Hydrogen-induced alterations in oil composition and reservoir properties: Experimental insights into underground hydrogen storage

Global energy demand continues to rise (+2.2% in 2023), with oil, coal, and gas still dominant. Hydrogen is increasingly viewed as a renewable fuel, yet its underground storage faces critical challenges due to its low density and uncertain geochemical interactions. This study presents the first systematic experimental investigation of hydrogen injection impacts on reservoir rock, caprock, and oil–brine systems under low- and high-pressure conditions, combining geochemical analysis, GC–MS (Gas Chromatography-Mass Spectrometry), viscosity, density, and advanced CT (Computed Tomography ) imaging at the Australian Synchrotron. Results reveal that in brine-saturated reservoir rocks, porosity increases slightly (∼10%), while caprock shows extreme variations (∼600%) caused by pyrite and calcite dissolution and grain expansion. In contrast, oil-saturated carbonate rocks undergo severe porosity loss (∼96%) and hydrocarbon restructuring: long-chain fractions are depleted, short-chain alkanes increase, crude oil becomes lighter, and hydrogen solubility declines. A dedicated image-processing workflow was also developed to quantify porosity in highly heterogeneous lithologies. These findings demonstrate, for the first time, hydrogen's dual behaviour: chemically inert in sandstones but strongly interactive in oil-bearing carbonate systems. This novelty provides critical mechanistic insights for the selection, risk assessment, and long-term design of underground hydrogen storage sites, ensuring safe deployment of hydrogen as a key energy carrier.