers. Modelling of geochemical reactions between CO2-charged water and aquifer mineralogy clearly identified siliciclastic aquifers as capable of trapping large quantities of CO2 by the precipitation of calcium-magnesium carbonate minerals. These conclusions are general and may be applied to any aquifer. These ideas were presented at the March 1993 IEA Carbon Dioxide Disposal Symposium at Oxford, England. The full context of the paper is contained in Appendix 11.5.2.
5. Aquifer Disposal should be Viewed from the Framework of a Sedimentary Basin
These mineral trapping reactions are kinetically very slow; but given the low regional flow rates (i.e. approximately 2 cm/year) and the large size of the Alberta Basin, the CO2 would be hydraulically topped for thousands of years; ample time for the mineral CO2 trapping reactions to take place. Thus CO2 disposal in aquifers should be focused on the regional flow in the siliciclastic aquifers in sedimentary basins not on sedimentary traps. The paradigm of mineral trapping, identified in this report, increases tremendously the potential volumes of the subsurface available for CO2 disposal.
6. More Detailed Studies of Aquifer Disposal is Warrant
This study has shown that the potential of subsurface CO2 capture is very large for certain types of aquifers in the Alberta Basin. This study's extensive mineral reaction work is to be strongly promoted, but further work on long term reaction kinetics is needed to more fully define the increased CO2 capture potential now indicated. Further geotechnical and geochemical work is essential to fully define the suitability of local aquifers and to select those most appropriate. As a first of a kind project, environmental control specifications, permitting, well design and system optimization will be extensive before an injection system can be put in place.