Kinetic study of methane hydrate development involving the role of self-preservation effect in frozen sandy sediments

The dissociation of natural gas hydrate in permafrost areas is a kinetic reaction process associated with the existence of ice and the accompanied self-preservation effect. The kinetic decomposition behaviors of methane hydrate below freezing point are investigated through both experimental and numerical simulations in this work. Dissociation experiments have been conducted in a high-pressure reactor (HPR) by depressurization and wellbore heating methods. Two kinetic models are employed for the prediction of gas recovery and hydrate dissociation in frozen sandy sediments. The modified new model which takes into account the protection effect of ice is found to predict more accurately than the traditional one. When the deposit is situated under frozen state, the hydrate particles can be maintained at a relatively stable state even when the external pressure has dropped below the equilibrium level. Such self-preservation effect is the main factor resulting in the undesirable recovery efficiency of frozen gas hydrate by pure depressurization, and it can be only eliminated by external heat injection. However, the heated wellbore has a limited influencing area, and lots of the provided heat is wasted outside through the boundary. Secondary ice formation exists in the cold regions far away from the well, which leads to incomplete decomposition of methane hydrate. The kinetic decomposition of frozen hydrate is mainly dominated by its self-preservation effect and the injected heat, and the dissociation process is not sensitive to the production pressure when it is adjusted below the equilibrium level.