While the concept of geologic storage seems simple enough, a CO2 injection well is a surprisingly complicated system. Multiple cement casings and provisions for monitoring are required to ensure that the supercritical fluid reaches only appropriate storage formations and stays there. In particular, steps must be taken to keep the CO2 from interfering with sources of drinking water at shallower depths. In the injection zone itself, special cement must be used to prevent damage to the casing from acids that form when CO2 reacts with the in situ saline solution.
A variety of measurement, monitoring, and verification (MMV) technologies will also need to be incorporated into a complete storage system to make sure the CO2is not leaking into the surrounding environment. Some off-the-shelf technologies,such as seismic imaging of subterranean formations, are already being used to trackthe underground migration of injected CO2, and sampling of groundwater couldprove useful for detecting leakage directly. Detecting small rates of leakage over long periods of time, however, will require higher-resolution measurements and the development of highly precise baseline data. More-sensitive MMV techniques thatcan measure the actual amount of CO2 in storage may also be needed for purposes of greenhouse gas mitigation reporting.
The issue of leakage is critical from both global and local perspectives. Even gradual leakage from numerous sites may provide enough CO2 reentering the atmosphere to undermine efforts to stabilize greenhouse gas concentrations. Locally, leakage from an underground storage site could present an immediate hazard to humans and ecosystems.
The most dramatic type of CO2 release would come from a blow-out at an injection well, which could produce high enough concentrations (7–10%) of the gas in the vicinity to endanger human life.
Undetected leakage from a faulty well or through ground fractures would probably be more diffuse and primarily affect groundwater and surface ecosystems. In particular, aquifers used as a source of drinking water could be harmed, either by acidification resulting from direct contact with large amounts of CO2 or by the seepage of brines displaced by CO2 during the injection process.
Because CO2 is heavier than air, it also could accumulate in lowlying geographic areas or in basements and potentially threaten human health.
Research is currently under way to improve CO2 leak detection and develop possible
remedial measures. Specifically, MMV technologies are needed that would detect
potential leaks long before they pose any danger to water supplies or surface ecosystems.
Seismic imaging, for example, can reveal deep subsurface faulting and abandoned
wells that might permit leakage by providing a route to the surface, and this
type of examination is expected to become a routine part of storage site evaluation. In addition, some experiments are under way to begin to investigate leakage rates for different types of storage and under a variety of injection conditions.
Several kinds of remediation techniques also need to be explored, including the extraction and purification of contaminated groundwater, the interception and reinjection of leaking CO2, and the removal of stored CO2 for injection elsewhere.
“The available monitoring methods are promising, but more experience is needed to establish detection levels and resolution."”