Decomposition thermokinetics and heat balance analysis of depressurized methane hydrate deposits under poor heat transfer conditions

Abstract

Methane hydrate is an important form of natural gas resource found in submarine deposits, with exploitation efficiency mainly determined by in-situ hydrate decomposition. During the depressurization exploitation process of large-scale hydrate deposits, the heat transfer situation of far-wellbore deposits is highly likely to be worse than that of near-wellbore deposits; However, the thermodynamic and kinetic characteristics of hydrate decomposition under poor heat transfer conditions are still unclear, particularly lacking heat balance analysis theory. This study adopted an external temperature control technique in experiments to worsen the heat transfer situation and investigate its effects on the decomposition thermokinetics of depressurized methane hydrate deposits to 2.9 MPa. The stable decomposition rates of methane hydrates under different heat transfer situations were found to be linear with temperature difference, and decomposition stagnation was observed as heat transfer worsened. Based on these results, a novel heat balance model of depressurized deposits with hydrate decomposition was developed, which can accurately predict the dynamic temperature response of hydrate decomposition deposits with a deviation of no >8.9 %. In addition, the low hydrate decomposition rate under a worse heat transfer condition induced synergistic fluctuations in pressure and temperature, which can be improved with progressive warming. This study reveals the thermodynamic and kinetic behaviors of methane hydrate decomposition under poor heat transfer conditions for the first time and can help predict the spot exploitation characteristics of far-wellbore hydrate deposits.