A Contribution to UHS – Overview of Different Approaches on Well Assessment in Depleted Reservoirs

Abstract

Hydrogen is a carbon-free energy carrier, and it will become one of the main energy carriers in the future sustainable energy system. In an energy system based on renewable energy resources hydrogen will be the energy carrier that can be cost-efficiently transported and stored to deliver renewable energy from remote resource areas, at the right time and place to the energy demand. Regarding the production of hydrogen from renewable energy sources, the problem of hydrogen storage arises. Underground hydrogen storage in salt caverns, deep aquifers, and depleted reservoirs for strategic or seasonal purposes is being considered today. H2 storage in depleted reservoirs is surely the solution that maximizes the environmental and economic benefits, as it would allow the reuse of existing wells and infrastructures after their exploitation lifetime, minimizing emissions and investment/operating costs. In this type of storage, well integrity plays a significant role. The wells must be able to withstand extreme conditions and various loads during the entire service life. Considering the small size of the molecule of H2 and its strong diffusion, the impact of hydrogen on steel materials, elastomers, and cement shall be deeply evaluated, to avoid corrosion phenomena or stresses led by a combination of H2 at different temperatures and pressures.The paper aims to assess the hydrogen impact on the different materials of wells (metals, elastomers, and cement), with a particular focus on finding technical gaps in materials installed in already existing wells. This scope of work will be carried out through the analysis of data available in the literature and technical discussions with some of the Oil & Gas main Service Contractors. At present, API steel alloys normally deployed in Underground Gas Storage facilities are not tested specifically for the storage of hydrogen or hydrogen-enriched natural gas mixtures.As per literature analysis, streams with H2 concentration less than 0.5% look not to be critical for standard well equipment. On the other hand, when percentages of hydrogen get higher than 10%, only a few studies are available and it looks that the H2 effect cannot be considered negligible, leading to the consideration that well components in direct contact with the fluid shall be hydrogen resistant. However, due to the lack of available data, ad hoc tests are recommended.