Understanding the Seismic Response to Varying CO2 Saturation through Concurrent Ultrasonic Monitoring and X-ray CT Imaging in Laboratory-scale Flow Experiments

Tracking CO2 injection and mapping fluid saturation in carbon sequestration sites are paramount to effective site management and long-term storage safety. Time-lapse active source seismic survey is one of the promising tools for monitoring CO2 migration in the subsurface. An assessment of CO2 saturation from seismic data necessitates establishing relations between the seismic attributes and rock and fluid properties. Controlled laboratory experiments provide high-quality data to enable building such relations. Here, we report on a series of laboratory-scale CO2-brine core flood experiments using a synthetic porous sample (sintered glass) at supercritical conditions to track and map CO2 phase saturations through different data acquisition modalities. Two shear wave ultrasonic transducers (center frequency ~ 500 kHz) embedded within an x-ray transparent Temco core holder enable the multi-modal measurements. The first experiment consists of injecting CO2 gas, brine, and live CO2-saturated brine at varying flow rates (ranging from 0.5 cc/min to 2 cc/min). The ultrasonic response and acoustic emission are recorded for each flow rate. This is followed by a supercritical CO2 flood (1 cc/min) to reach an irreducible brine state while conducting continuous ultrasonic monitoring. The second experiment consists of injecting supercritical CO2, brine, live CO2-saturated brine, a supercritical CO2 flood for reaching irreducible brine, and CO2-saturated brine flood for reaching residual supercritical CO2. At each stage, the sample is x-ray CT scanned and the ultrasonic response is recorded. In addition, the evolution of the states to reach irreducible brine, and finally to residual supercritical CO2 is continuously monitored through time-lapse active source ultrasonic monitoring. Our findings to date demonstrate that the injection of supercritical CO2 results in significant changes in the recorded ultrasonic signals. Specifically, we observe a stark reduction in signal amplitude and a significant phase shift. The ultimate goal is to relate the ultrasonic attributes to the saturation states of the medium extracted from x-ray CT images such that we can predict the extent of CO2 trapping from ultrasonic data, when x-ray scans are not available.