What is CO2 Sequestration


Inland Empire and San Bernardino Mountains

Inland Empire and San Bernardino Mountains

Nature has been storing carbon dioxide (CO2) and other fluids such as oil and natural gas in underground geologic formations for millions of years. These types of formations are now being put to use for long-term storage of CO2 from industrial sources. Storing CO2 in this way prevents it from entering the atmosphere and contributing to climate change.

Geologic formations considered for CO2 storage are layers of permeable, porous rock deep underground, such as sandstones, that are “capped” by a layer(s) of impermeable rock above them, such as shale or clay, which functions as a seal and prevents the movement of CO2 and other fluids into surrounding strata, especially the freshwater zones nearer the ground surface.

Sequestration practitioners drill a well down into the porous rock and inject pressurized CO2 into it. CO2 is buoyant and some flows upward until it encounters the layer of impermeable rock and becomes trapped. There are other mechanisms for CO2 trapping as well, CO2 molecules: can dissolve in brine, the salty water native to many storage formations; react with minerals in the rock to form solid carbonates; and adsorb into the pores of the formation. 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. There are many types of storage formations, or reservoirs, which can hold CO2, such as deep saline aquifers and depleted oil and gas reservoirs.

Saline aquifers

are layers of porous rock that are saturated with brine.

CO2 Storage

They are much more commonplace than oil- and gas-bearing rock, and represent an enormous potential for CO2 storage capacity. While not all saline formations in the United States have been examined, those documented thus far have an estimated sequestration potential ranging from 919 billion metric tons to more than 3,300 billion metric tons. However, much less is known about saline formations than is known about crude oil reservoirs, and there is greater 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, however, 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.

Saline Aquifers in Southern California

Saline Aquifers in Southern California (yellow areas)

Depleted oil and gas reservoirs

are formations that held crude oil and natural gas over geologic time frames.

CO2 injection into oil and gas reservoirs

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. Mature oil and gas reservoirs in the United States and Canada have been documented to have more than 82.4 million metric tons of sequestration potential. 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 (water flooding) produce 30-40 percent of a reservoir's original oil in place. A CO2 flood enables recovery of an additional 10-15 percent or more of the original oil in place. CO2 enhanced oil recovery (EOR) is a commercial process that is 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. Research in this area is now focused on CO2 EOR injection practices that maximize the amount of CO2 sequestered.

Oil & Gas Reservoirs in Southern California

Oil & Gas Reservoirs in Southern California (purple areas)