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USGS assesses carbon dioxide storage Says that U.S. underground rock formations could sequester more than 500 times the nation’s annual production of the greenhouse gas By Alan Bailey Petroleum News
In a turnaround from its usual oil and gas assessments, estimating the volumes of undiscovered hydrocarbon resources in geologic basins, the U.S. Geological Survey has issued a report focusing on the exhaust gas end of fossil fuel usage, assessing the quantities of carbon dioxide that might be sequestered in underground rock formations.
According to the report the United States has the potential to sequester some 3,000 metric gigatons of carbon dioxide in 36 geologic basins around the country, an amount equivalent to more than 500 times the total U.S. carbon dioxide emissions in 2011. Geologic reservoirs under Alaska’s North Slope could possibly contribute 270 gigatons to that overall U.S. total, the USGS report says.
Addressing climate change The USGS said that its report comes in the wake of a recent national plan announced by President Obama to address global warming. Many scientists view human generated carbon dioxide as the prime culprit in a rapid rate of warming of the earth’s climate and the underground sequestration of carbon dioxide exhaust from industrial facilities presents one possible means of addressing the climate change predicament.
“The USGS research is ground breaking because it is the first realistic view of technically accessible carbon storage capacity in these basins,” said Interior Secretary Sally Jewell when announcing publication of the report. “If enough of this capacity also proves to be environmentally and economically viable, then carbon sequestration could help us reduce carbon dioxide emissions that contribute to climate change.”
Technically feasible The assessment only considers reservoirs that appear technically feasible candidates for carbon dioxide storage. For example, the USGS scientists excluded the possibility of storing carbon dioxide in shales or low-permeability sandstones, rock types for which little is known about their carbon dioxide storage characteristics. Areas with the potential for contamination of underground freshwater resources were also excluded, the USGS report says.
However, the assessment did not consider the economic viability of carbon dioxide sequestration. Nor did it consider issues relating to land access or regulatory restrictions.
The economic viability of current operational carbon dioxide sequestration systems has tended to depend on countering the cost of sequestration by, in particular, using the carbon dioxide for enhanced oil recovery in producing oil fields. Also, the imposition of a cost to carbon dioxide emissions through, say, a carbon tax or a carbon cap-and-trade system can significantly impact the economics of carbon dioxide storage. Economic feasibility is also presumably related to the distance between the carbon dioxide source and a carbon dioxide storage location.
Assessment units In carrying out their assessment, USGS scientists identified a series of “storage assessment units,” volumes of porous reservoir rock bounded by impervious rocks that could trap carbon dioxide within the reservoir. And the scientists considered two types of potential carbon dioxide entrapment: buoyant trapping in which carbon dioxide “floats” towards the top of a geologic structure in a reservoir formation, with a seal rock preventing the gas from escaping; and residual trapping involving the holding of carbon dioxide by capillary action within individual rock pores.
For each assessment unit, the scientists specified possible ranges of values for various parameters that would determine the amount of carbon dioxide that the unit might store. The use of these ranges for making storage estimates led to a range of possible quantities of stored gas and a mean size for the unit. The estimates for individual assessment units were then statistically combined to derive estimated ranges and means for the possible storage capacities for different geologic basins and regions across the country, and for the nation as a whole.
Major regions It turned out that a region termed “the Coastal Plains,” along and inland from the Gulf Coast, showed by far the biggest potential of any major U.S. region for carbon dioxide sequestration, with a mean estimated storage quantity of 2,000 gigatons. The Alaska North Slope’s estimated potential of 270 gigatons shared second place in the ranking of U.S. regions with a region encompassing the Rocky Mountains and the northern Great Plains. The eastern and western mid-continental regions also showed major carbon dioxide sequestration potential.
And it appears that across the United States 89 percent of the storage capacity would come from the residual trapping of carbon dioxide, rather than from buoyant trapping, the report says.
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