Heat transfer analysis of methane hydrate dissociation by depressurization and thermal stimulation

The dissociation of natural gas hydrate is an endothermic reaction closely related with the heat transfer characteristics in porous media. This study mainly focuses on the three-dimensional heat transfer behaviors during hydrate dissociation by depressurization and thermal stimulation based on the experiments in a Cuboid Pressure Vessel (CPV). The evolution of various heat flows (including the heat transferred from the boundaries Q(B), the injected heat from the well Q(inj), the heat consumed by the hydrate dissociation Q(H), and the sensible heat change of the deposit Q(S)) and their relationships during hydrate dissociation are obtained through numerical simulation. The heat loss Q(L) during gas production is calculated and analyzed for the first time. It is found that the hydrate dissociation is mainly promoted by the driving forces of depressurization (F-dep) and thermal stimulation (F-ths), which are dependent on the heat flows of Q(B) and Q(inj), respectively. The effect of F-dep, will be weakened under higher F-ths. Part of Q(inj) and Q(B) are absorbed and stored as Q(S) by the porous media and the fluids of the deposit. Once Q(B) becomes negative, it starts to make contribution to the heat loss instead of the hydrate dissociation, resulting in a sharp increase of Q(L). In addition, a proper thermal stimulation rate q and production pressure P-W, should be selected so that the hydrate dissociation rate could be significantly enhanced while the thermal efficiency and energy efficiency are still favorable when compared with using single depressurization. (C) 2018 Elsevier Ltd. All rights reserved.