Self-adaptive gas flow and phase change behaviors during hydrate exploitation by alternate injection of N₂ and CO₂

Since hydrate resources play a part of the stratigraphic framework structure in sediments, establishing a safe and economic method for hydrates exploitation remains the primary challenge to this day. Among the proposed methods, the spontaneous displacement of CH₄ from hydrate cages by CO₂ seems to be a perfect mechanism to address gas production and CO₂ storage, especially in today's strong demand for carbon reduction and replacing clean energy. After extensive lab researches, in the past decade, injecting a mixture of CO₂ and small molecule gas has become a key means to enhance displacement efficiency and has great potential for application. However, there is a lack of in-depth research on gas flow in the reservoir, while the injected gas always passes through low-saturated hydrate areas with high permeability and then occurs gas channel in a short term, finally resulting in the decreases in gas production efficiency and produced gas quality. Therefore, we explored a new injection-production mode of alternate injection of N₂ and CO₂ in order to fully coordinate the advantages of N₂ in enhanced hydrate decomposition and CO₂ in solid storage and heat compensation. These alternate “taking” and “storing” processes perfectly repair the problem of the gas channel, achieving self-regulation effect of CH₄ recovery and CO₂ storage. The 3-D experimental results show that compared to the mixed gas injection, CH₄ recovery is increased by >50% and CO₂ storage is increased by >70%. Additionally, this alternate injection mode presented a better performance in CH₄ concentration of produced gas and showed outstanding N₂ utilization efficiency. Further, we analyzed its self-adaptive gas flow mechanism and proposed an application model of “one injection and multiple production”. We look forward to this study accelerating the application of CO₂-CH₄ replacement technology.