Methane Interface Desorption Mechanism in Geological Reservoirs After Natural Gas Hydrate Decomposition

The morphological transformation of methane hydrates during re-exploitation, caused by depressurization and changes in reservoir environment, significantly impacts hydrate extraction. The present study explores the adsorption and desorption behaviour of methane under different environmental factors by constructing a rock adsorption and desorption device, which can contribute to the efficient extraction of methane during the decomposition of methane hydrates. The findings demonstrate that reservoir temperature strongly promotes methane desorption. Desorption capacity increased markedly above $100^{\circ}\mathrm{C}$, reaching $9.4~\mu\mathrm{g/g}$ at $120^{\circ}\mathrm{C}$, consistent with reduced adsorption enthalpy ($\Delta H_{\text{ads}}$ from $-4.2$ to $-5.2\times10^{-3}$ kJ/mol) and increased Gibbs free energy favoring desorption. In contrast, higher reservoir pressure enhanced the drag force of the driving fluid, with methane desorption showing a sharp increase at pressures around $25$ MPa, although low pressures yielded only marginal improvements. In addition, methane desorption rose from $5.3~\mu\mathrm{g/g}$ in low-CO$_2$ formulations to $8.2~\mu\mathrm{g/g}$ at $90%$ CO$_2$ content, driven by increased drag force (from $2.5\times10^{-3}$ N to $4.3\times10^{-3}$ N) and reduced adsorption energy (from $6.2\times10^{-4}$ N to $4.8\times10^{-4}$ N). Among reservoir minerals, montmorillonite showed the highest methane desorption capacity due to its lowest adsorption enthalpy and strong electrostatic repulsion, followed by illite, while kaolinite exhibited the weakest desorption performance. In conclusion, the present study provides fundamental data for the desorption and efficient extraction of methane leaked into geological reservoirs after the decomposition of methane hydrates.