What is Carbon Capture and Storage (CCS)?
The level of CO2 in the atmosphere has risen from around 250 parts per million (ppm) in 1800 to today’s value of 380 ppm. In 2005 alone, the concentration rose by 2.6 ppm. To stabilise the concentration at 500 ppm by 2050 will mean man will have to limit emissions to the current level of 7 Gt CO2/year rather than the forecast 14 Gt CO2/year. Yet the Chinese have plans to build over 500 coal-fired power stations over the next 10 years. Also, the US has plans for 100 new coal fired power stations and in India they plan to double their electricity generation capacity over the next 10 years. At a time when we are being urged to cut down on CO2 emissions, we are set to see them increase at an even more rapid rate. However, as discussed by Steve Furnival (steve.furnival@senergyltd.com) of Senergy Ltd there maybe a strategy to deal with some of our CO2 emissions and that is Carbon Capture and Storage (CCS). Steve Furnival is author of ‘Burying Climate Change for Good’, published in Physics World, September 2006.
Basics
Simply put, in CCS we capture the CO2 generated from the burning of fossil fuels and then bury it so it cannot find its way into the biosphere. The three main components in this strategy are the capture, transportation and the storage of the CO2. Capturing the CO2 after combustion can be done using a chemical solvent as is done in many petrochemical plants across the world. Alternatively CO2 can be captured before combustion by mixing the fuel with steam and air to generate CO2 and Hydrogen which is used as the fuel. This technology is widely used in the manufacture of ammonia and fertilisers.
CO2 injection into deep geological formations is already widely used by the oil industry where it re-energises the remaining oil allowing it to be produced. Much of the CO2 used in this Enhanced Oil Recovery (EOR) scheme remains permanently trapped – an extension of this scheme is CCS. Large scale CCS projects are being undertaken by Statoil (Sleipner and Snøhvit) and BP (In Salah).
Technically, the main challenges we face are capture and storage. Currently it is estimated that CCS will cost between $25 and $50 per tonne of CO2 of which 80% is the cost of capture. Since a new 1 GW coal-fired power station will produce six million tonnes of CO2 per year, we will need better lower-cost technology to get widespread adoption.
Storage
With storage, the main issue is to ensure that once injected, the CO2 will not find its way back to the surface in any significant amount. Produced oil and gas has been trapped underground for tens of millions of years therefore it is reasonable to assume that CO2 injected into depleted hydrocarbon fields will be contained for millennia. Containment becomes a major problem if CCS is widely adopted as it is likely the depleted hydrocarbon fields will be filled within a few years. In which case, we would have to switch our attention to other potential underground repositories – the saline aquifers. These fields although large and numerous are not currently as well understood as hydrocarbon fields and therefore a substantial evaluation programme will be required.
One of the major drivers for the use of underground storage is that we already have considerable experience in the production and injection of fluids within the oil industry. Many countries use depleted hydrocarbon fields and salt caverns to store gas and oil for times of high demand, i.e. winter months using excess production from the summer months.
In the UK, depleted gas fields in the southern North Sea are used as swing producers at times of high demand. The Rough gas storage facility was designed to supply 10% of the UK gas market at peak demand. A recent fire closed the platform and along with other supply problems lead to the unprecedented 1-day rise in the spot price of UK gas from around £0.6/therm to over £2.3/therm.
Sleipner CCS Project
The Sleipner field is one of the largest gas producers in the Norwegian sector of the North Sea with an export rate of around 21 Mm3/day. However the gas contains between 4 and 10% CO2 which is too much for it to be fed directly into European gas sales pipelines; typically gas contracts require the CO2 content to be 2.5% or less.
Had the Sleipner field been located in almost any country’s territorial waters other than Norway, the field operator would have scrubbed the excess CO2 from the produced gas and vented it to the atmosphere. The Norwegian government, however, would have levied an annual carbon tax of around $50M if Statoil had followed this path. Instead, Statoil investigated options for scrubbing the excess CO2 and storing it in a nearby geological formation. In particular they studied the brine filled aquifer called Utsira which overlies the Sleipner field. Utsira is a massive formation some 500 km long, 50 km wide and 200 m thick.
After several years of experimental study by Statoil and its partners, a commercial plant was installed on the Sleipner platform. Two absorber columns were installed which have reduced the CO2 content of the sales gas to 2.25%, below the limit set by contracts. The near pure excess CO2 is then compressed to 80 bars before being injected into the base of the Utsira aquifer some 1000 m below the seabed. Since 1996, around 1 Mt of CO2 per year has been injected.
