LBM Simulation Of Methane Hydrate Decomposition Under Different Exploitation Methods

With the continuous development of global economy,human’s demand for energy is increasing day by day.At the same time,traditional fossil fuels(coal,oil)have caused serious environmental pollution and greenhouse effect,so it is urgent to find new clean energy.Methane hydrate reserves are huge,with high calorific value and no pollution.Exploration and exploitation of methane hydrate is of strategic significance for solving global energy and environmental problems.In nature,methane hydrate is mainly stored in seafloor sediments and permafrost.The decomposition of methane hydrate involves the coupling of fluid flow,mass transfer,the endothermic reaction of hydrate decomposition and the ablation of solid volume of hydrate.In this study,based on the Kim-Bishnoi kinetic model,the lattice Boltzmann method(LBM)was used to establish the fluid flow model,mass transfer model and coupled heat transfer model for methane hydrate decomposition.Solid phase renewal(VOP)method was used to track the solid ablation of methane hydrate.The variation of gas production rate and cumulative gas production under different exploitation methods(depressurization method,heat injection method and combined pressure injection method)and different influencing factors were studied,and the dissociation morphology and temperature variation characteristics of methane hydrate decomposition process were analyzed.The following conclusions are drawn:(1)In the process of decompression exploitation,when adiabatic boundary,unable to provide enough energy,hydrate and cannot be separated completely,the accumulative gas production package condition and state of suspension under the accumulative gas production is 5/6 and 3/8 of the total complete decomposition,respectively,show that parcel condition is more advantageous to hydrate decomposition,almost the same temperature change at the same time,It shows that the pressure of hydrate decomposition plays a major controlling role in the encapsulated state.When the boundary is heated,the hydrates near the opening and boundary are affected by pressure and temperature respectively,and the decomposition is completed first.With the increase of porosity,boundary temperature and initial temperature,the decomposition rate accelerates,and the time of hydrate decomposition decreases.(2)In the process of heat injection mining,hydrate decomposition will form a decomposition front with the transfer of temperature,which can be divided into dissociated zone and undissociated zone.Hydrate decomposition near the water injection port takes the lead,and the decomposition of hydrate continues to decompose towards the water outlet with the transfer of water injection temperature.The decomposition rate of methane hydrate increases with the increase of water injection rate and temperature.Compared with the water injection rate of 4ml/min,the decomposition time of methane hydrate at 6ml/min,8ml/min and 10ml/min is increased by 14.7%,28.2% and 35.5%,respectively.Compared with 288.15 K,the time shortened by 303.15 K,313.15 K and 333.15 K for methane hydrate decomposition was only increased by 4.1%,6.9% and 9.3%,respectively,indicating that the influence of water injection velocity was greater than that of water injection temperature,and the influence of thermal convection on hydrate decomposition was greater than that of heat conduction.The effect of porosity on hydrate decomposition is almost negligible.(3)In the process of heat injection combination mining hydrate,hydrate together due to the outlet pressure and water temperature control,mining hydrate is not as hot as a decomposition of a leading edge,when fully hydrate decomposition,injection temperature did not transfer to the hydrate area,shows that its pressure is controlled,the influence of the temperature of water is limited.The time of hydrate decomposition is shortened with the increase of water injection temperature,water injection velocity and porosity,and the effect of water injection velocity is greater than that of water injection temperature,while porosity has little effect on hydrate decomposition.