Multi-physical Field Evolutionary And Heat Mass Transfer During Gas Production From Hydrate Reservoir By Multi-branch Well

Since the 21 st century,the international community has paid more and more attention to the huge role of natural gas hydrate as a potential energy source,and has carried out many natural gas hydrate production tests.However,the current technological methods still cannot meet the demands of commercial exploitation.Multi-branch wells have the function of increasing hydrate decomposition area and expanding the horizontal channel of the reservoir,which is an effective way to improve the productivity of hydrate production.The heat and mass transfer process are one of the important factors determining the gas production efficiency of hydrate.Therefore,the design and optimization of multi-branch wells need to understand the evolution of the multi-physical field and the rule of heat and mass transfer in the process of hydrate exploitation.This paper analyzes the evolution of pressure field,temperature field,and resistivity field during the experiment by using the customized complex structural well experimental simulation device.In view of the formation process of natural gas hydrate,based on the Archie formula,parameters a,b,m,and n were measured and calibrated to establish the relationship between resistivity and hydrate saturation in coarse-grained sediments.The evolution of the hydrate saturation field was monitored by combining multiple Resistance measurement points.According to the decomposed stage of gas hydrate,based on the depressurization mechanism,simulation experiments with different pressure drop amplitude,different number of branches,different angle of branches and different sediment reservoirs were carried out.The monitoring results of the hydrate saturation field showed that the hydrate formation position is related to the gas injection mode,and multiple gas injection can significantly improve the uniformity of hydrate in the reactor.The hydrate saturation calculated based on the Archie formula and the gas state equation has the same changing trend in the later recrystallization stage,but the calculation result of the former is generally low due to the influence of the control range of the resistivity sensor.Therefore,the Archie formula is corrected by the gas state equation to calculate the overall hydrate saturation,which makes the calculation of the hydrate saturation field more accurate.Through experiments with different depressurization amplitude,it is found that in the hydrate decomposition stage,the sensible heat of sediments,latent heat released by ice,and heat transfer in the environment can provide heat for hydrate decomposition.Among them,the heat transfer of the environment is the most important factor in hydrate decomposition.But in the case of low hydrate saturation,the latent heat released by ice also plays an important role.Well pattern arrangement also has an impact on the hydrate exploitation process.In the free gas release stage,well pattern arrangement can control the fluid flow in sediments,thereby affecting reservoir temperature.In the early stage of hydrate decomposition,the branch wells increase the flow channel of methane gas produced by hydrate decomposition,which can control the range of hydrate decomposition and reduce the mass transfer limitation.However,with the environmental heat transfer,the influence of well type gradually weakens.According to the study of different reservoir types,it is found that fine sediments have high thermal conductivity,so the heat transfer rate is fast,and it is more susceptible to environmental temperature.In the free gas release stage,the gas flow increases the gas-water contact area,and the hydrate continues to form in the reactor.Thermal conductivity plays an important role in hydrate decomposition,so heat conduction is an important factor affecting hydrate decomposition.The above conclusions can help to understand the heat and mass transfer laws in the process of multi-branch well hydrate exploitation,and to provide theoretical and data basis for the safe and efficient mining of natural gas hydrate.