Hydrogen losses in gas reservoir in the presence of CH₄ cushion gas: Insight into geochemical, petrophysical and geomechanical analysis

Investigating rock fluid interactions is crucial for understanding the interplay between hydrogen, brine, rock formations, and residual gases, especially in the context of storing hydrogen in depleted gas reservoirs. This study investigates hydrogen storage in depleted gas reservoirs condition using Bandera Grey (BG) sandstone with a cushion gas mixture of 30 % CH₄, 5 % N₂, and 5 % CO₂ under controlled conditions of 42 °C temperature, 1350 psi partial pressure, and 5 wt% NaCl brine over 50 days. Comprehensive analyses were conducted to assess the reservoir's geochemical, petrophysical, and geomechanical properties via X-ray diffraction (XRD), X-ray fluorescence (XRF), Scanning Electron Microscopy (SEM), porosity and permeability analysis, and acoustic measurements. pH analysis evaluated fluid composition, while gas chromatography assessed the composition of injected and recovered gases. The SEM, XRD, and XRF analysis results confirmed dolomite dissolution, while illite and kaolinite caused pore blockage in the presence of hydrogen and cushion gas. Porosity increased by approximately 9 %, whereas permeability declined by 10 %. A decrease in acoustic velocity reduced Young's modulus and Poisson's ratio, while an increase had the opposite effect. Gas chromatography results showed minimal hydrogen loss of only 3 %, indicating limited dissolution under experimental conditions. These findings suggest that the selected cushion gas formulation effectively minimizes hydrogen loss, making it a viable option for hydrogen storage in depleted gas reservoirs. This work has significant implications for economically viable H₂ storage solutions and contributes to the advancement of clean energy technologies.