REVIEW ON THE MECHANICAL-THERMAL COMBINED EXPLOITATION METHODS OF DEEP SEA NATURAL GAS HYDRATE

Gas hydrates are an important sort of strategic energy resource in China because of their enormous reserves and small contamination compared to traditional fossil fuels. Many countries have accelerated the exploitation and research of gas hydrates. Profitable extraction methods, disaster control and environmental protection are two key issues that need to be addressed in the commercial exploitation of natural gas hydrates. Currently, the combined use of heat injection and pressure reduction methods is considered to be the most effective method of recovery of gas hydrates. In the method of depressurization and heat injection, natural gas hydrate production includes physical processes and effects such as heat transfer, phase change, seepage, and deformation. The heat transfer is the slowest and the phase change consumes a lot of heat, so it is impossible to directly use the conventional oil and gas extraction scheme that relies solely on the seepage principle to extract natural gas hydrates. Natural gas hydrates in the South China Sea occur mostly in silty clay and silty sand and other sediment types, with poor cementing and shallow burial depth. Conventional extraction methods are not suitable for hydrated extraction in the Southern Sea, and new methods of extraction have to be envisaged. Among these, the improvement of the efficiency of heat transfer in the sedimentary layer is the key to the exploitation of natural gas hydrate. Che-Min Cheng presents a mechanical-thermal combined recovery method, utilizing the heat of seawater, convective heat transfer, and pipe transportation, considering the stratum safety similar to coal mining. The mechanical-thermal combined method for gas hydrate exploitation is a new concept, which has the advantages of high efficiency and controllability, and meanwhile, the safety of formation can be reduced effectively. This paper presents a comprehensive estimation of the energy, apparatus, and economic feasibility of the new method demonstrates the advances in multiphase flow with phase transformation and stratum safety and gives some suggestions for the potential application of these results.