What is CCS?
Enhanced hydrocarbon recovery
Apart from pure storage, carbon dioxide that is captured can also be used for Enhanced Hydrocarbon Recovery. This includes Enhanced Oil Recovery (EOR), Enhanced Gas Recovery (EGR) and Enhanced Coalbed Methane Recovery (ECBM).
Any oil or gas that is recovered through these methods would otherwise not be extracted and therefore has an economic value, which offsets some of the costs of CO2 sequestration. A report published (in 2010) by the UK Department of Energy and Climate Change found that “The combination of carbon dioxide enhanced oil recovery (CO2-EOR) and permanent CO2 storage in oil reservoirs has the potential to provide a critical near-term solution for reducing greenhouse gas (GHG) emissions.”
Furthermore a study by Durham University found that CO2 captured through CCS and used for EOR could lead to £150 billion of oil to be obtained from existing oil fields in the North Sea that would not otherwise have been possible. This would also be of significant economic value to the UK, as it would add around £60 billion in revenue to the Treasury.
Enhanced Oil Recovery
Conventional oil production occurs in three phases: primary, secondary and tertiary. In the primary phase, natural pressure within the oil drives it towards the production wells and, with the help of pumps or other mechanisms, to the surface. Secondary production usually consists of injecting water into an oil reservoir to increase the pressure and again drive the oil towards the production wells. Enhanced Oil Recovery (EOR) is a tertiary method of oil recovery and can enable significant additional quantities of oil to be extracted.
There are two main methods for EOR using CO2: one uses just CO2, whilst the other process uses alternate injection of CO2 and water, to move the oil through the reservoir and towards production wells. The latter process is expected to be used in the majority of fields due to the payback time generally being shorter.
Enhanced Gas Recovery
Enhanced Gas Recovery (EGR) can be achieved using CO2 as it is heavier than natural gas. CO2 is injected into the base of a depleted gas reservoir and will tend to pool there, causing any remaining natural gas to “float” on top of it. This then drives the natural gas towards the production wells. However, since a high percentage of the natural gas contained in many gas fields can be recovered without using enhanced recovery techniques, the potential target for EGR is small.
Enhanced Coalbed Methane Recovery
Coal beds (also known as coal seams) can be reservoirs for gases, due to fractures and micropores in which natural gas, known as coalbed methane (CBM), can be found adsorbed onto the surface. However, CO2 has a greater adsorption affinity onto coal than methane. Thus, if CO2 is pumped into a coal seam towards the end of a coalbed methane production project, it displaces any remaining methane at the adsorption sites (allowing methane recovery jointly with CO2 storage).
Experiments have been conducted in the San Juan Basin showing that CO2 injection does appear to have enhanced CBM production. Smaller field trials of ECBM production using CO2 are under way in Europe, Canada and Japan.
However, there are issues with ECBM; for example the low permeability of seams means that a large number of wells may be needed to inject sufficient amounts of CO2. Moreover, the methane in coal represents only a small proportion of the energy value of the coal, and the remaining coal could not be mined or gasified underground without releasing the CO2 to the atmosphere. Finally methane is a far more potent greenhouse gas than CO2, so steps would have to be taken to ensure no methane leakage to the atmosphere took place.