Long - Term Storage of CO2
CO2 is a greenhouse gas and a major source of global warming, with the major emissions coming from power stations fired on fossil fuels. In the previous article we looked at carbon capture from a coal powered station, in this article we will examine the different methods and locations proposed and in current use for carbon storage.
Long term large quantities of CO2 storage must be in a secure and constantly monitored area, where will remain for a few thousand years.
We start by having a look at the methods of compressing the CO2 and some of the proposed locations for carbon storage……..
Methods of Preparation of CO2 for Injection
Once the anthropogenic CO2 has been captured from whatever source, it is piped to a drier where the water vapor is removed. This results a dry gas of 99% CO2 which is compressed, converting the gas into liquid CO2 for storage and transport.
It is worth mentioning at this stage that some CO2 liquid is sold to the chemistry and food industry and carbonated drinks manufacturers. A good percentage is used as oil enhancement, being injected into almost depleted oil wells enabling the last dregs of oil to be extracted from the reservoir.
I worked on a production/drilling rig modernization project away up on the West Coast in the Highlands of Scotland in the early eighties.
We installed a nitrogen plant on the deck which produced nitrogen for this very purpose. I remember at the time querying the cost of this against the enhanced oil recovery, but not why CO2 wasn’t used instead as the plant was very energy consuming (comments appreciated).
We shall now look at the different locations, then in the summary examine the advantages and disadvantages of carbon storage.
The Use of Depleted Offshore Oil & Gas Reservoirs
The most popular current method being proposed for storage is abandoned oil and gas reservoirs. The reservoirs were originally prevented from leaking hydrocarbons into the environment by a non-porous cap rock. This will again work as a barrier, preventing any escape of the CO2 into the environment.
These reservoirs will have had the wells sealed off; the production platform, steel jacket and the seabed BOP safety valves will all have been removed, or must be before any injection of CO2 is commenced. (See my article on offshore oil and gas – oil platform decommissioning.)
The subsea pipelines which were used to export the hydrocarbons ashore may still be in place. After testing the integrity of the pipes by using pressure tests or ultrasound equipment, these can be used to transfer the carbon from the land to the depleted reservoir via a subsea well head.
If the pipes fail the tests, or have been removed then a liquid gas tanker can be used to ship the carbon to the proposed site.
Once the carbon has been stored in the depleted reservoir, the remaining wells are capped and the storage area monitored. This can be carried out using seismic techniques, which was the original method of prospecting for the reservoir, only this time it will be done by constantly measuring the CO2 content in the reservoir, detecting any leaks with a change in volume.
Use of Inoperable Coal-mines
Un-mineable coal seams in deep underground pits are another method being proposed for the storage of CO2, this time underground.
The coal seams being porous will allow the carbon to seep into its surface which displaces the methane. This is another greenhouse gas that must not be allowed to escape to the atmosphere; instead it can be properly collected. Once collected and stored this gas can be sold off as fuel, with the profits going back into the CO2 storage process.
Use of Saline Aquifers
Saline aquifers, especially the really deep ones have been proposed for long-term storage of CO2. The water contained in these aquifers is salty and brackish, not at all potable. However, feasibility studies are being run on some of these aquifers to collect relevant information, as not a lot is known about the effect of CO2 on the rocks.
This method of storage is already being used in a saline aquifer under the North Sea. CO2 produced in natural gas processing in Statoil’s Sleipner gas field in the North Sea west of Norway is being injected into the aquifer, so time will tell.
Use of the Deep Oceans
The amount of storage space offered by the ocean is massive in comparison with the other storage methods we have looked at.
CO2 is transferred naturally from earth’s atmosphere into the seawater of the oceans in an ongoing process and scientists have predicted that three quarters of all CO2 (including anthropogenic produced CO2) will be transferred to the sea in 1000years.
There are several methods proposed for this type of storage, the main one being to inject the liquid CO2 into the medium depths of the ocean between 1000 and 3000m or into the deepest oceans at a depth of over 3000m, at this depth the CO2 becomes lighter than the seawater, sinking down to lie on the bottom of the ocean.
Please read on to see the advantages and disadvantages of the various CO2 storage locations….
CO2 levels in the atmosphere are at the highest ever recorded caused mainly, we are informed, by the combustion of fossil fuels to produce energy in the power stations worldwide.
At present the CO2 produced in this process goes unhindered into the atmosphere. Also with the supply of hydrocarbons predicted to run out relatively quickly, coal would seem to be the fuel to take over from the hydrocarbons of oil and gas. But coal has the highest CO2 emissions of all the fossil fuels, so we need to clean up the end result. To this end Carbon Capture and Storage (CCS) has been developed, with government legislation prohibiting any new coal-fired power stations being built without CCS.
We covered the capture of the CO2 in the previous article and here we have examined the different locations for the storage of CO2.
I have listed these below with their advantages and disadvantages, which we must weigh up in our own minds, against the very real threat of global warming.
- Depleted Oil and Gas Reservoirs
Advantages - These are readymade storage locations for the CO2, and provided the original oil and gas export pipelines are still located on the seabed and are in serviceable condition, can be used to pipe the CO2 to the location.
Disadvantages - CO2 reacts with water in the long term to produce carbolic acid. This could corrode any cement caps used to seal the old reservoir wells, in time allowing the CO2 to escape to the atmosphere. Existing steel well lining pipes would also be liable to corrode, but this can be overcome by lining these pipes internally with a non-corrosive metal such as nickel.
- Unworkable Coal Mines
Advantages - Again we have a readymade storage facility although the mine will need to be capped to prevent escape of the CO2 to the atmosphere should it vaporize. There is also the advantage of the byproduct of useful methane gas, which is displaced from the coal seams and cracks by the injection of CO2.
Disadvantages - Again there will CO2 contact with the water gathered at the bottom of the mine and although this can be pumped dry it will seep back in again through natural means. Also the coal tends to expand and swell in contact with CO2 and this can lead to problems.
- Deep Oceans
Advantages - The Ocean already continually absorbs quantities of CO2 from the atmosphere with no adverse effects. The oceans are the largest potential storage locations for CO2. At below 3000m, the CO2 becomes heavier than the seawater, sinking to the bottom.
Disadvantages - Some scientists are concerned about the possibility of the CO2 somehow converting back to gas and floating to the top of the ocean, thus escaping to the atmosphere in large quantities.
- Saline Aquifers.
Advantages - These aquifers contain brine absorbed in layers of soluble rocks. It is thought that the introduction of CO2 into the aquifer will result in a carbonate limestone–like rock, which will provide a lasting storage location.
Disadvantages - Not enough information is known about the effects of CO2 on the rock formation in saline aquifers, so more R & D is required. However Statoil’s natural gas field off the Norway coast has been pumping liquid CO2 into a saline aquifer under the sea since the mid-nineties with no known adverse effects.
In all the above cases there are pros and cons, but during my investigation, a couple of items stood out – capturing the CO2 was relatively easy, but expensive. Transporting either by pipeline or ship was again straightforward, but might cause a few problems due to the CO2’s liquid, solid, and gas phases being very close to each other. But one thing was certain– more research and development is required before we can safely store the captured CO2 in any of the proposed long-term locations.