Numerical And Experimental Studies On Natural Gas Hydrate Dissociation Via Depressurization In Reactor

With the increasing demand for energy,the world faces with increasingly serious problems such as energy shortage and environmental pollution.To solve these problems,the best choice for is promote energy transformation.Natural gas hydrate,as a new type of energy with high efficiency,energy-saving and environmental fridenly,has always been concerned by the energy industry,and it has significant potential development value.The depressurization has been widely used in field trials because of simple device demanded,no extra input energy and low cost.Therefore,it is a great significance to carry out experiments and numerical simulations of natural gas hydrate for commercial exploitation in the future.In this paper,the self-built hydrate experiment system is used as the research object to investigate the behavior of methane hydrate dissociation in the experiment and numerical simulation.Firstly,the decomposition of gas-saturated hydrate under three wellhead pressures are studied in the experiments,and the pressure and temperature changes during hydrate dissociation are systematically analyzed.Secondly,a 2-D axisymmetric mathematical model is established using TOHGH + HYDRATE with the reactor,and based on the laboratory reactor and relevant experimental parameters,the date is compared using the numerical simulation and the experimental results,which is consistent with each other and proves the accuracy of the model.The hydrate decomposition behaviors are investigated by depressurization under six production pressures with different pressure reduction and five reservoir permeability.The volume of gas production,gas production rate,temperature and pressure,and phase saturation spatial distribution of hydrate decomposition in the reactor in numerical simulations are analyzed,and the changes of phase saturation along the radial position in the reactor are specifically studied.The conclusion is summarized as follows:(1)The experimental result of NGH decompression indicate that: The wellhead pressure change in the reactor is rapidly decreased to back-pressure,and then remains the pressure unchanged throughout the hydrate dissociation.The average temperature change in the reactor first decreases rapidly and then increases.The tempreture change at the middle in the reactor first decreases rapidly and then increases slowly,and until the methane hydrate completely dissociate and finally reach the temperature of the water bath.At the same time,it is found that the boundary heat transfer has a great influence on the temperature rise in the reactor.(2)The numerical simulations result of NGH decompression indicate that: The six wellhead pressures are selected under the same initial pressure,the hydrate dissociation can be accelerated with the decrease of wellhead pressure,and lower production pressure results in more final cumulative gas production from the wellhead.At the same time,the gas production characteristics of methane hydrate decompression are compared under the same depressurizing range and different depressurizing range,the main influencing factor of hydrate decomposition is to reduce wellhaed pressure,and the lower wellhaed pressure leads to the greater the influence of this factor.When the wellhead pressure is 2.0 MPa and the decomposition is about 70 min,the gas production rate increases,it shows that the latent heat released by icing can accelerate the hydrate dissociation.The five permeability are selected under the same initial pressure and wellhead pressure,the final cumulative gas production at the wellhead is equal.With the decrease of permeability,it is found that no ice phase is generated at the open section,but secondary hydrate formation phenomenon occurs at the upper wall of the open section,and lower reservoir permeability leads to larger area of hydrate secondary formation.