Experimental And Numerical Research On Dissociation Characteristics Of Natural Gas Hydrate By Depressurization In Porous Media

Natural gas hydrate is a clathrate hydrate formed by water molecules and gas molecules under high pressure and low temperature conditions.It has many advantages,such as abundant reserves,high energy density and clean combustion products,and becomes one of the most important energy reserves to solve current energy insufficiency.Due to the simplicity and no need for additional energy,hydrate exploitation by depressurization has the most potential to become the main means of commercial hydrate exploitation.Based on the self-designed experimental system and the established numerical model of hydrate dissociation,the control strategies of production pressure for hydrate dissociation under different production targets and the upscaling techniques in the heterogeneous reservoirs simulations are explored in this dissertation.Referring to the design ideas of experimental systems for hydrate dissociation at home and abroad,an experimental system for hydrate dissociation by depressurization in porous media is designed and built.After construction and debugging,the system achieves the expected experimental needs.First,the design ideas are described and realized according to the functions that the experimental system needs to achieve,and the composition and working mode of each subsystem are introduced in detail.Secondly,the experimental materials and specific process for the generation of natural gas hydrate in the reactor and the dissociation experiment of the generated hydrate by depressurization are given.Finally,the calculation methods of key parameters in the experiment,such as porosity of porous media,phase equilibrium parameters,hydrate saturation,solution salinity,and gas production from hydrate dissociation,are derived,which provide basic preparation for subsequent hydrate dissociation experiments.With the experimental system,the hydrate dissociation experiments at different temperature and production pressures are carried out in the water system and the inhibition system,respectively,and the cumulative gas production,the gas production rate and the temperature variation in the reservoir are obtained.Besides,the hydrate dissociation experiments under different production periods are carried out and a new control strategy of production pressure aiming at a constant production rate based on the Kim-Bishnoi model is designed and tested.The experimental results show that in the water system and the inhibition system,the gas production rate of hydrate dissociation under higher reservoir temperature is higher.Similarly,the lower production pressure also leads to a higher gas production rate.Nevertheless,the overall dissociation process in the inhibition system is slower than that in the water system,because the salinity in the inhibition system decreases with the dissociation process,resulting in a decrease of equilibrium pressure.Under the same final production pressure,a smaller production period leads to a higher gas production rate due to its lower average production pressure.At the same time,in the process of hydrate dissociation by periodic depressurization,the reservoir temperature is more stable than that of single production pressure.Using the designed control strategy of production pressure,the gas production volume during each production period of the hydrate dissociation is nearly the same value and the expected target of constant gas production rate is achieved,which provides theoretical basis and technical support for the control strategy of production pressure in the situation that requires stable daily gas production in the commercial exploitation of natural gas hydrate.A two-dimensional axisymmetric numerical model matching the reactor in the experimental system is constructed,with which the hydrate dissociation characteristics under different production pressures,different thermal conductivity,and different initial pressures are investigated numerically.And the depressurization boundary is found when the hydrate reaches the maximum dissociation rate under icing conditions,and the influence of reservoir physical properties and initial conditions on the depressurization boundary is further discussed.The results show that a lower production pressure,a greater thermal conductivity or a higher initial pressure provides a faster gas production rate of hydrate dissociation,while the reservoir temperature has a lower minimum in the dissociation process.Under the combined influence of driving force and icing phenomenon,reducing gas production pressure to a certain extent still improves the hydrate dissociation rate.But there is a depressurization boundary that maximizes the hydrate dissociation rate and further pressure reduction will no longer result in a faster dissociation rate.The depressurization boundary is affected by the physical properties of the reservoir,that is,a higher depressurization boundary can be obtained under the condition of a greater thermal conductivity,while it has little relationship with the initial conditions.Based on the hydrate reservoir in nature,a heterogeneous reservoir model is built,with which the numerical simulation of hydrate dissociation in heterogeneous reservoirs is conducted.Furthermore,by exploring different upscaling techniques and homogenization methods,a homogenization method with the best prediction performance of hydrate dissociation characteristics in the coarse grid and its application scope are obtained.And the universality of this method in general heterogeneous reservoirs is also verified.The simulation results show that the hydrate dissociation rate in the heterogeneous reservoirs is relatively slower and the simulation time is also longer than that in the homogeneous reservoirs,due to the influence of local minimum permeability on the gas and water flow.It is found that the method of harmonic averaging in the axial and radial direction for the effective permeability(HHKe)in the situation of twice the basic grid scale shows the best accuracy on describing the gas production variation,gas production rate,reservoir temperature and pressure and hydrate saturation distribution in the coarse grid,with the maximum gas production error of 6.2%.Based on the average relative error of gas production,the recommended application range of HHKe method is 4 times the size of the basic grid.In addition,the upscaling simulations in two groups of random heterogeneous reservoirs using the HHKe method are carried out and the results indicates that the HHKe method shows a universality for the upscaling simulation of general heterogeneous hydrate reservoir,although it has better prediction performance for reservoirs with lower heterogeneity.Therefore,it can be used for the research of gas production prediction in large-scale heterogeneous hydrate reservoirs in nature.