CO-OPTIMIZING GROUNDWATER AND GEOLOGIC CO2 STORAGE RESOURCES IN THE UNITED STATES

A growing number of studies echo the conclusion that deep, saline-filled geologic formations (DSFs) likely account for a large fraction of the available geologic carbon dioxide (CO₂) storage capacity in the United States and across much of the world. The distribution of this formation class is such that, coupled with their very large capacities, DSFs likely represent the bulk of the resource for CO₂ capture and geologic storage (CCS) undertaken as part of future societal efforts to address climate change. However, beyond their potential for storage of various waste materials, few other uses currently exist for these DSFs or their largely non-potable waters.

In the U.S., waters with total dissolved solids (TDS) below 10,000 mg/L cannot be used for CCS because they fall below this minimum salinity threshold set by the statutes governing CO₂ injection. Preliminary analysis suggests that low TDS waters may account for over 300 billion tons of potential storage capacity, much of it in areas where there is little existing demand for these waters, and sometimes in places where there are few other storage options. In such cases, potential CCS operators may choose to petition for exemptions to de minimus TDS rules if these waters are also unlikely to be used in the future. At the same time, climate models now suggest an increased likelihood of water stress in certain areas of the U.S. Desalination plants are being built at an increasing rate, with total operational capacity up 41 percent between 2000 and 2005, to augment water supplies in areas facing scarcity concerns. As growing populations and demand outstrip freshwater supplies, and as aquifer drawdown rates begin to exceed recharge rates by increasing margins, these drivers may be pushing society toward a future in which the marginal and lower quality waters present in these candidate DSFs may also be valued as potential supply resources.

Quantitative estimation of the impacts of this type of co-optimization on the available geologic storage capacity will be presented for case studies in the Rocky Mountains and Southwest U.S. regions. These will be used to illustrate the importance of a region-specific approach to resource allocation, and to support a discussion of the broader technical and policy implications of competing demands for the nation’s geologic CO₂ storage resource.