Seepage Behavior of CO ₂ Hydrate Bearing Sands Regulated by <scp>l</scp> -Methionine: Insights into Hydrate-Based CO ₂ Sequestration

Hydrate-based CO2 sequestration in submarine sediments is a promising large-scale carbon mitigation strategy. This study examined the effects of sediment grain size on CO2 hydrate formation kinetics, seepage behavior, and morphology evolution regulated by l-Methionine (l-Met) as a kinetic promoter, under controlled conditions of 3.6 MPa and 277.15 K, simulating the harsh environment of submarine organic-rich sediments. The experimental results demonstrate a nonunidirectional relationship between grain size reduction and sequestration enhancement. While finer sediments generally promote formation kinetics, the 98–138 μm quartz sand fraction exhibited optimal overall performance, achieving a CO2 consumption of 0.369 mol and the fastest growth rate (t90 = 58.027 min). This optimum reflects a balance between kinetic promotion and mass transfer, as evidenced by seepage tests showing that excessively fine sediments cause severe pore-throat clogging, leading to a marked reduction in effective permeability, whereas medium-grained systems exhibit the highest effective permeability. Mixed-grain systems display enhanced injectivity due to preferential flow pathways formed by coarse fractions. Visual observations reveal that CO2 hydrate in natural sea sands forms heterogeneously with nodular-like morphology. These findings offer theoretical guidance for optimizing kinetic promoter-regulated CO2 hydrate formation and continuous CO2 injection strategies in submarine sediments.