In geologic sequestration, carbon dioxide would be stored in deep underground formations, such as depleted oil and gas reservoirs, unmineable coal seams, and deep saline formations.  Many of these formations have naturally stored carbon dioxide and other gases and fluids (i.e., petroleum) for millions of years, and have the potential to store hundreds of years’ worth of human-generated carbon dioxide emissions.

Ideally, it would be best to avoid creating CO₂ emissions.  But right now our energy systems are based on fossil fuels, and burning fossil fuels generates CO₂ emissions.  That means when we drive a car, mow the grass, heat and cool our homes, or use electricity (unless it’s hydropower or nuclear power), we’re generating CO₂.  U.S. policies such as the Global Climate Change Initiative are aimed at determining what we can do now to reduce CO₂ through the use of sequestration, energy conservation, and renewables while we assess and develop improved energy systems that will put out less CO₂.  The magnitude of greenhouse gas (GHG) emissions that must be offset to achieve atmospheric stabilization is daunting and requires that all options jointly contribute toward this reduction.  Sequestration and a host of other GHG mitigation options — as well as adaptive measures — must be aggressively explored in parallel so that the best mix of cost-effective solutions are found.

CO₂ is not a poison like CO (carbon monoxide) and poses no health risk at ambient levels (350–500 parts per million, or ppm) or modestly elevated concentrations.  In fact, low levels of CO₂ are necessary for all life.  Health organization guidelines for prolonged indoor exposure cite a maximum average CO₂ concentration of 5,000 ppm.  Like other heavier-than-air gases that can accumulate and displace oxygen in sub-grade or enclosed areas, elevated concentrations of CO₂ can cause asphyxiation.*  The severity of health effects depends on the actual CO2 concentration and length of exposure.  Exposure to concentrations of above 5,000 ppm to 30,000 ppm may cause headaches, dizziness, and other reversible side effects.  Unconsciousness can occur at concentrations above 50,000 ppm and concentrations above 100,000 ppm are considered to be life threatening.  There are safe-guards in place for industries that handle CO₂.  Those safe-guards would protect workers who would likely come in contact with CO₂, either in handling or during transport.

First of all, the CO₂ will be injected into sites at which the chance of leaking is very remote, based on field test results and site selection.  It is well known that CO₂ makes water more acidic, which can dissolve minerals including some containing heavy metals.  In addition to dissolving some minerals, this type of geochemical environment can promote the deposition of other minerals in the sequestration host formation.  Reservoir studies at a test site in Weyburn, in Saskatchewan, Canada, have shown that material dissolving in one place has deposited in other places within the storage site deep underground.  If there is any movement out of the storage reservoir, it is likely to be further neutralized by dilution with brine in the intermediate buffer zones.  Since the formations used for sequestration are very deep and located so as to be remote and isolated from potable water aquifers, the chances of any mobilized chemicals reaching the drinking water zones are remote, if any at all.