Study Explores Storage of Hydrogen/Natural Gas Mixtures in Underground Gas Repositories

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225606, “Numerical Study of Storage of Hydrogen/Natural Gas Mixtures in Underground Gas Repositories: Effect of the Gas Composition on System Behavior and Energy Recovery,” by Tianjia Huang, SPE, George J. Moridis, SPE, and Thomas A. Blasingame, SPE, Texas A&M University. The paper has not been peer-reviewed. _ The objective of this study is to numerically investigate system behavior of H2 or natural-gas (CH4) mixtures (HNM) in aquifer-related underground gas storage (UGS), and the effect of the gas composition and salinity on energy-recovery efficiency. The findings provide valuable insights into engineering decisions associated with the injection, production, and overall behavior of HNM in aquifer-based UGS facilities. Introduction Storing HNM requires careful considerations of reservoir properties, gas behavior, and operational feasibility. H2 is lighter and more buoyant than CH4, which can cause gas stratification within the reservoir. Furthermore, the low density of H2 affects reservoir pressure and gas-expansion rate, requiring greater optimization of storage strategies when compared with CH4-dominated storage. Previous work has identified one of the most important engineering choices as the H2 fraction in HNM. Because of the difference in heating value and density between H2 and CH4, the total injected mass and volume will vary according to the mass fraction of gas components to achieve the same level of energy content. Additionally, different reservoir dynamics are expected as the result of various flow properties of HNM; these dynamics must be evaluated from the perspective of reservoir simulation. Salinity also plays a crucial role in the performance of UGS, particularly in deep saline aquifers. Salinity can affect gas dissolution, wettability, and liquid/gas interfacial tension, further altering gas displacement and gas saturation. Increasing salinity results in lower gas solubility, greater brine viscosity, and higher gas/brine interfacial tension values, all of which influence gas injection, storage, and withdrawal efficiency. Numerical simulation studies aim to investigate the underground storage of HNM using high-resolution grids. A complete 1-year storage cycle, encompassing gas injection, resting, and withdrawal phases, is simulated to, first, compare the reservoir response and energy-recovery efficiency of underground storage of HNM with varying H2 mass fractions but equivalent energy content, and second, to analyze the effect of aquifer salinity on HNM storage operations. This synopsis concentrates on the study’s numerical simulation and problem description, with the results of the study described in the “Conclusions” section; a detailed discussion of the simulation results is provided in the complete paper.