Kinetic reaction performance of methane hydrate under different unsaturated seepage scenarios in high-pressure sediments

Methane hydrates (MH) represent a massive unconventional clean energy resource, making the understanding of their multiphase phase-transition dynamics within porous media critical for safe extraction and geological carbon storage. While macroscopic static systems and steady-state flows have been extensively studied, existing kinetic models largely rely on full-saturation assumptions, which fail to capture the complex, unsaturated multiphase seepage environments characteristic of natural MH reservoirs. To address this limitation, we investigate the deep coupling mechanisms between unsaturated multiphase hydrodynamics and crystallization kinetics by simulating the spatiotemporal evolution of MHs under designed boundary and thermodynamic conditions. Employing a comprehensive high-pressure multiphase flow-reaction framework, we integrate the kinetic model with van Genuchten soil–water characteristics (swcc) and multiphase heat and mass transfer equations. We systematically evaluate the hydrate formation process over different periods (days), analyzing the differential impacts of temperature, medium porosity, boundary fluid fluxes, and injection phase states on matrix evolution. Our results reveal that kinetic enhancement at higher temperatures (4 °C) dominates over thermodynamic subcooling, driving greater overall MH accumulation. Furthermore, high-porosity matrices sustain superior late-stage reaction rates by preserving seepage connectivity, whereas forced high-velocity gas injection overcomes capillary resistance but inhibits crystal growth by disrupting interfacial stability. Crucially, water injection into gas-bearing media tends to induce frontal self-sealing, leading to localized MH accumulation near the injection end; In contrast, gas injection into water-bearing media leverages the high mobility of the gas phase to effectively overcome pore-throat blockage, enabling extensive and deep MH formation within the porous medium. These results provide critical insights into the numerical prediction of MH formation and serve as a scientific basis for evaluating MH resource potential.