Carbon sequestration

The carbon cycle Natural and man-made processes Credit: U.S. Geological Survey

Atmospheric levels of CO 2 have risen from pre-industrial levels of 280 parts per million (ppm) to present levels of 375 ppm. Evidence suggests that the rise in carbon emissions is closely linked to our increased usage of fossil fuels. Credit: U.S. Geological Survey

There is a clear link between the rise in CO 2 levels and the rise of global temperatures although this is not yet fully understood. Scientists are still working on it! Data from Vostok ice core Data from NASA

Predictions of global energy use in the next century suggest a continued increase in carbon emissions and rising concentrations of CO 2 in the atmosphere unless major changes are made in the way we produce and use energy - in particular, how we manage carbon. One way to manage carbon is to use energy more efficiently to reduce our need for a major energy and carbon source—fossil fuel combustion. Credit: U.S. Geological Survey

Another way is to increase our use of low-carbon and carbon -free fuels and technologies (nuclear power and renewable sources such as solar energy, wind power, and biomass fuels). Images 1, 2, 3 – Credit: U.S. Geological Survey 1 2 3

The newest way to manage carbon is through……. “carbon sequestration”

Carbon sequestration Credit: U.S. Geological Survey

Carbon sequestration refers to the provision of long-term storage of carbon in the terrestrial biosphere, underground, or in the oceans so that the build up of carbon dioxide (the principal greenhouse gas) concentration in the atmosphere will reduce or slow. In some cases, this is accomplished by maintaining or enhancing natural processes; in other cases, novel techniques are developed to dispose of carbon.

Types of carbon sequestration Credit: U.S. Geological Survey

There are different types of geological formations in which CO 2 can be stored, and each has different opportunities and challenges. Suitable formations are found in three main geological situations: Depleted oil and gas reservoirs Unmineable coal beds Saline formations

1. Depleted oil and gas reservoirs as possible CO 2 repositories. Locations considered for CO 2 storage are layers of permeable and porous rock deep underground that are “capped” by a layer or multiple layers of non-porous/permeable rock above them. ie. These are the same places where “oil and gas” are found !!!!!!

Depleted oil and gas reservoirs that hold crude oil and natural gas over long geological time frames are ideal. In general, these involve layers of permeable/porous rock with layers of impermeable/non-porous rock above such that they form a dome. It is the dome shape that traps the hydrocarbons. This same dome offers great potential to trap CO 2 and makes these formations excellent for sequestration.

Q. Which situation (s) would be most suitable for carbon sequestration and why ? Answer : All of them are suitable as long as the gas is injected into the right place !

There are other mechanisms for CO 2 trapping as well: CO 2 molecules can dissolve in brine, react with minerals to form solid carbonates, or adsorb in the pores of the porous rock. The technique : Sequestration involves drilling a well down into the reservoir rock and injecting pressurized CO 2 into it. Under high pressure, CO 2 turns to liquid and can move through a formation as a fluid. Once injected, the liquid CO 2 tends to be buoyant and will flow upward until it encounters a barrier of impermeable rock, which can trap the CO 2 and prevent further upward migration.

The degree to which a specific underground formation is amenable to CO 2 storage can be difficult to discern. Research is aimed at developing the ability to characterise a formation before CO 2 -injection to be able to predict its CO 2 storage capacity. Another area of research is the development of CO 2 injection techniques that achieve broad dispersion of CO 2 throughout the formation, overcome low diffusion rates, and avoid fracturing the cap rock. Site characterisation and injection techniques are inter-related because improved formation characterisation will help determine the best injection procedure.

In these operations, CO 2 is separated from the fuel and captured either before or after the combustion of coal. It is then compressed to a super critical liquid, transported by pipeline to an injection well and then pumped underground to depths sufficient to maintain critical temperatures and pressures. The CO 2 seeps into the pore spaces in the surrounding rock and its escape to the surface is blocked by a caprock, or overlaying impermeable layer.

