Study on the mechanisms and injection-production characteristics of converting depleted reservoirs into oil storage facilities

In the late stage of oil development, a significant number of depleted reservoirs approach to be exhausted. These reservoirs not only occupy subsurface space but also pose safety and environmental risks. Inspired by the model of gas storage facilities, converting such reservoirs into oil storage facilities offers dual benefits of enhancing resource utilization and mitigating environmental hazards. However, substantial differences exist in the injection-production mechanisms and dynamic features between oil and gas storage facilities, with limited systematic research conducted domestically or internationally. This study systematically investigates the feasibility and key injection-production characteristics of converting such three typical depleted reservoirs as water-flooded sandstone reservoirs, fractured buried-hill reservoirs, and heavy oil reservoirs (post-SAGD development) into oil storage facilities, through oil-water mutual displacement seepage simulation experiments and 3D physical simulation experiments. The findings demonstrate that multi-cycle oil-water mutual displacement lead to systematic downward shift in oil-water relative permeability curves of water-flooded sandstone and fractured buried-hill reservoirs, continuous declines in endpoint effective permeability, and gradual reduction in injection-production capacity, as well as progressive increases in residual oil and irreducible water saturation, which narrows the co-percolation range and reduces effective pore utilization. During oil injection, residual oil and irreducible water retention in pores causes declining injection-production volumes, yet the oil recovery efficiency remains above 90%, with higher permeability correlating to superior recovery efficiency. For thick massive heavy oil reservoirs, three-dimensional physical simulations experiments of post-SAGD injection-production confirmed stable oil production after two cycles, achieving a recovery efficiency exceeding 84%, thereby validating the technical feasibility of converting high-porosity, high-permeability heavy oil reservoirs into oil storage facilities. This research establishes a theoretical foundation for site selection and parameter design in converting depleted reservoirs into oil storage facilities.