Abstract
Depleted porous sandstone reservoirs have historically been the preferred option for underground gas storage (UGS). However, in mature gas provinces most suitable sandstone reservoirs have already been converted or allocated, leaving naturally fractured carbonate and tight clastic reservoirs as the principal remaining candidates for conversion. These reservoirs behave in a fundamentally different way from the porous media on which conventional storage design practice was built: storage volume and flow capacity are decoupled, deliverability is controlled by a small number of fault-associated fracture corridors, and the same corridors are simultaneously the pathway for water invasion, the locus of stress-sensitive permeability loss and the plane on which reactivation and leakage risk concentrate. This review synthesises the published evidence on whether, and under what conditions, a fractured gas reservoir can be converted into a technically sound and economically viable storage facility. We first re-derive the site-screening criteria used for porous reservoirs in terms appropriate to fractured media, and compile the documented cases—the Xiangguosi and Sulige carbonate facilities in China, the Loenhout fractured Viséan limestone aquifer store in Belgium and the cancelled Castor project in Spain—together with the Cornegliano facility in Italy, a depleted clastic reservoir included as a benchmark for pre-injection monitoring practice. We then review the multi-scale characterisation workflow (core and vug statistics, image logs, drilling-loss records, seismic attribute and machine learning fracture prediction, discrete-fracture and dual-continuum modelling), and illustrate its geological inputs with a single field example, the Upper Permian Changxing fracture-dominated carbonate reservoir at Tongluoxia in the eastern Sichuan Basin; only geological characteristics are reported, operational data being proprietary. The advantages of fractured reservoirs—very high deliverability per well, rapid pressure communication, small well counts and surface footprints, and a trap and seal already proven by the original accumulation—are set against five recurring challenges: connected-volume and capacity uncertainty, multi-cycle stress-sensitive permeability degradation, fault reactivation and induced seismicity, fracture-guided water invasion, and drilling-induced damage and well integrity. Finally, we review the economics and argue that the unit economics of fractured storage are governed by capacity attainment and cycling frequency rather than by capital cost, and that volumetric storage tariffs systematically under-reward the deliverability that fractured reservoirs uniquely supply. We conclude that conversion is feasible and, in several documented cases, already routine, but that it requires a design philosophy built on fracture-network behaviour rather than on matrix properties, and a monitoring commitment proportionate to the geomechanical exposure of a faulted storage reservoir.