Measuring Ultrasonic Velocities and Attenuation of H2 and CO2-Saturated Sands for Deep Saline Reservoirs

Implementing subsurface storage for greenhouse gas (CO2) and clean energy carrier (H2) involves repurposing saline sandstone reservoirs to support clean energy transition. Time-lapse acoustic monitoring, including seismic surveys, sonic well-logging, and vertical seismic profiling, can delineate fluid plume migration and monitor fluid containment through changes in acoustic velocities, acoustic impedance, and attenuation. In addition, attenuation can be used as a seismic attribute to explore, detect, and quantify potential subsurface natural H2 accumulations (white H2). Measuring acoustic properties for monitoring fluids in time-lapse seismic surveys is challenging primarily due to weak seismic signatures, sub-seismic saturation heterogeneity, and lack of calibrated rock physics models. This study investigates changes in ultrasonic velocities and attenuation in H2 and CO2-saturated Fontainebleau sandstone under varying effective stress and temperature conditions. Acoustic waveforms were captured using the pulse transmission method, while attenuation was quantified using the spectral ratio technique. We also conducted NMR, Micro-CT scans, thin-section photographs, porosity, and permeability measurements to characterize the microstructure. Fontainebleau sandstones had a densely cemented matrix (quartz overgrowths) with subrounded to euhedral quartz grains (200-300 𝑢m) with planar to sutured grain contacts. Under isostatic stress conditions, the increase in elastic velocities and reduction in attenuation is attributed to the closure of compliant pores due to increased matrix rigidity as fluid-flow relaxation effects diminish. Attenuation increases with porosity due to the higher wave-induced fluid-flow dissipation associated with a higher volume of compliant pores and fewer grain contacts. Higher applied temperature decreased the elastic velocities and increased attenuation due to reduced gas modulus and increased molecular collisions. Attenuation increases due to increased viscosity when transitioning from the subcritical to the supercritical phase of CO2 at constant effective stress and increasing temperatures. Generally, CO2 exhibited slightly higher attenuation than H2 due to higher viscosity. The low ultrasonic attenuation exhibited by H2 and different phases of CO2 saturation is dominated by minimal microscale wave-induced flow of weakly viscous gases between the compliant and stiffer intergranular pores in a densely cemented matrix. Partial saturation of the supercritical CO2-brine and H2-brine systems led to slightly higher attenuation, attributed to mesoscopic- patchy wave-induced fluid flow. 1.