Numerical Studies of Hydrogen Loss During Underground Hydrogen Storage in Saline Aquifers

Abstract This study presents a three-dimensional (3D) numerical model developed to quantify hydrogen loss during underground hydrogen storage (UHS) in saline aquifers. The primary objective is to evaluate the efficiency of UHS by analyzing the hydrogen recovery factor, which serves as a key metric for assessing storage performance. The model simulates a heterogeneous, isothermal saline aquifer composed of 16 layers and includes two wells, one injection and one production well. A total of 100 storage-retrieval cycles were simulated, each consisting of an injection and a shut-in period. The model accounts for permeability anisotropy and fluid-rock interaction effects, particularly through relative permeability functions, to examine their influence on hydrogen recovery. This study focuses on quantifying the amount of lost hydrogen, distinguishing between recoverable and non-recoverable losses based on the connate gas saturation threshold. Hydrogen recovery was evaluated using both cyclic and cumulative recovery factors, calculated from the injected and produced hydrogen masses. The cyclic recovery factor (CyRF) was consistently higher than the cumulative recovery (CRF) factor, while the latter exhibited a monotonically increasing trend throughout the cycles. The difference between cumulative injected and produced masses defines the total hydrogen loss within the reservoir. Not all the lost hydrogen was recoverable; classification based on connate gas saturation revealed that a significant portion of the lost hydrogen remained non-recoverable due to residual trapping and other reservoir retention mechanisms. The novelty of this work lies in applying classical reservoir engineering principles to quantify and classify hydrogen losses during UHS operations. The findings highlight the substantial role of residual trapping in the retention of hydrogen, emphasizing its contribution to the non-recoverable fraction.