Geologic Risk and Uncertainty for Underground Storage of Buoyant Fluids, Lessons Learned in Illinois

Abstract This paper describes selected natural gas storage and CCS case studies from Illinois USA, their general applicability to subsurface buoyant fluid storage, and summarizes lessons learned. In Illinois, a 70-year history of sustained natural gas storage has provided a foundational understanding of risk associated with using the subsurface for storing buoyant fluids. In addition to natural gas storage, over 3.0 million tonnes of CO 2 have been injected into the Cambrian Mt. Simon Sandstone at the Decatur, Illinois CCS project site. In Illinois many of the storage projects are in the Mt. Simon Sandstone, a saline (aquifer) reservoir. In gas storage fields the characterization of faulting and seal properties has historically not been adequate. Identification of Precambrian paleotopography is essential when completing initial site assessment in reservoirs close to the Precambrian contact. The presence of basement paleotopographic highs increase the risk of not having a storage reservoir and in an increased risk of induced seismicity. The Cambrian Mt. Simon Sandstone of the Illinois Basin has some of the most suitable strata in the midwestern USA for the sequestration of carbon dioxide (CO 2 ) and storage of energy in the form of natural gas. The improved understanding gained from natural gas storage and from carbon sequestration is also directly applicable to assessing the potential of emerging energy storage technologies such as compressed air energy storage (CAES) and geological hydrogen storage.