Investigation Into The Dissociation Behavior Of Natural Gas Hydrate In Porous Media By Depressurization

Natural gas hydrate is a kind of solid, non-stoichiometric crystalline compound in which gas molecules are trapped within a lattice of ice-like crystal structure. It mainly occurs in two distinct geographic settings:in the permafrost and beneath the sea floor under high-pressure and low-temperature conditions. And its sheer volume demands that it be evaluated as a potential energy source. Considering the feasibility of exploiting different reservoirs and the economic problems encountered in natural gas production, depressurization is believed to be the most effective of the three methods and can be used either alone or in combination with the other two techniques. Thus, investigation of the dissociation behavior during gas production from porous sediment by depressurization is critical for the utilization of hydrate accumulations.In this work, experiments are conducted to investigate the effect of gas production pressure and thermal conductivity of the porous media on methane hydrate dissociation induced by depressurization. In addition, the reason for the occurrence of methane hydrate reformation and ice generation is analyzed. Finally, heat transfer is more thoroughly characterized by identifying and quantifying the effects of both the sensible heat of the reservoir and ambient heat transfer on hydrate dissociation. Results show that the decrease of gas production pressure and the raise of the thermal conductivity of porous media can effectively accelerate the rate of hydrate dissociation. The gas production process can be divided into three main stages:free gas liberation, consumption of the sensible heat of the reservoir, and hydrate dissociation driven by ambient heat transfer. In the first stage,the free gas in reservoir is liberated, yet the methane hydrate remains stable. In the second stage, hydrate dissociation occurs at the phase equilibrium simultaneously throughout the hydrate zone. While in the third the pressure is reduced to the production pressure, and the hydrate dissociation spreads radially from the outside as a result of ambient heat transfer. Hydrate reformation and ice generation always occur in the reservoir interior due to insufficient heat transfer from the reservoir sensible heat and the ambient environment. However, these phenomena are effectively eliminated in porous media with high thermal conductivity. Furthermore, the Ste number and dissociation rate constant (Kd) are employed to evaluate the impact of the sensible heat of the reservoir and ambient heat transfer. The results indicate that these factors are the main driving forces for hydrate dissociation induced by depressurization, and that both are dependent on production pressure.Meanwhile, the numerical model of hydrate dissociation induced by depressurization is constructed in this work. The influence of the gravity on the penetration rate of gas and water is considered in the Darcy law. The simulation results agree well with experiment, which verifies the accuracy of the dissociation model. The profiles of the pressure, temperature, and hydrate saturation in the reservoir are analyzed. The results indicate that the heat supplied by the ambient environment is mainly transferred from the the base layer and the lateral side of the reservoir. It is also confirmed that the complex dissociation process which the hydrate dissociation occurs initially throughout the sample, and then from the outside in as a result of ambient heat transfer.