Background

The goal of this study was to compare the volumetric and dynamic CO2 storage resource estimation methodologies used to evaluate the storage potential of deep saline formations (DSFs). This comparison was carried out to investigate the applicability and validity of using volumetric methods, which typically require less data and time to apply, to estimate the CO2 storage resource potential of a given saline formation or saline system. The project has showed how different variables including saline extraction (pressure management), geological uncertainty, boundary conditions and trapping mechanisms affect storage capacity. Dynamic modelling also revealed how CO2 storage capacity changes over time.

The project goals were accomplished by applying both volumetric and dynamic CO2 storage resource estimation methodologies to the open-system upper Minnelusa Formation in the Powder River Basin, of the United States, and a closed-system comprising the Qingshankou and Yaojia Formations in the Songliao Basin, of north-east China. These two saline systems were selected because they are representative examples of an open and a closed system. The upper Minnelusa Formation consists of aeolian sand dunes cemented and interspersed with carbonates which act as a single flow unit. The Qingshankou and Yaojia Formations consist of deltaic–fluvial deposits, with good storage properties, separated by lacustrine muds with low storage potential. These formations are representative of a linked stacked storage system and were modeled as one system. Both study areas are in intermontane basins; however, the Qingshankou and Yaojia system does not have areas of discharge and recharge while the Minnelusa does. This results in the Minnelusa Formation acting more as an open system, while the Qingshankou and Yaojia system is expected to behave in more of a closed or semiclosed manner. This contrast adds a further dimension and provides a better comparison between the volumetric and dynamic approaches. The volumetric methodology and open-system storage efficiency terms are described in the U.S. Department of Energy (DOE) Carbon Sequestration Atlas of the United States and Canada (U.S. Department of Energy National Energy Technology Laboratory, 2010, Carbon sequestration atlas of the United States and Canada [3rd ed.]) and the closed-system efficiency terms are described by Zhou and others (Zhou, Q., Birkholzer, J.T., Tsang, C.-F., and Rutqvist, J., 2008, A method for quick assessment of CO2 storage capacity in closed and semiclosed saline formations: International Journal of Greenhouse Gas Control, v. 2, no. 4, p. 626–639). Both these terms were used to estimate the effective CO2 storage resource potential and efficiency in both the upper Minnelusa and Qingshankou–Yaojia systems.

Key Messages

• CO2 storage efficiency starts low, rises quickly, and then levels off in an asymptotic trend to a maximum in much the same way as oil recovery changes in an oil field through time. There is a distinct contrast between an open system, represented by the Minnelusa Formation, and a closed system represented by the Qingshankou and Yaojia Formations. In the Minnelusa Formation it would take 500 years to reach over 50% of the estimated storage capacity whereas this level of capacity could be reached in approximately 50 years in the Qingshankou and Yaojia Formations.

• Care needs to be applied to dynamic storage estimates. The dynamic efficiency method shows that the open aquifer cumulative injection capacity in 50 years is not significantly larger than the closed aquifer one. Consequently, there is a risk that storage capacities could be over-estimated if dynamic conditions are not applied and the properties of ‘open’ and ‘closed’ formations are not taken into account.

• Results from this study clearly show that storage capacity estimates are strongly time-dependent. However, it is important to recognize that a key objective of this study was to determine the maximum storage resource without an arbitrary limited time restriction.

• Additional optimisation operations can be implemented to1) increase the rate at which storage efficiency increases or 2) increase the maximum storage efficiency.
• The dynamic results become roughly equivalent to the volumetric efficiency values after about 500 years. Volumetric efficiency values could be used if enough time were given for CO2 to be injected.

• The biggest single factor that increases storage capacity is extraction of formation saline. There are much bigger differences (P10 – P90) in the modelled capacity of an Open system compared with a Closed system.

• Between 15% to 33% of the injected CO2 could end up in solution in the first 50 years of injection, and this percentage could further increase by up to 16% to 41% after 2,000 years.