A thermodynamic and technical feasibility study of the subsurface storage of energy in the North Sea abandoned reservoirs

Abstract

The oil and gas extraction from the Danish sector of the North Sea will soon end. Simultaneously, the existing and in development offshore windmills in the North Sea will ensure the availability of abundant and cheap electricity in the region yet fail to address the intermittent nature of the wind resources. This paper hypothesizes that the surplus electricity in the windy days and off-peak time can be converted to chemical forms (synthetic fuels, e.g., hydrogen, ammonia, methanol, or methane) and stored in the vast space of the abandoned oil and gas reservoirs under the North Sea. The stored energy can be extracted and consumed as carbon-neutral fuels or converted back to electricity when there is a shortage of wind. This work studies the technical and thermodynamic feasibility of offshore conversion of electricity to chemical energy sources and their storage/extraction in/from the North Sea oil and gas reservoirs. The technical feasibility study briefly deals with the sufficiency of the existing infrastructures including platforms and surface facilities to accommodate the process equipment for the conversion of electricity to chemical storable forms. Several processes including nitrogen and carbon dioxide separation from the atmosphere, electrolysis of seawater, and reduction of CO 2 and N 2 to synthetic fuels are simulated in a commercial process simulator. The thermodynamic analysis quantifies the exergy loss during the offshore conversion and storage/extraction of chemical energy in the reservoirs based on the results of an in-house open-source dynamic model that simulates the multi-component non-isothermal flow of fluids in the subsurface. The results show that the gas reservoirs with a higher permeability can store the large amount of energy that is required to address the wind energy fluctuation. However, depending on the future supply of wind electricity, a large energy loss in the process should be expected. Several technical problems regarding the electrolysis capacity, hydrogen turbines, and efficiency of compressors need to be addressed. Moreover, the estimated discharge rate from the subsurface storage can be too slow for addressing urgent surges of the electricity demand. Offshore subsurface storage seems to be a more suitable solution for the storage of chemicals to be used as transport fuel or raw materials that do not require high discharge rates.