Heat and mass transfer analysis of depressurization-induced hydrate decomposition with different temperatures of over- and underburden

Natural Gas Hydrate (NGH) reserves are large and regarded as an important alternative future energy source. Safely and efficiently exploiting natural gas hydrates has recently become a hot issue around the world. The decomposition of natural gas hydrate requires a great deal of absorbed heat. The decomposition process is a complicated heat and mass transfer process with phase changes, such that heat and mass transfer will have a great impact on the decomposition results. Focusing on the heat transfer effect, a two dimensional mathematical model of natural gas hydrate decomposition was established. Using a numerical simulation method, the thermal conductivities of different porous mediums were selected and the bottom hole pressure (BHP) was changed under different overburden and underburden temperatures to simulate the depressurization exploitation of natural gas hydrates. The reservoir temperature distribution, hydrate decomposition front, gas production rate, and cumulative gas production under different cases were obtained. The results show that the incoming heat directly influenced the reservoir temperature distribution, hydrate decomposition rate, and gas production and the temperature at any point on the decomposition front is constant under certain BHP. When there was no external heat introduced into the hydrate-bearing layers, the impact of thermal conductivity on decomposition was greater, but decreased with increasing temperature of overburden and underburden. When no external heat was introduced into the hydrate-bearing layers, the BHP became the main controlling factor of the cumulative gas production, and the effect of the BHP on the cumulative gas production was gradually weakened with increasing temperatures of the overburden and underburden.