Simulation Study On Optimizing Exploitation Efficiency Of Natural Gas Hydrate By Depressurization

With the transformation of national economic development mode and the implementation of carbon peaking and neutrality goals,it has become an inevitable trend to develop clean energy to replace traditional fossil energy.Natural gas hydrate,as an unconventional natural gas resource with huge storage,is considered as a new energy with great development potential due to its wide distribution area and clean combustion.Until now,the commercial exploitation of gas hydrate has not been carried out on a large scale due to the limited of low efficiency and poor reservoir environment.Therefore,it is necessary to carry out the research on optimizing exploitation methods and identifying the impact of reservoir physical properties based on the challenges of hydrate development.In this paper,the mathematical models for the decomposition of pure methane hydrate are established based on the equations of mass and energy conservation,decomposition kinetics,and gas-water flow.And the simulation study of natural gas hydrate production performance through depressurization is conducted.Firstly,the comparative study is carried out on the exploitation of hydrate by various depressurization methods,and several indicators are used to evaluate each depressurization mode.Secondly,based on the optimal depressurization method and the reservoir parameters of the South China Sea,the influence of the seepage and heat transfer characteristics of hydrate sediments on gas production is investigated.In addition,the best value of each physical property is determined through the evaluation index,and the sensitivity analysis is used to quantify the impact of each parameter on the exploitation efficiency.Then,genetic algorithm is used to optimize the depressurization rate,and the configuration of depressurization and constant pressure sections is adjusted to optimize the segmentation strategy and improve the exploitation efficiency.Finally,the depressurization method is combined with auxiliary methods such as thermal stimulation and fracturing to further increase the exploitation efficiency.The results show that the exploitation economy can be increased by nearly 5 % by using stepwise depressurization with the same depressurization time,and then the exploitation efficiency can be improved.This method makes full use of the heat transfer around the hydrate sediment to make up for the heat loss caused by the hydrate decomposition,avoid ice generation and hydrate reformation,which makes the exploitation trial more safely.Moreover,the porosity,thermal conductivity,gas relative permeability and absolute permeability of hydrate reservoirs are the main factors affecting the production efficiency,and the sensitive factor of them are 0.52,0.22,0.1 and 0.04 respectively;then the permeability anisotropy and the specific heat capacity take the second place;and the water relative permeability has little effect on exploitation efficiency.The conclusion is instructive to the site selection of exploitation wells.The optimization results show that the comprehensive exploitation efficiency can be increased by 14%-20% by adopting faster depressurization rate.In addition,the results of the optimization of stepwise depressurization show that the exploitation efficiency can be improved about 2 % by reducing the depressurization steps and the time of constant pressure section on the premise of increasing the depressurization rate and ensuring development safely.However,in order to improve the exploitation safety,it can be considered to increase depressurization steps and the duration of constant pressure section.Furthermore,increasing the temperature and time of thermal stimulation will significantly improve the exploitation efficiency,while it will cause the decrease of the ratio of energy efficiency by up to 68.65 %.Finally,the promotion of exploitation efficiency will be enlarged by improving the fracturing area and strength.Thus,the combination of depressurization and external auxiliary methods can further enhance the efficiency of hydrate exploitation.