Storing carbon dioxide underground
Once captured, the CO2 must then be stored forever - longer than nuclear waste.
Carbon dioxide can be stored underground under pressure or reacted with minerals. At 800 M the hydrostatic pressure is enough to keep it at supercritical state. Neither a liquid or gas, It has the density of the liquid, but the viscosity of gas.
Underground, CO2 can be stored in:
(1 tonne of coal or oil produces about 3 tonnes of CO2)
When an oil well is pumped "dry", only 20-40 percent of the oil has been extracted. Extra oil can be recovered by pushing it out with CO2. This way the total extraction is 30-60%.
CO2 is used because it is cheap, and is miscible with oil. As it mixes with the oil, it swells the oil, and reduces it's viscosity making it easier to pump.
The CO2 is recovered and sent back for more oil recovery.
Once the recovery is compete, the CO2 stays in the empty oil formation and is stored in a proven gas tight formation.
The advantages of depleted oil fields is that the geology is well known, and they are gas tight.
In the Otway trial project it was found that 60% of the space occupied by the original gas was available for CO2.
The potential storage capacity in Australia is estimated to be 16,500 MT CO2.
In the US the total CO2 stored after many years of EOR, is about 500 MT. Their yearly production of CO2 is 5,000 MT per year.
1 t of CO2 pushes out 0.5 t of oil which when burnt will produce 1.5 t of CO2
So it is an economic benefit to the oil company, but it is a loss for the atmosphere. Overall it produces 50% more CO2 than it saves.
There are several projects underway in Australia at present, each is testing different methods of capturing CO2. CO2CRC projects.
Oil is extracted from a deposit in three stages:
But - Most CCS wells will fail!
100 per cent of oil and gas wells that are “cased and capped” eventually leak their remaining gases over the following 100 years, deflating like all old soccer and basketballs.
Recently, a study by Cornell University titled 'Assessment and Risk Analysis of Casing and Cement Impairment in Oil and Gas Wells in Pennsylvania, 2000-2012',and published in the journal Proceedings of the National Academy of Sciences in July, shows that “in the Marcellus shale region of north-east Pennsylvania of 41,381 wells over 40 per cent of them will fail catastrophically and leak”. The study was undertaken after there had been “numerous contamination complaints and explosions nationally in areas with high concentrations of unconventional oil and gas development”, and because of the increased awareness of the role of methane.
Matthew Wright - executive director of Zero Emissions Australia
|Abstract: Proceedings of the national Academy of Sciences of USA|
Deep Saline Aquifer - DSA
Saline aquifers are far more extensive than oil fields and offer the greatest hope for underground storage. They are old sedimentary basins made of porous material, usually sandstone, capped with a layer of impermeable material such as shale. They can extend over thousands of kilometres and be anything from a few hundred metres to thousands of metres thick.The sandstone is saturated with water too salty to use for farming or drinking. The water flow is extremely slow at just a few metres per year.
The CO2 dissolves in the water which becomes denser and sinks deeper into the basin. Some CO2 is trapped undissolved.
Care must be taken to not crack the overlying layer with too much pressure.
Since 1995 a million tonnes of CO2 per year has been injected into an undersea DSA at Sleipner in Norway.
The CO2 needs to be stored more than 800 metres below the surface so that the hyrostatic pressure will keep it in a dense supercritical state. In this state it has the density of a liquid and the viscosity of a gas. So it can permeate into fine pores in the rock.
Maps of CO2 storage potential
Click on maps to enlarge
Long term fate of CO2
CO2 moves upwards to the top seal. Over time the CO2 dissolves into the formation water. This makes it denser and it moves downwards.
Storing CO2 in coal
Just as methane is stored in coal seams, so too is CO2.
Not all coals are suited. Too deep, and the pores will be compacted closed, too shallow, and someone may want to mine it. The usual target will be CSG (CBM) wells that are in between.
CO2 is preferentially adsorbed by the coal, and is used to recover more methane from a CSG well. In the USA this process is called in Enhanced Coal Bed Methane - ECBM.
The proportions vary but generally 3 molecules of CO2 are used to displace 1 molecule of CH4 (methane) which will produce 1 mole of CO2 when burnt.
3 moles of CO2 are stored for every mole CO2 produced from burning CH4 released. Climate-wise, it is profitable.
CSG operators will use as little CO2 as possible because at present they have to buy it, but if CO2 had a carbon price, they would inject as much as possible. Ref p 50 The process will be affected by the carbon price.
CSIRO has found that CO2 could be stored for many millions of years in coal seams. The capacity at the high temperatures of the deep coal is better than expected.
This is important for Australian coal fired power stations, as most are located close to coal seams but a long way from other potential geological storage sites, such as saline aquifers.
The research is ongoing. (some of it contradictory)
3 Moles of CO2 ---> 1 mole CH4
1 mole CH4 ---> 1 mole CO2
3 Moles of CO2 ---> 1 mole CO2 from burning methane
The Gorgon LNG Project
The Gorgon LNG Project in Western Australia will soon be one of the largest storage projects in the world, storing over 3 million tonnes of CO2 per year.
Starting in 2014-15, Chevron will begin injecting 120 million tonnes of pressurised supercritical carbon dioxide 2.5km underground as part of its giant Gorgon LNG project.
Raw gas from the Gorgon field contains about 14 per cent carbon dioxide, which must be separated out and safely disposed of before the purified methane can be liquefied and sold.
Under an ambitious program agreed with the state and federal governments back in 2009, Chevron will inject the CO2 into a saline aquifer beneath Barrow Island off the coast of Western Australia.
Chevron is spending $2 billion on the world's largest CO2-injection facility, which will store over 3 million tonnes per year, making it by far the world's largest CO2 storage project, and creating a unique opportunity to study how injected CO2 behaves underground in saline aquifers.
The Norwegian carbon capture and storage nightmare
IPCC special report on CCS 2005 - Lead author Peter Cook
Carbon capture and storage – a vital part of our climate change response - The Conversation.