Introduction

CO2 Capture and Storage (CCS) from large point source emissions has tended to concentrate on depleted oil and gas reservoirs and deep saline basins. Reservoir conditions are favourable for large scale injection and retention of CO2, however these repositories may not necessarily be in close proximity to emission sources. There are countries, notably South Africa and India, which are heavily reliant on coal-fired power generation where oil and gas fields, and deep saline aquifers, are hundreds of kilometres from CO2 sources. The viability of alternative geological options, with low matrix permeability, is therefore warranted especially if alternative reservoirs are located in close proximity to point sources of CO2.

This review has summarised a detailed assessment of South Africa’s geological storage potential which is the most comprehensive for a country with both conventional and alternative (low permeability) options. The relevance of the experimental Longyearbyen site has been reviewed partly because of its relevance to South Africa. Brief consideration of India and South Korea has also been included because they too face the dilemma of limited, or unproven, alternatives to large scale conventional reservoirs.

Conclusions

Locating suitable sites for CO2 sequestration in low porosity / permeability formations presents a series of challenges:

• The identification and evaluation of suitable sites for CO2 storage will need to be detailed and focused on sites with potentially limited capacity by comparison with depleted oil and gas reservoirs and deep saline aquifers.

• Detailed site characterisation could be challenging and expensive to ensure accurate quantification of capacity and compliance requirements by regulators.

• To store large quantities of CO2 several different sites may need to be identified in one area.

• The economic viability of formations with low porosity / permeability properties, or restricted lateral homogeneity, needs to be determined. Research suggests that in comparison with sites that have good characteristics (high porosity / permeability) low porosity / permeability conditions require a higher well concentration and injection pressure leading to disproportionately higher development costs. This deduction needs to be treated with some caution because it is based on one study which assumed a permeability differential of an order of magnitude.

• Geophysical techniques will need to be tested or developed to improve resolution beneath sills. Without adequate interpretation it will not be possible to accurately delineate site characteristics.

• There is some experimental evidence that suggests clay minerals, and especially kerogen within organic rich shale formations, could adsorb injected CO2. The retention capacity of organic rich shales depends on thermal maturity as well as the kerogen type. A major limitation of this lithology is its low permeability. CO2 injection would require high pressure injection, fracturing and low rates that might be impracticable.

• The use of CO2 for ECBM is potentially viable but more research, including field test evaluation, is necessary to monitor and quantify CO2 retention in coal seams. The relatively rapid adsorption of CO2 induces swelling in coal adjacent to injection points causing reservoir pressures to increase. Pressure drops can then occur as CO2 displaces CH4. The rate of this transition is likely to restrict the rate of CO2 injection. The effects of free hydrocarbon mobilisation, caused by injection of supercritical CO2 at field scale, also need to be understood.

• Extruded basalt is another low permeable lithology which has the potential for CO2 storage. Its ferro-magnesium mineral composition reacts to form carbonate minerals causing effective mineral trapping.

• There is only limited experimental data on CO2 diffusion and adsorption in low permeability lithologies. Initial conclusions suggest CO2 could slowly diffuse through argillaceous caprocks over periods of thousands of years.

• The most promising reservoirs with low matrix permeability have significant fracture zones which do not compromise the caprock. Dolerite intrusions can induce fracture propagation but need to be differentiated to delineate a fracture dominated reservoir.

• The technical and economic constraints of formations with low matrix permeability may preclude them from CO2 storage. Depleted oil and gas fields, and deep saline aquifers, may still be the only viable options especially for countries like South Africa and India which have limited, or unproven, geological conditions in close proximity to large point sources of carbon. Thorough research is needed to determine whether onshore and unproven reservoirs are technically and economically viable by comparison with offshore deep saline aquifers and depleted oil and gas reservoirs. Geological conditions in some countries, for example South Korea, suggest limited onshore CO2 storage but much larger potential capacity offshore either in depleted oil and gas fields or saline aquifers.