An Introduction
Storage is the placement of CO2 into a repository in such a way that it will remain sequestered for hundreds of thousands of years. Efforts are focused on storage in geologic formations.
Geologic formations considered for CO2 storage are layers of porous rock deep underground that are "capped" by a layer of non-porous rock above them. Sequestration practitioners drill a well down into the porous rock and inject pressurized CO2 into it. The CO2 is buoyant and flows upward until it encounters the layer of non-porous rock and becomes trapped. There are other mechanisms for CO2 trapping as well. CO2 molecules can dissolve in brine, react with minerals to form solid carbonates, or adsorb in the pores of the porous rock. The degree to which a specific underground formation is amenable to CO2 storage can be difficult to discern. Research is aimed at developing the ability to characterize a formation before CO2-injection to be able to predict its CO2 storage capacity. Another area of research is the development of CO2 injection techniques that achieve broad dispersion of CO2 throughout the formation, overcome low diffusion rates, and avoid fracturing the cap rock. These two areas, site characterization and injection techniques, are interrelated because improved formation characterization will help determine the best injection procedure.
There are three priority types of geologic formations in which CO2 can be stored, and each has different opportunities and challenges: Depleted oil and gas reservoirs, Unmineable coal seams and Saline formations.
These are formations that held crude oil and natural gas over geologic time frames. In general they are a layer of porous rock with a layer of non-porous rock above such that the non-porous layer forms a dome. It is the dome shape that trapped the hydrocarbons. This same dome offers great potential to trap CO2 and makes these formations excellent sequestration opportunities.
As a value-added benefit, CO2 injected into a depleting oil reservoir can enable incremental oil to be recovered. The CO2 lowers the viscosity of the oil enabling it to slip through the pores in the rock and flow with the 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 in place (OOIP). A CO2 flood enables recovery of an additional 10-15% of the OOIP. CO2 enhanced oil recovery (EOR) is a commercial process and in demand recently with high crude oil prices. However, commercial practitioners operate their injections with the goal of minimizing the amount of CO2 left in the ground so that the CO2 can be used for another well.
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 depressurization, leave a fair amount of CBM in the reservoir. Additional CBM recovery can be achieved by sweeping the coalbed with nitrogen. CO2 offers an alternative to nitrogen. It preferentially adsorbs onto the surface of the coal, releasing the methane. Two or three molecules of CO2 are adsorbed for each molecule of methane released, thereby providing an excellent storage sink for CO2. The maximum capacity in the USA for CO2 enhanced coalbed methane (ECBM) has been estimated at 90 billion metric tons CO2, 40 billion metric tons of which are in the state of Alaska . Like depleting oil reservoirs, unmineable coal beds are a good early opportunity for CO2 storage.
Coal swelling is a potential barrier to CO2 ECBM. It has been observed that when coal adsorbs CO2, it swells in volume. In an underground formation swelling can cause a sharp drop in permeability, which not only restricts the flow of CO2 into the formation but also impedes the recovery of displaced CBM.
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 rock, and represent an enormous potential for CO2 storage capacity. However, much less is known about saline formations than is known about crude oil reservoirs and coal seams and there is a greater amount of uncertainty associated with their amenability to CO2 storage.
Saline formations tend to have a lower permeability than do hydrocarbon-bearing formations, and work is directed at hydraulic fracturing and other field practices to increase injectivity. Saline formations contain minerals that could react with injected CO2 to form solid carbonates. The carbonate reactions have the potential to be both a positive and a negative. They can increase permanence but they also may plug up the formation in the immediate vicinity of an injection well. Researchers seek injection techniques that promote advantageous mineralization reactions.
Related links:
The U.S. Department of Energy's created the National Carbon Explorer, an interactive atlas that identifies sources that produce CO2 and sinks where the CO2 can be stored.
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