A GEOLOGICALLY-BASED PROBABILISTIC METHOD FOR ASSESSING TECHNICALLY ACCESSIBLE CO2 STORAGE RESOURCES

For geologic sequestration to play a significant role in reducing CO₂ emissions typical estimates indicate that at least 1 billion tonnes (GT) of CO₂ will need to be stored in the U.S. per year by the year 2050. To store this amount of CO₂ and deploy carbon capture and storage (CCS) at this scale, methods are needed to rapidly assess regions with large potential storage sites that may have a high probability of CO₂ retention for thousands of years. Once these regions are identified and assessed, then individual sites can be evaluated in detail to identify the storage capacity for individual carbon capture and storage CCS projects.

The USGS has developed a geologically-based probabilistic method for assessing the size and number of potential storage sites within a storage assessment unit (SAU) at the regional or basin scale. An SAU consists of both a porous flow unit for injection and storage of CO₂ and an overlying seal that is a barrier for vertical migration of a buoyant CO₂-rich phase. The methodology for assessing the potential storage of an SAU is based on the storage resource volume of physical traps (PTs, e.g. oil and gas reservoirs) as determined on both a fluid production and volumetric basis as well as the storage resource volume of the saline formation (SF) which is the porous flow unit between PTs within an SAU. The model produces results that can be used to estimate the amount of technically accessible storage resources volumes available using present-day technology.

Probabilistic ranges of storage resources volumes within an SAU are calculated using Monte Carlo simulations of potential storage size in individual PTs and the SF. These calculations include the range of uncertainty for all volumetric parameters. Monte Carlo methods are also used to aggregate the storage resource in all PTs and to evaluate the probability of successful storage, which is defined as storage of a mass of CO₂ greater than a prescribed minimum storage size in both PTs and the SF.

The concept of minimum storage size is a critical component of the assessment methodology, as it allows for the evaluation of SAUs based on the probability of occurrence of at least one storage site greater than the minimum and an estimate of the number of sites greater than the minimum. The minimum size can be based on an estimate of the total storage required by an individual CCS project.