Chemically Modified Hydrate Swapping and Hydrate Stability during Multistage CO₂-N₂ Injection Schemes

A core-flooding setup under high-pressure and low-temperature conditions was utilized to create artificial sedimentary CH₄ hydrates, that were subjected to multistage injections of fresh CO₂-N₂ gas mixtures to study CH₄ recovery yield, CO₂ storage, and hydrate stability in response to various chemical treatments. The effect of chemical type (methyl alcohol, sodium dodecyl sulfate, and L-methionine), number of fresh injections, injection pressures, and influence of CO₂ concentration were examined to support the sustainability and efficacy of the process. Electrical resistivity was also measured to investigate hydrate stability under different injection schemes and chemicals. Higher recovery and storage were observed in the presence of additives (compared to pure water) was observed, suggesting that enhanced but delayed formation kinetics at the gas/liquid interface and unstable mixed hydrate reformation were the major contributing factors. During gas injection, the gas/liquid boundary moved within the pore space, and nucleation initiated at this interface. Therefore, chemical additives were used to enhance, but delay, formation kinetics at the injected-gas/residual-water boundary. This caused release of additional in-situ heat, that dissociated CH₄ hydrates, increased the total contact time, and improved gas diffusion. A porous morphology in the presence of chemicals may also provide additional pathways through hydrate films for improved gas migration. The number of fresh CO₂-N₂ injections increased CH₄ recovery, and the efficiency of the injection scheme was dependent on initial hydrate saturation. No significant reduction in hydrate stability was observed after the first injection of dilute CO₂ gas mixture. Multiple injections of fresh CO₂-N₂ gas may further enhance the hydrate stability.