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

Abstract

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.