Underground hydrogen storage (UHS) in porous reservoirs offers a promising solution for addressing the temporal and spatial variability of renewable energy sources. To safely and efficiently utilize these reservoirs for UHS at an affordable cost, it is essential to understand the diverse disciplines involved at various scales. This study combines large-scale techno-economic evaluations with micro- scale laboratory research to assess the commercial and technical viability of UHS in porous media. A techno-economic case study of commercial-scale UHS in a depleted gas field in the Dutch nearshore is investigated, defining the integrated scope of infrastructure and facilities and estimating associated costs. Additionally, the multiphase flow behavior and redistribution of hydrogen in sandstone rock are analyzed through experimental assessment of relative permeability, capillary trapping, dissolution, and Ostwald ripening using CT imagery. The case study compares various surface facility design concepts, revealing that the Levelized Cost of Hydrogen Storage (LCOHS)
ranges from [cannot be disclosed for confidentiality reasons] , depending on the required purification scope of the production stream. Cushion gas cost significantly impacts the LCOHS. The multiphase flow behavior was examined on a micro-scale by co-injecting hydrogen and brine at varying fractional flows into a vertically placed 17 cm Berea sandstone rock sample at 25°C and 50 bar, while observing under CT imaging during both drainage and imbibition. High- resolution CT imaging visualized Ostwald ripening at the pore scale after several periods without flow. The experimental results indicate a hydrogen end-point relative permeability of 0.043 and a linear trapping coefficient of 0.725. No preferential flow paths were observed, however, dissolution was shown to have an impact the saturation profiles. Pore-scale image analysis demonstrated that Ostwald ripening leads to the fragmentation of mid-sized hydrogen ganglia and the growth of larger ganglia over time. These findings provide valuable insights into optimizing UHS and provide input for large-scale reservoir simulations, emphasizing the importance of integrating techno-economic assessments with detailed laboratory research for the commercial success of UHS.

Overcoming the intermittency problem of renewable energy is an issue that has to be addressed in order to achieve an efficient energy system. Hydrogen has been gaining interests, as green energy carrier, which is included in the energy transition plans of not only the Netherlands, but worldwide. Due to its physical characteristics, large scale storage of hydrogen requires volumes that can only be provided by porous media in the subsurface. Underground Hydrogen Storage (UHS) in depleted gas fields is a large scale storage possibility that is generating attention and increased research. However, it is currently in a Low Technology Readiness Level, meaning that more research, and especially field scale projects are necessary. There is very few literature that tries to use alternative gases as a cushion gas for a UHS. Therefore, this investigation is relevant for the future development of UHS. This investigation will use CMG GEM, a commercial compositional reservoir simulator to model the fluid interactions in the subsurface when hydrogen is stored in depleted gas fields. This will be done by means of a sensitivity analysis, where the hydrodynamic behaviour between hydrogen and possible alternative cushion gases, such as methane, carbon dioxide and nitrogen are studied. Apart from the technical analysis, a simplified economic evaluation is used to calculate a levelized cost to store hydrogen, which will allow for an economic optimized selection of cushion gases in an UHS. The two main constraints for an UHS system are the purity of the extracted hydrogen and the rate at which it is extracted from the subsurface. The results of this investigation show that the mixing of hydrogen with an alternative cushion gas will change drastically based on the degree of the reservoir heterogeneity and the location of the perforated interval. This ever-increasing mixing will have an effect on the capability of the system of delivering pure hydrogen for the expected time.