Quantifying the Limits of CH4–CO2 Hydrate Replacement: Impact of Critical Replacement Thickness, Particle Size, and Flow Rate on Recovery Efficiency

The CH4–CO2 hydrate replacement exploitation method has gained high attention, but its low recovery efficiency hampers its application, and the reported limits of CH4–CO2 replacement vary greatly among different studies. This study introduces the concept of critical replacement thickness to quantify the exploitation limits of CH4–CO2 hydrate replacement, based on a series of CH4–CO2 replacement experiments conducted in a one-dimensional high-pressure reactor with well-defined hydrate-bearing sediments. Real-time hydrate composition analysis was employed to calculate the CH4–CO2 replacement limitation and critical replacement thickness at various stages of CO2 flooding. The results show that during the initial stage, the replacement thickness is approximately 2–4 μm, while it decreases to 0.1–0.3 μm in the final stage. Additionally, the study systematically examines the impact of the hydrate particle size and fluid flow rate on CH4 recovery and CO2 sequestration. It is found that smaller particle sizes and higher flow rates significantly improve recovery efficiency, with CH4 recovery increasing from 39.1 to 63.4% through optimization of these factors. Moreover, the mass transfer resistance created by the reformed CH4–CO2 hydrate film restricts the critical replacement thickness to no more than 7 μm for a particle size distribution of 0–250 μm without additional stimulation. The findings provide a clearer understanding of the factors influencing CH4 recovery and offer insights into optimizing the replacement method for improved efficiency. These results contribute to the development of more effective strategies for the CH4–CO2 replacement exploitation in hydrate reservoirs.