Assessing the impact of oxygen on rock mineralogy and fluid composition for subsurface biomethane storage in porous reservoirs

Biomethane is an environmentally friendly alternative to natural gas and is regarded as a key energy source for aiding the decarbonization of the energy system. The urgent need to transition to clean energy has driven the demand for large-scale storage of alternative energy carriers, such as biomethane, in subsurface porous reservoirs. Biomethane typically contains oxygen as an impurity (up to 1%), yet the potential impact of oxygen on reservoir rock integrity and subsurface fluid composition during storage remains poorly understood. This study presents a comprehensive geochemical investigation, combining experimental and modelling approaches, to evaluate oxygen’s impact on rock mineralogy and fluid composition at two potential subsurface storage sites with distinct rock properties and mineralogy.

Batch-reaction experiments were conducted under worst-case scenarios, including a high fluid-to-rock ratio and elevated oxygen partial pressures (~3%). Three different experiments were performed for each site: (1) oxygen-brine-rock, to directly evaluate oxygen-brine-rock reactions; (2) nitrogen-brine-rock, to isolate the influence of oxygen; and (3) oxygen-brine, to assess oxygen’s impact on fluid composition alone. Fluid samples were collected regularly during the experiments and analysed alongside pre- and post-experimental fluids to assess changes in ion concentrations. Mineralogical analyses of pre- and post-experimental rock samples were also performed to identify any changes in rock composition.

Fluid analysis shows relatively higher increases in potassium and iron concentrations in the oxygen-brine-rock experiments compared to the nitrogen-brine-rock experiments, suggesting slight dissolution of K⁺-bearing minerals. However, the changes were marginal considering the amount of these minerals present in the rock. Other ions, including Ca²⁺, Mg²⁺, Na⁺, and SO₄²⁻ , exhibit minimal changes, primarily attributed to brine-rock interactions rather than reactions involving oxygen.

Mineralogical analysis shows negligible changes in bulk rock composition, with major minerals such as quartz, calcite, and K-feldspar remaining stable. Minor changes in clay minerals, such as slightly increased kaolinite and decreased illite/smectite, were consistent across both gas-brine-rock experiments, indicating that oxygen does not cause significant mineralogical alterations. Geochemical modelling corroborated the experimental findings, showing that oxygen has no long-term negative impact on rock mineralogy.

These results demonstrate that the presence of oxygen in biomethane has a minimal effect on reservoir rock and fluid stability, supporting the geochemical feasibility of subsurface biomethane storage. Moreover, the findings suggest that existing regulatory oxygen limits could be slightly relaxed for subsurface biomethane storage, facilitating a smoother transition to this alternative energy source.