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Geothermal resources represent precious energy sources to ensure sustainable power generation. As proposed in the majority of the future sustainable energy scenarios, geothermal energy exploitation is going to play a significant role in the energy mix to meet carbon neutrality target. Upon the different technologies involved, geothermal wells constitute the core and turning point for proper fluid/heat mining. Indeed, the number of suitable candidates for geothermal applications could be significantly enhanced by overcoming a series of wells related technological issues. Therefore, the object of this work is to provide a general overview of the principal challenges that characterized well design and construction in geothermal applications which are mainly related to the type of geological system and its relative temperature level. As a matter of fact, reservoir temperature guides most of the choices referring to geothermal systems not only in the selection of the final energy application purpose (direct use, power generation, combined heat and power) but also in well design definition. Based on temperature range, geothermal fields are usually grouped in enthalpy classes (low, medium and high) referring to fields characterized by similar energy potential. From a well design and construction perspective, the low and medium enthalpy classes, in the range of temperature lower than 150 °C, do not present specific criticalities. On the contrary, high enthalpy scenarios, for temperatures higher than 170 °C, present many challenges for most of the current drilling and completion technologies. Even though some field applications exist in high/ultra-high enthalpy scenarios, they still present an elevated risk of failure. Therefore, dedicated studies shall be conducted for all the elements involved in the well construction process such as: drilling fluids, cement slurry, metallurgy, drilling and completion equipment to properly account for their specific technical limitations. In this framework, a clear picture of the actual technical gaps constitutes the starting point for current and next research activities. In the close future, the growing interest in geothermal applications will surely boost the born and development of dedicated tools to unlock the enormous potential of geothermal energy.

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.

Eni, like all the Energy companies, aims to accelerate the reduction of its carbon footprint by implementing the best applicable low-carbon solutions, and hydrogen is surely playing a fundamental role. Hydrogen is the simplest and most abundant element on the planet and an energy vector that has shown great promise worldwide as a solution for meeting climate challenges. This is because it can store and supply large quantities of energy without producing CO2 emissions during combustion. Once produced, the hydrogen needs to be stored for later consumption and here it becomes clear that the importance of Underground Hydrogen Storage (UHS) rises significantly. The main goal of any company is to find all the possible synergies with current activity and business. Even though injection in salt caverns has unquestionable benefits, hydrogen injection in depleted oil and gas reservoirs could be the solution to maximize efficiency and implement a circular economy approach. From an engineering point of view, before developing any Underground Hydrogen Storage (UHS) project, the first step is to analyze and study all the challenges related to this activity. Well suitability must be guaranteed during all the reinjection, in the following document a detail of this topic and especially of one of the main elements that have the role to safeguard the integrity of the well, which is the cement. This document aims to analyze the interaction of standard cement slurries used in oil and gas fields with hydrogen at standard reservoir conditions. The cement-hydrogen interaction tests shown in this document were conducted using autoclave as key instrumentation to simulate reservoir temperature and pressure conditions. The tests were designed and conducted using the methodological approach typical of the materials/fluids compatibility tests. Preliminary autoclave experimentation results show that hydrogen does not alter the chemical and physical characteristics of cement samples. This compatibility study of hydrogen is the first important step to further de-risk activities, the assurance of well integrity is critical and important in all stages to avoid any loss uncontrolled of hydrogen.

Depleted gas reservoirs represent a great opportunity for Hydrogen underground storage(UHS). Eni’s portfolio contains several late-life assets that indeed may provide significant storage capacity. These fields, identified as potential storage sites, are producing for several decades, thus they are accompanied by deep reservoir knowledge and infrastructure availability. Moreover, the caprock, that was able to store hydrocarbon during the past centuries, represents a containment potentially effective also when the reservoir acts as H2 storage site. In the framework of the energy transition, Eni has adopted a robust workflow allowing for the identification of UHS opportunities among its depleted assets portfolio. Starting from a first selection of the sites based on multidisciplinary available data, the most promising opportunities are then deepened through lab analyses, 3D numerical modelling and special studies. The special studies are devoted to understanding interactions among injected H2 and the reservoir fluids/rock but also with the active communities of microorganisms. Accurate studies are also needed to evaluate the effects of H2 during interaction with wells materials. In this paper, we will show the steps that lead Eni to identify and mature UHS opportunities.