Analytical Investigation of Subsurface Gas Storage Potential in Depleted Reservoirs in the Anambra Basin

Underground gas storage (UGS) in depleted reservoirs provides a strategic mechanism for seasonal supply balancing and supply security in gas distribution systems, and the Anambra Basin's geological characteristics warrant systematic evaluation as a potential storage location for southeastern Nigerian gas supply management. This study analytically investigates the subsurface gas storage potential of depleted gas reservoirs in the Anambra Basin, drawing on well completion data, production history, and seismic interpretation from the basin's exploration and production record. The research employs a reservoir assessment methodology combining volumetric estimation of storage capacity, injectivity potential modelling using reservoir simulation, geological risk assessment for cap rock integrity, and proximity analysis of the assessed structures relative to the national gas pipeline grid. Specific attention is given to the storage development challenges unique to the Anambra Basin, including shallow reservoir depths limiting working gas pressure ratios, the presence of multiple thin pay zones requiring selective perforation management, and the limited well inventory available for conversion to injection and withdrawal service. The study fills a critical gap in Nigerian gas infrastructure planning literature, as subsurface gas storage has been absent from national gas sector development plans despite growing supply variability challenges. Findings identify three depleted gas accumulations as technically viable candidates for pilot storage development, with combined working gas capacity estimated at 8 to 12 billion standard cubic feet. Economic modelling suggests storage development is commercially viable under long-term seasonal gas price differentials above 0.15 USD per MMBtu. Recommendations include detailed feasibility studies for the identified structures and integration of UGS development into the National Gas Policy framework. Keywords: underground gas storage, depleted reservoir, Anambra Basin, reservoir simulation, gas supply security.