Applicability of the Vertical Equilibrium model to underground hydrogen injection and withdrawal

Underground hydrogen storage (UHS) has emerged as a significant focus for the<br/>energy transition from fossil fuels to renewable energy. Saline aquifers, akin to<br/>those used in geological CO₂ storage, are proposed due to their potential as reservoirs for hydrogen injection and withdrawal over extended periods. Modeling the drainage and imbibition processes at the reservoir scale is critical for efficient UHS implementation. To address the excessive computational costs of three dimensional simulations over hundreds of cycles, low-order models such as the Vertical Equilibrium (VE) models offer practicality. This study centers on the VE model’s applicability to predict hydrogen plume shapes during a single injection and withdrawal cycle, considering varying viscous, capillary, and buoyancy forces, captured by Capillary number (<i>Ca</i>) and Bond number (<i>Bo</i>). Examining<i> Ca </i>values from 1.5 × 10⁻⁹to 1.5 × 10⁻⁷and <i>Bo</i> numbers from 0.1 to 0.3,<br/>we compared VE results to a 2D two-phase simulation, serving as the reference<br/>solution. For small <i>Ca</i>, the VE method accurately predicted both vertical and<br/>horizontal hydrogen plume distributions, although it occasionally overestimated<br/>lateral spread at high injection rates. As the vertical permeability and <i>Bo</i> increase, the VE predictions become more aligned with the reference solution. This is because greater density-driven flow and improved vertical connectivity make the phase segregation easier. Our study also demonstrated the VE model’s promising performance in hydrogen recovery during withdrawal at small <i>Ca</i> and large <i>Bo</i>.