Compressing the CO2 to around 80 bars is significant. As with all fluids, we can change its phase state, i.e. solid, liquid or vapour by changing pressure and/or temperature. Beyond the so-called critical point which for CO2 is at the critical temperature of 31 °C and critical pressure of 74 bars, the fluid exists in a super-critical state whose state cannot be changed by a change in pressure or in temperature alone. In this super-critical state, the density of CO2 is around 700 kg/m3, i.e. about 0.7 times that of water.
Therefore, even though the CO2 is injected into the base of the Utsira formation, we can expect it to migrate upwards in time. Statoil have seen this effect via the use of 4D seismic surveys. If these surveys are run at regular times going forward, one would hope to see a 4D picture showing fluid movement. This is what has been done by Statoil in Utsira. Figure 1 shows clearly how the CO2 injected into the Utsira has gravity segregated upward and then spread laterally as the injection has continued.
Figure 1: Cross-Sections Through the 4D Seismic Images of the Sleipner/Utsera Formation (courtesy Andy Chadwick, BGS)
The major concern is the seal at the top of the formation, which is just above the top of the Utsira sand, is not broken by the effect of the CO2 injection. This certainly has not happened to date and there is confidence this will remain intact. Even if the seal is eventually broken, one would hope that progressively shallower seals exist which minimise the amount of CO2 that finally reaches the geosphere. In time, it is believed the CO2 will start to dissolve in the brine. This CO2-brine mixture will then become heavier than the unsaturated brine and will consequently move downward. This will set up a convective role that will further promote dissolution of the CO2 into the brine increasing the probability the CO2 will be locked in for the long term. On an even longer timeframe it is believed there will be a mineralisation of the CO2/brine mixture which will permanently lock the CO2 into the formation.
There are two other major CCS projects, both involving the separation of CO2 from produced gas. These are at Snøhvit in the far north of the Norwegian Sea (Statoil) and at In Salah in the Algerian desert (BP, Statoil and Sonatrach).
Enhanced Oil Recovery
Why aren’t more companies rushing to adopt CCS? In a word, cost! At a time when the worldwide demand for energy is rising rapidly, supply appears limited and hence energy prices are rising, are consumers willing to pay the extra costs imposed by CCS? One way to offset the cost of CCS is if it can be used to boost oil recovery through Enhanced Oil Recovery (EOR).
In a well managed water flood, it is not uncommon to recover 50% of the original oil in place but that means 50% is left behind and at 2006 oil prices of $70 per barrel, that’s a valuable resource. The problem is secondary recovery by water injection is largely a recycling exercise – I’ve heard this called the ‘washing machine’ effect – instead we need another way to re-energise the reservoir which is where CO2 EOR comes in. When mixed with depleted oil, CO2 mimics the hydrocarbon gas that was originally dissolved in the oil by re-swelling it and reducing its viscosity. First deployed in 1972 in Scurry County, Texas, CO2 injection has been widely used throughout the Permian basin of West Texas and eastern New Mexico.
Most projects use naturally occurring CO2, but a major project using anthropogenic (manmade) CO2 takes CO2 produced from the Great Plains Synfuels coal gasification plant in North Dakota and pipes it 200 miles north across the US-Canada border into Saskatchewan where it is used by EnCana in the Weyburn field. It is envisaged that CO2 EOR will extend the life of this 50 year old field by a further 25 years in which time it will produce an additional 130 million barrels of oil and sequester 14 million tons of CO2.
To date no one has tried CO2 EOR in an offshore environment but that is due to change in the near future. Shell and Statoil have announced plans to use CO2 in the central section of the Norwegian North Sea as part of Statoil’s plans to build a gas-fired power station. BP is planning to use CO2 in the Miller field in the UK sector of the North Sea. This project which goes under the name of DF-1 for Decarbonised Fuel-One will convert North Sea gas to hydrogen and CO2 using pre-combustion capture with the hydrogen being burnt in a power station on the Aberdeenshire coast at Peterhead and the CO2 sent offshore to the near-depleted Miller field.
In all the CO2 EOR projects, some of the injected CO2 will be re-produced along with the oil. However, as the CO2 is a major component of the cost of the tertiary recovery scheme it is usually stripped out of the production stream and re-injected.
Deep Ocean Storage
As an alternative to underground disposal, there have been suggestions that CO2 could be injected into the deep oceans. To make this scheme work requires the CO2 to be bubbled into the oceanic brine so that it is immediately dissolved. Since a CO2-brine mixture is heavier than unsaturated brine it will sink to the seabed, whereas if the CO2 remains as a separate phase it will migrate upwards because of its lower density.
Although the volume of oceanic storage is considered to be much greater than that of underground reservoirs, there are major uncertainties on the environmental impact and retention time.