This study presents a field trial of hydrogen storage and geo-methanation in a depleted hydrocarbon reservoir (Lehen, Austria). The trial stored 119,353 m³ of hydrogen mixed with natural gas, achieving a recovery rate of 84.3% after 285 days. Microbial activity converted hydrogen and carbon dioxide to methane, with a conversion rate of 0.26 mmol l⁻¹ h⁻¹ over 14 cycles. This rate could produce 114,648 m³ of methane annually, equivalent to ~1.08 GWh. The study demonstrates the feasibility of hydrogen storage and the importance of geo-methanation in depleted reservoirs for renewable energy storage. The research highlights the need for efficient storage solutions for surplus renewable energy, as peak production and consumption are out of phase. Power-to-gas technology, which converts electricity into hydrogen or methane, is a promising approach. Depleted hydrocarbon reservoirs are suitable for storing gaseous energy carriers, but practical experience with hydrogen storage is limited. However, microbial conversion of hydrogen to methane in these reservoirs is feasible, offering a viable alternative for renewable energy storage. The study shows that hydrogen can be stored in depleted reservoirs with minimal loss, and microbial activity can convert hydrogen and carbon dioxide to methane. The process is stable, efficient, and compatible with existing infrastructure. The results suggest that depleted reservoirs can serve as large-scale storage sites for renewable energy in the form of methane, contributing to a net-zero carbon emissions future. The study also addresses potential challenges, such as hydrogen reactivity with reservoir rock and brine, and the compatibility of methane with existing infrastructure. The results indicate that hydrogen is unlikely to react with reservoir rock or brine under storage conditions, and that microbial activity is effective in converting hydrogen to methane. The study provides insights into the technical feasibility of hydrogen storage and geo-methanation in depleted reservoirs, demonstrating their potential for renewable energy storage.This study presents a field trial of hydrogen storage and geo-methanation in a depleted hydrocarbon reservoir (Lehen, Austria). The trial stored 119,353 m³ of hydrogen mixed with natural gas, achieving a recovery rate of 84.3% after 285 days. Microbial activity converted hydrogen and carbon dioxide to methane, with a conversion rate of 0.26 mmol l⁻¹ h⁻¹ over 14 cycles. This rate could produce 114,648 m³ of methane annually, equivalent to ~1.08 GWh. The study demonstrates the feasibility of hydrogen storage and the importance of geo-methanation in depleted reservoirs for renewable energy storage. The research highlights the need for efficient storage solutions for surplus renewable energy, as peak production and consumption are out of phase. Power-to-gas technology, which converts electricity into hydrogen or methane, is a promising approach. Depleted hydrocarbon reservoirs are suitable for storing gaseous energy carriers, but practical experience with hydrogen storage is limited. However, microbial conversion of hydrogen to methane in these reservoirs is feasible, offering a viable alternative for renewable energy storage. The study shows that hydrogen can be stored in depleted reservoirs with minimal loss, and microbial activity can convert hydrogen and carbon dioxide to methane. The process is stable, efficient, and compatible with existing infrastructure. The results suggest that depleted reservoirs can serve as large-scale storage sites for renewable energy in the form of methane, contributing to a net-zero carbon emissions future. The study also addresses potential challenges, such as hydrogen reactivity with reservoir rock and brine, and the compatibility of methane with existing infrastructure. The results indicate that hydrogen is unlikely to react with reservoir rock or brine under storage conditions, and that microbial activity is effective in converting hydrogen to methane. The study provides insights into the technical feasibility of hydrogen storage and geo-methanation in depleted reservoirs, demonstrating their potential for renewable energy storage.