Carbon Capture & Storage Technologies
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Geological Storage
Geological features being considered for CO2 storage fall into three categories
- deep saline formations
- depleted oil and gasfields
- unmineable coal seams
As CO2 is pumped deep underground, it is compressed by the higher pressures and becomes essentially a liquid. There are a number of different types of geological trapping mechanisms (depending on the physical and chemical characteristics of the rocks and fluids) which can be utilised for CO2 storage.
| Geological Trapping Mechanisms | |
|---|---|
| Structural Storage | When the CO2 is pumped deep underground, it is initially more buoyant than water and will rise up through the porous rocks until it reaches the top of the formation where it can become trapped by an impermeable layer of cap-rock, such as shale. The wells that were drilled to place the CO2 in storage can be sealed with plugs made of steel and cement. |
| Residual Storage | Reservoir rocks act like a tight, rigid sponge. Air in a sponge is residually trapped and the sponge usually has to be squeezed several times to replace the air with water. When liquid CO2 is pumped into a rock formation, much of it becomes stuck within the pore spaces of the rock and does not move. |
| Dissolution Storage | CO2 dissolves in salty water, just like sugar dissolves in tea. The water with CO2 dissolved in it is then heavier than the water around it (without CO2) and so sinks to the bottom of the rock formation. |
| Mineral Storage | CO2 dissolved in salt water is weakly acidic and can react with the minerals in the surrounding rocks, forming new minerals, as a coating on the rock (much like shellfish use calcium and carbon from seawater to form their shells). This process can be rapid or very slow (depending on the chemistry of the rocks and water) and it effectively binds the CO2 to the rocks. |
Source: IEAGHG/WCI 2007
Deep saline formations are underground formations of permeable reservoir rock, such as sandstones, that are saturated with very salty water (which would never be used as drinking water) and covered by a layer of impermeable cap rock (e.g. shale or clay) which acts as a seal. In the case of gas and oilfields, it was this cap rock that trapped the oil and gas underground for millions of years.
CO2 injected into the formation is contained beneath the cap rock and in the groundwater flow and, in time, dissolves into the saline water in the reservoir. CO2 storage in deep saline formations is expected to take place at depths below 800m. Saline aquifers have the largest storage potential globally but are the least well-explored and researched of the geological options. However, a number of storage projects are now using saline formations and have proven their viability and potential (see Otway Project case study).
Depleted oil and gas fields are well-explored and geologically well-defined and have a proven ability to store hydrocarbons over geological time spans of millions of years.
CO2 is already widely used in the oil industry for Enhanced Oil Recovery (EOR) from mature oilfields. When CO2 is injected into an oilfield it can mix with the crude oil causing it to swell and thereby reducing its viscosity, helping to maintain or increase the pressure in the reservoir. The combination of these processes allows more of the crude oil to flow to the production wells.
In other situations, the CO2 is not soluble in the oil. Here, injection of CO2 raises the pressure in the reservoir, helping to sweep the oil towards the production well. In EOR, the CO2 can therefore have a positive commercial value.
Schematic Diagram of CO2- Enhanced Oil Recovery
Coal seam storage involves another form of trapping in which the injected CO2 is adsorbed onto (accumulates on) the surface of the in situ coal in preference to other gases (such as methane) which are displaced.
The effectiveness of the technique depends on the permeability of the coal seam. It is generally accepted that coal seam storage is most likely to be feasible when undertaken in conjunction with enhanced coalbed methane recovery (ECBM) in which the commercial production of coal seam methane is assisted by the displacement effect of the CO2.
Mapping & Monitoring
Storage projects are carefully tracked through measurement, monitoring and verification (MM&V) procedures both during and after the period when the CO2 is being injected. These procedures address the effectiveness and safety of storage activities and the behaviour of the injected CO2 underground.
MM&V are used to measure the amount of CO2 stored at a specific geological storage site, to ensure the CO2 is behaving as expected. The techniques used for MM&V are largely new applications of existing technologies. These technologies now monitor oil and gas fields and waste storage sites. They measure injection rates and pressures, subsurface distributions of CO2, injection well integrity and local environmental impacts.
The IPCC found that the risk of leakage from geological storage was very likely to be less than 1% over 100 years; and likely to be less than 1% over 1000 years.
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Safe Storage - Closing the Carbon Loop
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