As a value-added benefit, CO 2 injected into a depleting oil reservoir can enable recovery of additional oil known as : Enhanced Oil Recovery - EOR When injected into a depleted oil bearing formation, the CO 2 dissolves in the trapped oil and reduces its viscosity. This “frees” more of the oil by improving its ability to move through the pores in the rock and flow with a pressure differential toward a recovery well. Typically, primary oil recovery and secondary recovery via a water flood produce 30–40% of a reservoir's original oil. A CO 2 flood enables recovery of an additional 10–15% of the oil.

2. Unmineable coal seams. Unmineable coal seams are too deep or too thin to be mined economically. All coals have varying amounts of methane adsorbed onto pore surfaces, and wells can be drilled into unmineable coal beds to recover this coal bed methane (CBM). Initial CBM recovery methods, dewatering and depressurisation, leave a fair amount of CBM in the reservoir. Additional CBM recovery can be achieved by sweeping the coal bed with nitrogen. CO 2 offers an alternative to nitrogen. It preferentially adsorbs onto the surface of the coal, releasing the methane. Two or three molecules of CO 2 are adsorbed for each molecule of methane released, thereby providing an excellent storage sink for CO 2.

3. Saline formations. Saline formations are layers of porous rock that are saturated with brine. They are much more commonplace than coal seams or oil and gas bearing rocks, and represent an enormous potential for CO 2 storage capacity. Saline formations tend to have a lower permeability than hydrocarbon-bearing formations, and work is directed at hydraulic fracturing and other field practices to increase the potential injection. Saline formations contain minerals that could react with injected CO 2 to form solid carbonates. The carbonate reactions have the potential to be both a positive and a negative.

4. Other geological formations : a) Shale. Shale, the most common type of sedimentary rock, is characterized by thin horizontal layers of rock with very low permeability in the vertical direction. Many types of shale contain 1–5 percent organic material, and this hydrocarbon material provides an adsorption substrate for CO 2 storage, similar to where CO 2 can be stored in coal seams. Given the generally low permeability of shale, research is focused on achieving economically viable CO 2 injection rates.

b) Basalt formations. Basalts are of solidified lava. They have a unique chemical makeup that could potentially convert all of the injected CO 2 to a solid mineral form, thus permanently isolating it from the atmosphere. Research is currently being focused on enhancing and utilizing the mineralisation reactions and increasing CO 2 flow within a basalt formation. Research is in its infancy, but these formations may, in the future, prove to be optimal storage sites for stranded CO 2 emissions.

5) Other options : a) Terrestrial and Marine Ecosystems Terrestrial sequestration is the enhancement of CO 2 uptake by plants that grow on land and in freshwater and, importantly, the enhancement of carbon storage in soils where it may remain more permanently stored. Terrestrial sequestration provides an opportunity for low-cost CO 2 emissions offsets. Early efforts include tree-plantings, no-till farming, and forest preservation. More advanced research is being conducted to develop fast-growing trees and grasses, in deciphering the genomes of carbon-storing soil microbes and in nutrient enrichment to enhance algal growth in the oceans. All of these are potential carbon stores of the future. Credit: U.S. Geological Survey

b) Carbon Capture Technologies A new coal-based generation technology known as Integrated Gasification Combined Cycle Process offers promise as a pathway to capture CO 2 before combustion at coal plants and sequester it downstream. IGCC plants are able to capture emissions more cost-effectively than methods currently used at more conventional plants—such as supercritical pulverized coal—because they do not rely on direct combustion and instead convert coal feedstocks using gasification. The current carbon capture rate for IGCC plants is believed to be around 85 percent. Efforts are underway to develop capture technologies for traditional pulverized coal power plants. At these plants, CO 2 would need to be captured from flue gases after combustion through a chilled ammonia or amine stripping process. CO 2 capture at conventional plants is likely to be more costly than at IGCC plants but has advantages, particularly in the re-fit of existing plants.