AN INVESTIGATION ON CH₄ HYDRATE TRANSITION AND GAS BUBBLE EVOLUTION ON A MICROFLUIDIC CHIP

Methane hydrate (MH) have been considered the future energy source due to its vast resource volume and high energy density. Understanding the pore-scale MH formation and dissociation behavior and gas-liquid-MH distribution at pores, and its effect on gas-liquid flow is significant for designing effective production strategies and safe exploiting of MH reservoirs. In this study, we employed a novel microfluidic chip technology (P_max = 19.0 MPa) that is capable of directly observation of pore-scale MH formation and dissociation behavior. Firstly, we observed the MH nucleation and growth behaviors under same conditions with Shenhu Sea, South China Sea (P = 15.5 MPa, T = 276.2 K). In addition, the MH dissociation behavior and gas bubble evolution via thermal stimulation with three heating rates (0.5, 2.0 and 8.0 K/h) were examined. Our experimental results reveal that two types MH formation mechanisms co-exist in pores: (a) porous-type MH with gas-liquid-hydrate co-exist formed from CH₄ gas bubbles; and (b) crystal-type MH formed from dissolved CH₄ gas. The crystal-type MH is wrapped around the porous-type MH with dendritic shape. The growth of crystal-type MH can trigger the nucleation of porous-type MH. In addition, the gas-liquid-hydrate coexistence of porous-type MH preferential dissociation under thermal stimulation. Crystal-type MH is relatively stable and its dissociation temperature is higher than that of porous-type MH about 0.3 K. MH dissociation in the pores produce obvious gas bubble aggregation and coalescence, and the average diameter of gas bubbles is 60 ~ 100 μm. Increasing the heating rate significantly enhanced the MH dissociation rate, which was more favorable for gas bubble aggregation and coalescence, resulting in the distribution of larger gas bubbles within the pores. The results provide direct pore scale observation of MH formation and dissociation behavior, gas bubble evolution, which provide fundamental understanding of MH transition phase kinetic and gas-liquid seepage in natural gas hydrate-bearing sediments.