Intermittent Ultrasonic-Assisted CO₂ Replacement for CH₄ Extraction from Hydrate-Bearing Clay-Silt Sediments

Natural gas hydrate (NGH) mining via CH4/CO2 exchange integrates CH4 extraction with CO2 geological sequestration, thereby facilitating the achievement of carbon neutrality. However, the relatively low replacement cost remains a significant bottleneck that hinders its large-scale application. In this study, we investigated the potential of intermittent ultrasonic assistance to enhance CH4/CO2 replacement in hydrate-bearing sediments. The effects of ultrasonic power, experimental pressure, and temperature, as well as initial water and hydrate saturations, were systematically analyzed. Results demonstrated that both CH4 recovery and CO2 sequestration rates exhibited an increasing trend with higher ultrasonic power. Specifically, at 100% ultrasonic power, the CH4 recovery and CO2 sequestration rates reached 79.06% and 55.94%, respectively. Additionally, both rates increased as the pressure decreased and the temperature rose. Notably, ultrasonic assistance significantly improved CH4/CO2 replacement even under conditions where the experimental pressure exceeded the CH4 hydrate phase boundary, or the temperature was below the freezing point. Increased initial water saturation was found to be beneficial for the ultrasonic-assisted CH4/CO2 replacement, whereas a higher hydrate saturation negatively impacted the process. These findings provide valuable insights into enhancing CH4/CO2 swapping and contribute to the advancement of NGH mining and CO2 geological sequestration technologies.