The U.S. has least 3,000 metric gigatons (Gt) of subsurface carbon dioxide storage capacity that is technically accessible below onshore areas and state waters—500 times more than previously estimated—the U.S. Geological Survey (USGS) suggests in a new assessment released on Wednesday.

The Interior Department’s scientific bureau, which studies U.S. natural resources and natural hazards, records its estimates in its first " National Assessment of Geologic Carbon Dioxide Storage Resources." The evaluation of the technically accessible storage resource (TASR) for CO2 for 36 sedimentary basins reveals CO2 storage capacity is far more than the 5.5 Gt estimated by the Energy Information Administration (EIA) in 2011. For comparison, global emissions of energy-related CO2 totaled 31.6 Gt in 2011.

The TASR is an estimate of the geologic storage resource that may be available for CO2 injection and storage and is based on current technology, pressurization, and injection practices, the USGS explains. The evaluation shows the largest potential by far (65%) is in the Coastal Plains region, an area that covers most of Texas, Louisiana, Mississippi, Alabama, and parts of Florida. The Alaska region and Rocky Mountains and Northern Great Plains region each comprise another 9% of storage capacity, while the Eastern Mid-Continent region comprises 8%.

The assessment eliminates all areas with groundwater sources that are considered freshwater. It is also limited to rock layers that have "sufficient" natural seals to prevent CO2 from escaping and is focused only on rock layers at depths at which CO2 would stay under sufficient pressure to remain liquid. However, the study "did not evaluate economic viability or accessibility due to land-management or regulatory restrictions for geologic carbon sequestration within these basins," the USGS admits.

Human-engineered geologic carbon sequestration most often takes the form of pressurizing carbon dioxide into a liquid, then injecting it into the pores of rock formations. The CO2 is injected into only those rock layers with an appropriate seal, so as to ensure long-term storage.

Currently, CO2 is most commonly captured for injection near production wells to purify methane for delivery to consumers and by installing systems in coal-fired power plants to collect the CO2 as it is created and before it escapes into the atmosphere.

The Energy Independence and Security Act of 2007 directed the USGS, in consultation with the Environmental Protection Agency, Department of Energy, and state geological surveys, to conduct a national assessment of geologic storage resources.

According to the USGS, while the Wednesday-released assessment goes further than all previous assessments in considering the viability of sequestration, more research is needed. "This assessment estimates the technically accessible pore space for carbon dioxide storage, meaning it can be developed using today’s technology and standard engineering practices," it says.

"It does not, however, incorporate economic values. An economic analysis of these results will help policy makers and other assessment users better understand the potential development of the resource under various economic conditions." The agency says it will undertake an economic analysis as well as the potential for incremental oil recovery and associated CO2 sequestration in oil fields in the lower 48 states that pass an engineering and geologic screening test for CO2-enhanced oil recovery application.

The USGS will also pair its geothermal energy and earthquake hazards research with the geologic carbon sequestration studies to determine to what extent induced seismicity is a concern. "Considering the sheer quantity of potential CO2 injections into the subsurface rock layers, there is a potential seismic hazard in the form of triggered earthquakes. Other forms of fluid injection have been associated with induced seismicity in the past," the USGS says, adding that it is studying what the possible frequency of triggered earthquakes might be, how large they might be, and how certain engineering practices could reduce or control such earthquakes.

Sources: POWERnews, POWER, DOI, USGS

Sonal Patel, Senior Writer (@POWERmagazine, @sonalcpatel)