This article discusses the problem of creating a gas storage facility for mixed hydrogen with methane (underground hydrogen storage - UHS) in an aquifer. At the same time, hydrogen is the most expensive valuable substance in the mixture. It is assumed that the hydrogen injection process and its storage life cycle should ensure the most complete re-production of the injected hydrogen even after several years. The distinctive features of injected gas mixture propagation within an aquifer are considered in this paper, taking into account a number of phenomena. For example, the effects of degassing though the caprocks and migration due to natural water flows from aquifers associated with the target formation.

In our previous article SPE 201999 (Abukova et al., 2020) we proposed an efficient storage and subsequent production system that allows to control the geometry and volume of a gas bubble in an aquifer. At the same time, our article (Abukova et al., 2020) considered only flow processes occurring throughout the entire life cycle of the storage facility. Previously, diffusion losses of hydrogen especially through the caprocks were estimated by a number of researchers as being significant. According to the results of large-scale simulation presented in the current article, those losses are estimated as being negligible.

The results of this paper were obtained on the basis of large-scale 3D geological and flow modeling of a multicomponent system in the form of a mixture of hydrogen, methane and water in a porous medium for synthetic 3D models. A compositional approach was applied to describe fluid state. The studies were carried out on a certified flow simulator with an additional option of gas diffusion.

On a 3D reservoir model the losses of hydrogen through the caprocks under conditions of its low permeability were estimated. Similar losses due to multicomponent diffusion were also evaluated. Upper bounds for diffusion losses were obtained straightforwardly. Corresponding study was carried out for the model from paper (Abukova et al., 2020) with maximum spreading of the injected gas over the top of the reservoir. Estimating calculations showed that if the caprocks remains sealing even with its non-zero permeability, the losses of hydrogen through the caprocks were insignificant. Much greater hydrogen reserves spreading were facilitated by its rapid movement along the top of the reservoir. That is why there is a need for a previously substantiated system of various wells on UHS, which support and control the shape of the storage facility.

This article confirms that by creating a pressure barrier controlled due to the concomitant injection of water, it is possible to effectively store hydrogen together with methane in an aquifer. The amount of diffusion losses of hydrogen, as well as its losses through the caprocks, even under the condition of its low permeability, were insignificant. In this regard, the efficiency of creating UHS practically does not differ from the efficiency of creating traditional underground natural gas storage facilities.

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