Numerical Simulation of the Effect of Porous Media Permeability on the Decomposition Characteristics of Natural Gas Hydrate at the Core Scale

As a new type of clean energy, natural gas hydrates are widely distributed in ocean and permafrost. Moreover, it is important to understand the gas production characteristics of gas hydrate resources for the commercial exploitation in the future. The natural gas produced from hydrate bearing sediment is influenced by heat transfer, multiphase flow, and hydrate decomposition phase transition in the reservoir. This paper investigates the impact of permeability on the decomposition characteristics of hydrate-bearing sediments at laboratory scale by using a validated two-dimensional axisymmetric model. The results showed that in the initial depressurization stage for the low-permeability hydrate core the pressure-lowering end and right boundary exhibit a relatively large pressure difference; as the decomposition advances, the effective thermal conductivity within the reservoir decreases. Subsequently, the Joule–Thomson effect and hydrate decomposition phase change absorb sensible heat in the reservoir, contributing to a further decrease in the center temperature of the core; in areas far from the pressure-lowering end, the hydrate decomposition rate is slower than that in cores with high-permeability hydrates. The hydrate decomposition can lead to an increase in permeability, and hydrate cores with high permeability exhibit faster instantaneous gas production rate than hydrate cores with low permeability, which peak at 12 min. The variation of the effective thermal conductivity has a relatively minimal influence on the gas production characteristics of the hydrate core by depressurization. The gas production of the hydrate core under different conditions exhibits relatively small differences; for the low-permeability hydrate core, hydrate decomposition with thermal injection and the depressurization method exhibits obvious improvement in gas production, and for the higher permeability hydrate core, the acceleration effect is weaker.