Hydrogen loss in depleted carbonate reservoirs: Experimental study of geochemical and mechanical effects

Hydrogen loss during subsurface storage due to physical leakage, geochemical reactions or biological activity, represents a key challenge for the security of large-scale storage operations. Among these processes, geochemical and geomechanical interactions strongly influence the integrity of the host formation. Previous studies have predominantly examined hydrogen behavior in dry or brine-only systems and pure hydrogen environments, often neglecting the effects of cushion gases and the presence of residual hydrocarbons characteristic of depleted oil reservoirs. This work examines the geochemical and geomechanical influences of storing Hydrogen (H₂) using Methane (CH₄) as a cushion gas in a depleted carbonate oil reservoir. Limestone cores containing residual oil and brine were aged in a 5 wt% NaCl brine solution at 70 °C and 2000 psi for two months under a 60:40 hydrogen-methane gas mixture. Analytical assessments performed before and after gas exposure included petrographic, mineralogical, petrophysical measurements, acoustic velocity testing, and fluid compositional analyses. Scanning Electron Microscope (SEM) analysis revealed partial pore coverage by micritic films and fine-grained deposits, while Energy Dispersive Spectrometry (EDS) confirmed secondary calcite overgrowth. Petrophysical analyses indicated porosity reductions of (4–9 %) and moderate permeability increases, with Nuclear Magnetic Resonance (NMR) revealed notable pore occlusion. Gas composition analysis indicated H₂ enrichment (+35.8 %) and CH₄ depletion (−61.8 %). Fluid chemistry exhibited a marked pH increase and elevated Ca²⁺ concentrations, consistent with carbonate surface dissolution. Geomechanical testing recorded only minor variations in Young's modulus and Poisson's ratio, within experimental uncertainty, indicating mechanical stability. Overall, the findings demonstrate that H₂–CH₄ mixture induce limited geochemical reactivity and negligible mechanical weakening in carbonate reservoirs, supporting the feasibility of depleted oil formations as stable and chemically compatible sites for underground hydrogen storage.