Effect Of Ice And Methane Hydrate Reformation On Development Of Hydrate Reservoir By Depressurization

The depressurization is an economical and effective method for the development of natural gas hydrate reservoirs.However,there are few studies on the formation of ice below the quadruple point and secondary hydrate formation,and the current numerical simulation software cannot simulate the influence of secondary hydrate and ice on physical parameters well such as effective porosity and effective permeability.Therefore,it is important to establish a relatively complete numerical simulation software for hydrate reservoir development and analyze the effects of ice and secondary hydrate on depressurization.Based on the existing hydrate reservoir numerical simulation software HydrateResSim,the formation and dissociation of ice and secondary hydrate were considered,and the porosity permeability model,relative permeability model and surface area participating in the reaction were modified.Then,the accuracy was verified by laboratory and field-scale hydrate reservoir development tests.On this basis,the effects of ice and secondary hydrate on depressurization during the development of hydrate reservoirs at laboratory scale and field-scale scale were studied,and the factors affecting the formation of ice and secondary hydrate were analyzed.Finally,the development mode of field-scale hydration reservoir was studied,and the depressurization amplitude,depressurization speed and cyclic depressurization were conducted.Studies on ice formation at laboratory scales have shown that when a small amount of ice is formed near the production well,the dissociation of the hydrate is delayed.The distribution of ice is more uniform at high permeability,while ice is more likely to accumulate near production well at low permeability.Studies on ice formation at the field-scale indicate that the ice is mainly distributed in the middle layer of the hydrate reservoir in the longitudinal direction and S_I is higher at the dissociation front of the hydrate on the plane.The same characteristics to both scales are that:(1)when there is a large amount of ice formation,although the effective porosity and effective permeability decrease,the latent heat released by the formation of ice would provide in-situ heat for hydrate dissociation.Overall,the formation of ice has a positive effect on the dissociation of hydrates.(2)the lower the production pressure,the earlier the ice starts to form and the higher the average ice saturation.Studies on the hydrate reformation at the laboratory scale have shown that the hydrate reformation rate near the wellbore is large at the initial stage of shut-in,but it is gradually reduced in the later stage.After the formation of the secondary hydrate,it also leads to a decrease in effective porosity and effective permeability,but the effect is small.The lower the initial intrinsic permeability,the higher the average hydrate reservoir pressure and the greater the hydrate reformation rate.At the field-scale reservoir,secondary hydrate tend to form in the upper layers near the production well.The occasion can be predicted by the temperature and pressure variation.The secondary hydrate could decrease the permeability remarkably and which could be removed by well heating or hot water injection,so that the gas production capacity can be promoted.The higher the permeability(?75×10^-3?m²),the earlier the secondary hydrate form,the higher the secondary hydrate saturation;the greater the initial hydrate saturation(?0.4),the later the secondary hydrate form;the higher the production pressure,the later the secondary hydrate form,and the lower the maximum secondary hydrate saturation.When considering the design of the depressurization amplitude and the depressurization speed with ice generation,the larger the better,as long as the actual production condition is allowed.In the cyclic depressurization mode,the total volume of hydrates decreases"stepwise"for high initial intrinsic permeability hydrate reservoirs.During the shut-in phase,the hydrate at the distal end will continue to dissociate,and secondary hydrate is easily formed near the production well.Therefore,the operation of shut-in could"transport"the far-end hydrate to the near well,which is beneficial to rapidly gas production during the next cycle.Although the production pressure is below the quadruple point pressure,the temperature is still higher than quadruple point temperature,so there is no ice formation during the production process.Moreover,the average gas production rate is greater during the effective well opening duration.However,the cyclic depressurization of low permeability hydrate reservoirs has no obvious advantages.Therefore,it is recommended that high permeability hydrate reservoirs can be developed using a cyclic depressurization mode.