Fracture seismic response to CO2 injection: Ultrasonic and low-frequency shear measurements of a natural dolomite fracture from Kevin Dome, north central Montana

Time-lapse seismic monitoring (4D) is currently the primary technique available for tracking sequestered CO2 in a geologic storage reservoir away from monitoring wells. The main seismic responses to injection are those due to direct fluid substitution, changes in differential pressure, and chemical interactions with reservoir rocks; the importance of each depends on reservoir/injection properties and temporal/spatial scales of interest. Many proposed reservoirs are dominated by fracture permeability but few laboratory scale measurements have been made on natural fractures under reservoir conditions. We use ultrasonic measurements under reservoir confining and pore pressure conditions to understand how the seismic properties of natural fractures change with stress and fluid substitution due to CO2 injection. We also utilize a unique low-frequency shear modulus and attenuation apparatus to compare the stress and frictional seismic signature between natural and artificial (well-mated) fractures. These measurements can be applied directly to monitoring the injection and fate of sequestered CO2 in fractured carbonate reservoirs, such as the source of our core - Kevin Dome, Montana.