Enhancing the methane production from methane hydrate by cyclic N2–CO2 gas injection and soaking method: Significance of the slow diffusion-controlled process

Methane hydrate (MH) is a crucial lower-carbon energy resource in climate mitigation and current energy transition. A depressurization technique has been mainly attempted to produce methane gas. Simultaneously, the exchange of CH₄ from gas hydrates by N₂–CO₂ has been studied to enhance methane gas production and CO₂ sequestration. We conducted experiments with hydrate-bearing cores by applying the cyclic injection and soaking method. An N₂–CO₂ gas (ca. 60 mol% CO₂) was injected into a core during the injection period, and the core was allowed to stand stationary during the soaking period. Concurrently, a numerical model was developed to simulate the gas production performance of the cyclic injection and soaking method. This model considers a two-stage process for the gas replacement phenomena between CH₄ and (N₂ + CO₂) in the hydrate: an almost immediate replacement at the hydrate surface followed by a more gradual diffusion-controlled replacement in the subsurface hydrate layer. The rapid replacement is modeled by phase equilibrium between the vapor phase and the hydrate surface. The diffusion due to the difference in concentration of each gas component between the surface hydrate layer and the inner hydrate describes the slow gas replacement phenomenon. By repeating four cycles of soaking and injection, the experiments achieved a high CH₄ recovery factor of 67.7 % and a high exchange ratio of 54.7 %. About 18 % of the CO₂ injected gas was sequestrated. The soaking process enhanced methane recovery by 1.5 times in the recovery factor compared to the first injection production and almost half of the CH₄ molecules in MH were extracted. Our simulations demonstrated excellent agreement with experimental results, confirming the soaking process is very efficient for methane recovery. From the production history matching, the diffusion coefficient of CH₄ molecules in the solid-state MH during slow replacement phenomena was estimated to be on the order of 10⁻¹⁹ m²/s, significantly smaller than those of previous research.