Seismic Responses for Monitoring of Underground Storage of H2-CH4

Currently, large volumes of natural gas are stored annually to allow for rapid withdrawl when needed, and is probable that underground storage of H2 in aquifers and depleted hydrocarbon fields can follow this same path. Indeed, at least initially H2 will be mixed with natural gas to allow for use within the existing municipal pipeline distribution systems. As with CO2 sequestration, it is also likely that active source seismic reflection methods will play a role in monitoring the growth and subsurface conditions within such reservoirs. Fully interpreting these seismic responses, however, also requires full knowledge of the rock and fluid properties. With regards to the latter this is further complicated in that the thermodynamic properties of mixed real gases, here H2 and CH4 say, is not easily determined for the expected subsurface temperature (T) and pressure (P) environment. After reviewing the essential elements of active source seismic monitoring and the seismic properties of mixed fluids, we develop a protocol to extract the fluid density (which for H2 is quite low) and adiabatic bulk modulus, both of which are critical in controlling the seismic wave speeds and impedances. Low frequency Biot-Gassmann theory is then used to calculate the relevant seismic properties of a hypothetical sandstone reservoir subject to differing H2-CH4 concentrations and representative P-T ranges. These are then used with the full Zoeppritz equations to calculate expected seismic responses that would occur in a typical amplitude-versus-offset seismic monitoring program. Generally, the models show that detection of zones saturated with the mixed gas is possible relative to original brine saturated zones, and that the low density of H2 can be influential at high concentrations. However, the weighting of the higher density of CH4 within the mixture will make discrimination of relative concentrations of the gases in situ problematic.