Effects of geological heterogeneity on gas mixing during underground hydrogen storage (UHS) in braided-fluvial reservoirs

Hydrogen, as an energy carrier, is at the centre of research attention for its potential advantages over electricity to transport and store excessive renewable energy at the GW scale as part of the energy transition. To store energy at such a large scale and in a seasonal manner, energy storage technologies such as compressed air storage and high-temperature aquifer thermal storage are proposed, where Underground Hydrogen Storage (UHS) in porous reservoirs may be an important technology for hydrogen economy. Research studies suggest the necessity of using alternative gas to hydrogen as cushion gas due to the very low density at reservoir conditions and the high production cost of hydrogen. In addition, the potentially lower development cost of UHS in existing depleted natural gas reservoirs or former sites for underground gas storage compared to that of saline aquifers makes gas mixing a real possibility for future UHS operations. However, this topic is rarely studied, let alone its geological governing factors. In this study, we focus on the most likely geological settings for early UHS projects (Depleted gas fields in high permeability braided fluvial reservoirs) to understand the potential impacts of geological heterogeneity on project economics. To quantify the possible gas mixing induced by macro-scale heterogeneity and find the dominant factors that affect storage performance, in this study, we start by building synthetic high permeability water-gas reservoir model with geological characteristics of braided-fluvial systems often encountered in the oil and gas industry. Then we examine the effects of structural and litho-facies heterogeneity on gas mixing processes during typical UHS projects (10 mol% hydrogen-methane mixture as stored gas) via compositional numerical simulation. Homogeneous cases with different injection/production rates are also part of the sensitivity analysis. The cumulative days hydrogen fraction in produced stream is used as the metric for quantifying gas mixing during this process. Our results show that, compared to the homogeneous cases, macro-scale geological heterogeneity will intensify gas mixing and degrade the hydrogen fraction in the produced stream, affecting up to 15.8% of the recovery in 10 years (6% more than the homogeneous cases). Geological structure (reservoir dip angle and closure area) is a first-order determining factor above facies heterogeneity (braided channel dimensions). It determines the level of methane breakthrough during UHS projects in all the test cases, leading to contrasting gas mixing behaviors. Our study hereby provides a systematic method for evaluating gas mixing in UHS projects and facilitates future UHS techno-economic analysis.