A critical assessment of the applicability of the Storage Resources Management System (SRMS) to subsurface energy storage

The storage of large quantities of various gases, including hydrocarbons, carbon dioxide and hydrogen, is going to be increasingly important as the energy transition progresses. Large-scale gas storage capacity, which will require some form of underground (geological) storage, will be critical. The necessity to finance various storage schemes requires a rigorous estimation of the ‘bookable’ storage capacity using recognized, auditable standards that a financial institution can depend on. Ostensibly, the requirements of any underground store of gas are similar. However, there are key differences between the requirements of an underground store for carbon dioxide (CO 2 ) in perpetuity when compared to the requirements of an underground ‘energy store’ (where the ability to inject and recover the gas cyclically to meet demand is critical), whether that is natural gas, hydrogen or, even, compressed air. The purpose of this paper is three-fold: (a) to highlight certain key aspects of the existing storage guidelines in the Society of Petroleum Engineers (SPE) CO 2 Storage Resources Management System (SRMS) that were developed specifically for CO 2 ; (b) to highlight key differences between CO 2 storage in perpetuity and underground ‘energy storage’; and (c) to discuss whether the guidelines in the SRMS may be applicable to other types of gas storage such as ‘energy storage’. Similar to hydrocarbon reserves classification (the SPE Petroleum Resource Management System (PRMS)), the SRMS is a project-based system that rigorously defines CO 2 storage in major storage resource classes: Stored, Capacity, Contingent Storage Resources and Prospective Storage Resources, as well as Inaccessible Storage Resources. The basic classification requires the establishment of criteria for the discovery of storable quantities and, thereafter, the distinction between commercial and sub-commercial projects (and hence between Capacity and Contingent Storage Resources). Again, as with the PRMS, there is a range of uncertainty in volume assigned to each class. In all cases, the key attributes of the geological store are the ability to receive the gas efficiently and to trap it effectively. For both natural gas and hydrogen, the ability to recover the stored gas efficiently, and economically, is imperative. In all cases, understanding the characteristics of the host rock (whether a depleted hydrocarbon reservoir, saline aquifer or salt cavern) and modelling the flow of gas and/or fluids through the porous and permeable host is critical. A further key difference is the likely efficiency requirements to inject CO 2 as a supercritical fluid, even though it is extracted from the atmosphere as a gas.