Ultrasonic Waves as a Monitoring Tool in Underground Hydrogen Storage Reservoirs

Underground Hydrogen (H2) Storage (UHS) in porous media such as depleted oil and gas reservoirs and saline aquifers has emerged as the scalable and cost-effective solution to buffer intermittency in renewable energy production. As H2 is injected into a brine saturated porous reservoir, brine is displaced by H2, which impacts the propagation of elastic waves through the reservoir. This change in the elastic wave signature, if detectable, can be utilized to monitor the gas plume inside the reservoir. Due to the novelty of UHS, the change in elastic wave properties due to the presence of H2 in rock pores has not yet been characterized. It is crucial to experimentally measure the change in elastic wave signatures due to pore fluid substitution by H2 in reservoir rock for successful monitoring of the H2 plume as well as to detect leakage in a UHS reservoir. As such, we measured the sensitivity of P-wave signatures to the presence of H2 in a Berea sandstone specimen. Our results show that P-wave velocity decreased by 3.5% when H2 saturation inside the sample increased to 36%, compared to a 3 to 5% velocity decrease in the presence of CO2. For the same level of increase in H2 saturation, the P-wave attenuation increased by 66% and the dominant frequency decreased by 7.3%. This degree of change in P-wave signatures due to the presence of H2 is measurable with current seismic monitoring techniques. Combined, our results showed that current 4D seismic techniques could be applied to monitor the migration of gas plumes and detect leakage in a UHS reservoir. Change in ultrasonic P- wave signatures collected in this study at different H2 saturation levels will provide a baseline for future field-scale seismic models.