Temperature response mechanism of methane hydrate decomposition coupled with icing and melting under variational thermodynamic conditions

Abstract

High-efficient decomposition is the precondition of commercial production of methane hydrates. Under variational thermodynamic conditions, icing and melting may occur during hydrate decomposition, inducing complex heat and mass transfer. In this study, hydrates formed under temperature gradient of 0.012, 0.020 and 0.026 °C/mm in a 2 L reactor, decomposed by long depressurization to 1.0 MPa at constant exhaust rate from 0.11 to 1.07 ln/min. Temperature of hydrate-bearing area was in a specific relationship called thermodynamic non-equilibrium effect with pressure, controlled by hydrate decomposition. When pressure reached around 2.1–2.3 MPa, instant icing caused rapid temperature rise, and accelerated hydrate decomposition generating pressure rise of 2.36 MPa at most coordinated by limited exhaust rate. Isothermy stage occurred in double phase transition system with hydrate decomposition and ice melting, temperature slightly raised after most hydrates had decomposed. Temperature gradient only worked on non-hydrate/ice area because the temperature response of hydrate decomposition couple with icing and melting is first dominated by thermodynamics and then affected by heat and mass transfer. The control effect of ice melting in thermodynamic is stronger than hydrate decomposition. These results are significant for systematic analysis among gas production, hydrate decomposition, temperature gradient and ice behavior of actual hydrate exploitation.