Static storage estimates for depleted gas fields using volumetric and production approaches

• Provides a comparative analysis of volumetric and production-based methods for estimating static storage in depleted gas fields. • Demonstrates practical application using real field data from a mature gas reservoir. • Identifies key uncertainties and limitations associated with each estimation approach. • Supports decision-making for subsurface storage projects such as CO 2 sequestration or gas storage. Depleted gas fields provide a promising option for geological CO 2 storage because of their proven trap integrity, large pore volumes, and existing infrastructure. In this study, static CO 2 storage capacity was estimated for a representative mature gas reservoir using two complementary approaches: the volumetric method and the production-based method. The volumetric approach integrates structural dimensions, petrophysical properties, and reservoir fluid parameters to calculate effective pore volume. At the same time, the production-based method quantifies storage potential as the pore space vacated by produced gas. Both methods were applied using consistent input ranges for porosity, initial water saturation, formation volume factor, recovery factor, and CO 2 density under reservoir conditions. The estimated capacities were 4.58–5.99 Mt for the volumetric method and 4.53–6.76 Mt for the production-based method. The close agreement between the two approaches increases confidence in the results and highlights the key influence of porosity, water saturation, and recovery factor on uncertainty. These findings provide a transparent and reproducible framework for preliminary screening of depleted gas reservoirs as candidate CO 2 storage sites. It is emphasized that the estimated capacities represent static, upper-bound values suitable for preliminary screening and do not account for dynamic constraints such as pressure buildup, injectivity, plume migration, or geomechanical effects.