| The movement of dissociation front is closely related with the production performance of hydrate reservoirs.The hydrate reservoir would be divided into two zones(dissociated zones and hydrate zones)by the dissociation front during production.There exist great differences in the properties of fluids and stability of reservoirs between these two zones,and therefore it is significant to study the movement of dissociation front during hydrate reservoir production.The dissociation mode as well as characteristic of dissociation front movement under the comprehensive effect of multiple controlling mechanisms has been investigated with numerical simulation and experimental methods.The decoupling analysis of different mechanisms affecting the dissociation mode and movement of dissociation front has been conducted,and predicting models of dissociation front movement have been established based on controlling mechanisms of fluid flow,hydrate dissociation,heat transfer,flow-dissociation,flow-heat transfer and dissociation-heat transfer.A set of discriminant method for different controlling mechanisms has been propsed by means of characteristic time based on the results of established models.Methods of experimental study,numerical simulation and theoretical model analysis have been adopted to study the sensitivity of parameters affecting the movement of dissociation front.It is revealed from the numerical simulation that the dissociation model and characteristic of dissociation front movement are determined by the comprehensive effect of fluid flow,heat transfer and kinetics of hydrate dissocation: when the production of hydrate reservoir is controlled by fluid flow,a piston dissociation front would come into being and the fitted power exponent of dissociation front movement curve ranges from 0.5 to 1;when the production of hydrate reservoir is controlled by heat transfer,a piston dissociation front would come into being and the fitted power exponent of dissociation front movement curve is around 0.5;when the production of hydrate reservoir is controlled by hydrate dissocaition,an extending non-piston dissociation front would come into being and the movement rate of dissociation front would grow in the late stage;when the production of hydrate reservoirs is controlled by both fluid flow and heat transfer and the fluid flow is the dominant controlling mechanism,a stable non-piston dissociation front would come into being,and the dissociation front advances linearly;when the production of hydrate reservoirs is controlled by both fluid flow and heat transfer and heat transfer is the dominant controlling mechanism,an extending non-piston dissociation front would come into being,and the movement rate of dissociation front increases in the late stage;when the production of hydrate reservoirs is controlled by both fluid flow and hydrate dissociation and the fluid flow is the dominant controlling mechanism,a stable non-piston dissociation front would come into being,and the dissociation front advances linearly;when the production of hydrate reservoirs is controlled by both fluid flow and hydrate dissociation and hydrate dissociation is the dominant controlling mechanism,an extending non-piston dissociation front would come into being,and the movement rate of dissociation front increases in the late stage;when the production of hydrate reservoirs is controlled by both heat transfer and hydrate dissociation and heat transfer is the dominant controlling mechanism,a stable non-piston dissociation front would come into being,and the fitted power exponent of dissociation front movement curve ranges from 0.5 to 1,and would be more closer to 0.5;when the production of hydrate reservoirs is controlled by both heat transfer and hydrate dissociation and hydrate dissociation is the dominant controlling mechanism,an extending non-piston dissociation front would come into being and the fitted power exponent of dissociation front movement curve ranges from 0.5 to 1,and would be more closer to 1.According to the experimental results of 1D model,the dissociation front advances linearly,while the dissociation front movement of the 2D model would decrease gradually.It could be predicted from the results of numerical simulation that the 1D model is controlled by both hydrate dissociation and fluid flow and fluid flow is the dominant controlling mechanism,while the 2D model is controlled by both hydrate dissociation and heat transfer and heat transfer is the dominant controlling mechanism.The decoupling analysis reveals that the movement of dissociation front is strongly affected by different controlling mechanisms,and therefore it is significant to identify the controlling mechanisms of hydrate reservoirs before the prediction of dissociation front movement.It is found that characteristic time could be used for the discrimination of different controlling mechanism: the characteristic time of fluid flow controlling mechanism is defined as the time it takes for the gas phase to flow through the whole length of hydrate reservoir;the characteristic time of hydrate dissociation controlling mechanism is defined as the time it takes to dissociate the hydrate in a unit volume of hydrate reservoir;the characteristic time of heat transfer controlling mechanism is defined as the time it takes for the gas phase to transfer the heat in the unit length of hydrate reservoir through the unit area of reservoir by heat conduction.The sensitivity analysis of parameters affecting the movement rate of dissociation front shows that:(1)in the hot water injection experiments,the movement of dissociation front is more sensitive to initial hydrate saturation,intrinsic permeability,hot water temperature and injection rate,while less sensitive to the initial temperature and salinity of injected water;(2)according to numerical simulation of the 10 m hydrate reservoir production by hot water flooding,the movement of dissociation front is more sensitive to the initial hydrate saturation,temperature and injection rate of injected hot water,and production pressure,while less sensitive to the initial temperature and intrinsic permeability of the hydrate reservoir;(3)based on results of the theoretical model analysis,the movement rate of dissociation front is more sensitive to wellbore heating temperature,conductivity of dissociated zone,initial temperature and heat capacity of dissociated zone,and then is the production pressure,and less sensitive to porosity,conductivity and heat capacity of hydrate zone.When the hydrate reservoir is produced by hot water injection,the sensitivity of intrinsic permeability to the movement of dissociation front is larger for smaller sized hydrate reservoirs,and less for larger sized ones.The sensitivity of initial temperature of hydrate reservoirs is little when hydrate reservoirs are produced by hot water injection,while large when produced by depressurization combined with wellbore heating. |
