The physics of seismic propagation in aquifers partially saturated with H2. Comparison with the cases of CO2 and CH4

This study presents a methodology to model the acoustic properties of a siliciclastic rock partially saturated with hydrogen (H2), carbon dioxide (CO2) and methane (CH4). The gas properties are obtained with the Peng-Robinson equation of state, and pressure-temperature conditions, based on linear basin modelling with a constant geothermal gradient, are taken into consideration. The dry-rock bulk and shear moduli are obtained with the Krief model, while the Athy equation is used to calculate porosity as a function of depth. Mesoscopic attenuation and velocity dispersion due to fluid effects are quantified using the Johnson model. The viscoelastic Cole-Cole model is used to describe the velocities and attenuation predicted by the Johnson model when synthetic seismograms, with an equivalent viscoelastic rheology, are to be calculated. In this case, the P-wave velocity is calculated as a function of gas saturation using the Gassmann equation and an effective fluid modulus based on the Brie equation. This method can be used for feasibility studies on the geological storage of H2 and CO2 in aquifers and depleted CH4 fields.