FACTORS AFFECTING CARBON DIOXIDE STORAGE POTENTIAL IN UNMINEABLE COAL BEDS

Global atmospheric carbon dioxide (CO₂) concentration has increased from about 280 ppm in pre-industrial time to more than 390 ppm today. This increase is expected to continue as energy demand continues to increase worldwide. Capture and geologic storage of CO₂ is one approach to reduce the atmospheric CO₂ concentration and its effects on global climate. The U.S. Geological Survey (USGS) is currently assessing the potential national geologic CO₂ storage resource, as directed by the 2007 Energy Independence and Security Act (EISA, Public Law 110–140). Although the present assessment will not address potential CO₂ storage in unmineable coal beds, future assessments may. For this reason, the USGS is currently researching factors that affect the CO₂ storage potential of coal.

Because long-term storage of CO₂ in coal essentially precludes use of the coal as fuel, EISA specifies that only unmineable coal seams will be considered for CO₂ storage. The term “unmineable” is problematic, however, as its definition changes based on economics and technology. A consensus definition of unmineable coal is needed before its potential for CO₂ storage can be estimated.

Carbon dioxide can be stored in coal by sorption and trapping in pore spaces. Fracture and pore permeability in coals allows injection and subsequent filtration of CO₂ into coal beds for storage. Adsorption of injected CO₂ gas causes coal to swell, reducing its permeability and making further injection of CO₂ more difficult. Coal permeability also decreases with depth. At pressure and temperature conditions that occur deeper than about 800 m, CO₂ is a supercritical fluid rather than a gas. Supercritical CO₂ is an organic solvent that can diffuse into and plasticize coal, reducing its permeability and porosity. Research into sorption-induced strain, its effect on permeability and porosity, and supercritical CO₂-coal interaction is ongoing by the USGS, other agencies and/or industry.

Several active and completed field CO₂ injection tests and subsequent monitoring are allowing researchers to build on modeling and laboratory studies and better understand the geologic and engineering factors affecting CO₂ storage in coal. This understanding will help the USGS develop a future methodology for the assessment of CO₂ storage potential in unmineable coal beds.