Evaluating the Impact of Biochemical Reactions on H2 Storage in Depleted Gas Fields

Underground hydrogen storage (UHS) is an emerging technology to store energy, produced by renewable sources, in subsurface porous formations. UHS efficiency in depleted gas reservoirs can be affected by H-2 biochemical degradation due to interactions with rock, brine, and gas. In the reservoir, subsurface microorganisms can metabolize H-2 with possible hydrogen losses, H2S production, clogging, and formation damage. In this work, we investigate the impact of hydrogen losses due to microbial activities on UHS operations in depleted gas reservoirs lying in sandstone formations. We developed a workflow to exploit the chemical reactive transport functionalities of a commercial reservoir simulator, to model biochemical processes occurring in UHS. Kinetic chemical reaction formulation was used to replicate a Monod's type microorganism growth, using PHREEQC to tune reaction parameters by matching a 0D process in an ideal reactor. Then, we applied the methodology to evaluate the impact of biotic reactions on UHS operations in depleted gas fields. Eventually, various sensitivities were carried out considering injection/production cycle lengths, cushion gas volumes, and microbial model parameters. Benchmark against PHREEQC demonstrated that, by properly tuning the kinetic reaction model coefficients, we are capable of adequately reproducing Monod- like growth and competition of different microbial community species. Field- scale results showed that hydrogen losses due to biochemistry are limited, even though this may depend on the availability of reactants in the specific environment: In this work, we focus on gas reservoirs where the molar fraction of the key nutrient, CO2, is small (<= 2%) and the formation is a typical sandstone. Operational parameters (e.g., storage cycle length) have an impact on the biochemical dynamics and, then, on the hydrogen degradation and generation of undesired byproducts. Similar considerations hold for the model microbial growth kinetic parameters: In this study, they were established using available literature data for calibration, but we envisage tuning them using experimental results on specific reservoirs. The current model setup does not account for rock- fluid geochemical interactions, which may result in mineral precipitation/dissolution affecting the concentration of substrates available for biotic reactions. Nonetheless, it can provide an estimate of hydrogen consumption during storage in depleted gas reservoirs due to microbial activities. This study is among the first attempts to evaluate the impact of hydrogen losses by the presence of in- situ microbial populations during hydrogen storage in a realistic depleted gas field. The assessment was performed by implementing a novel workflow to encapsulate biochemical reactions and bacterial dynamic growth in commercial reservoir simulators, which may be applied to estimate the efficiency and associated risks of future UHS projects.