Occurrence frequencies and uncertainties for US underground natural gas storage facilities by state

The frequency of occurrences per facility-year, their probabilities, and their uncertainties are critical inputs to hazard and risk management plans for underground storage facilities. We used Bayesian analysis to investigate and quantify the occurrence frequencies at US underground natural gas storage facilities in porous rock and salt cavern storage facilities. Occurrences for each of the 31 hosting states were classified by storage type, probable cause, and severity. States having the largest number of occurrences at the lowest, nuisance-group level of severity are California and Pennsylvania (for oil-and-gas storage), Iowa and Illinois (for aquifer storage), and Texas (for salt-cavern storage). Those states having the longest operational experience in natural-gas storage, as inferred by the number of facility-years, are Pennsylvania (for oil-and-gas storage), Illinois (for aquifer storage), and Texas (for salt-cavern storage). Bayesian nuisance-group occurrence frequencies are generally within a two order-of magnitude range, 10(-3) to 10(-1) (P5-P95), with greater variability for salt-cavern storage. Seriousand catastrophic-group occurrence frequencies for depleted oil-and-gas storages decrease to 10(-4) to 10(-2) for all states except for California, Colorado, and, for aquifer storage, in Illinois, that remain consistent with their nuisance-group levels. The data do not support previous correlations of high occurrence frequencies with well age or construction practices. Instead, anomalously high occurrence frequencies for individual states might be associated with increased testing of wells and related infrastructure, or particular subsurface conditions that might promote location-specific hazards such as corrosion of well components.