Deep Geological CO2 Storage: Principles Reviewed, and Prospecting for Bio-energy Disposal Sites

The principles of hydrocarbon exploration and production provide well-established and tested principles and technologies to investigate storage of fluids in the subsurface. CO₂ can be stored in the subsurface using settings of: (A) thick permeable coal seams; (B) depleted oil and gas fields; (C) saline aquifers of regional extent, with an overlying seal. The North Sea Sleipner project shows that CO₂ can be injected into the pore space of deep geological aquifers deeper than 800 m at 1 Mt/yr, using established technology. Suitable sediment sequences of saline aquifers exist in all hydrocarbon-producing areas, are volumetrically much larger than exploited oil and gas fields, and hold the potential to easily store all worldwide CO₂ emissions until 2050. Geological principles are established to assess entire continents for candidate sites of CO₂ storage. This shows that opportunity may be widespread, but needs more specific local investigations. Onshore sub-Saharan Africa is considered the most problematic region – but even here there are potentially viable sediment sequences. No demonstration projects currently exist for CO₂ capture and storage using small-scale onshore facilities. A simple estimate, assuming CO₂ value of $20 per ton, suggests that single boreholes onshore may be viable over 20 years with supply rates of 100,000 ton CO₂ per year. In principle, atmospheric CO₂ could be captured by cultivated biomass, and co-fired in existing power stations. Or energy crops could be grown, CO₂ to be used, and stored deep below ground, in a country distant from an original fossil-fuel CO₂ emission site.