Study On Mining Characteristics And Efficiency Optimization Of Naturel Gas Hydrate Resources Via Depressurization

Natural gas hydrate is an unconventional natural gas resource with great development potentialities.Due to its advantages of high energy density,clean combustion,wide distribution and large reserves of resources,natural gas hydrate has attracted the attention of scientists from all over the world and has been regarded as an alternative energy source for coal,oil and natural gas.It is estimated that the reserves of natural gas hydrate resources in China's sea area are about 80 billion tons of oil equivalent,which is twice the proven oil reserves.It has great resource potential and development prospects.Realizing the exploitation and utilization of natural gas hydrate resources is of great significance in alleviating China's resource shortage,optimizing energy structure,and ensuring energy security.However,due to the unique reservoir environment of natural gas hydrate resources,reservoir deformation caused by natural gas hydrate resource exploitation may induce geological disasters and the mining efficiency problem is also an important factor limiting its commercial exploitation.Therefore,focusing on the safe and efficient utilization of natural gas hydrate resources,this work studies the reservoir response characteristics and gas production characteristics while exploiting and utilizing natural gas hydrate resources via depressurization,and analyzes the faced issues while mining natural gas hydrate resources from the aspects of safety and efficiency.Then,a framework based on production pressure control and thermal stimulation is proposed to optimize the gas production efficiency of natural gas hydrate resources utilization via depressurization.Natural gas hydrate dissociation changes the heat transfer characteristics and mechanical properties of reservoir,affecting the efficiency and safety of natural gas hydrate utilization.The influencing mechanism of component ratio and component properties on the heat transfer characteristics of natural gas hydrate reservoirs is experimentally investigated using a thermistor based thermal conductivity measuring method,and a hybrid model for the prediction of effective thermal conductivity of hydrate-bearing sediments is proposed based on the experimental data obtained.The CT visualization was used to study the characteristics of sand migration and reservoir compaction caused by gas-water migration,skeletal structure change and reservoir effective stress change during natural gas hydrate dissociation process.It was found that the hydrate-bearing sediments exhibited shrinkage deformation during natural gas hydrate decomposition.The maximum body variable can reach?1.4%.After natural gas hydrate dissociation,the sediment particles lose cementation and the reservoir strength decreases,which affects the reservoir's stability.By analyzing the pressure-temperature response characteristics of gas hydrate production,the gas production process during hydrate dissociation was analyzed,and the temperature buffer phenomenon was found.Through analyzing the energy sources of natural gas hydrate dissociation,it is found that the dissociation characteristics of natural gas hydrates above freezing point and below freezing point are different The gas production characteristics during natural gas hydrate dissociation shows that the contribution of reservoir's sensible heat increases as the decrease of gas production pressure,which is benefit for the gas production efficiency,however,under the condition of low gas production pressure,the sensible heat of the reservoir is excessively consumed,which leads to the problematic ice generation,affecting the gas production efficiency.The proposed efficiency optimization method is analyzed from both experimental and simulation aspects.The experimental results indicate that gas hydrate dissociation rate can be controlled through adjusting the gas production pressure,therefore governing the consumption rate of reservoir's sensible heat and strengthening the contribution of environmental heat transfer to hydrate dissociation,to prevent the problematic ice generation.Additional thermal stimulation can directly supply heat to the natural gas hydrate reservoir,therefore effectively promoting the gas production efficiency of natural gas hydrate dissociation via depressurization.The simulation results also show that the contribution of reservoir's sensible heat to the dissociation of natural gas hydrate increases with the decrease of gas production pressure,but the contribution of reservoir's sensible heat is limited.Ambient heat transfer is the important factor affecting the gas production efficiency of natural gas hydrate utilization.The increase of reservoir's specific heat capacity and thermal conductivity is beneficial to enhance the contribution of reservoir's sensible heat and ambient heat transfer to the dissociation of natural gas hydrate.The auxiliary methods of boundary heat injection and spatial microwave heating can provide energy supplement for the depressurization induced natural gas hydrate dissociation,and effectively promote the efficiency of natural gas hydrate dissociation and gas production.The higher the boundary heat injection temperature and the microwave radiation energy density,the more obvious the contribution of heat stimulation on the dissociation of natural gas hydrate.