Multi-scale Joint Study On Dynamic Evolution Characteristics Of Reservoir Seepage Parameters During Hydrate Dissociation

Natural gas hydrate（NGH）is a widely distributed strategic resource,which has attracted the attention of scholars and governments around the world due to its advantagdes of abundant reserves and clean.The seepage characteristic of hydrate bearing sediments is the key factor to control heat and mass flow.On the one hand,it affects the thermal dynamic conditions during hydrate dissociation,and on the other hand,it affects the gas and water production rate during hydrate exploitation.Different from traditional oil and gas reservoirs,as a part of sediments framework,solid hydrate forms complex pore space together with rock particles,the hydrate dissociation will change the pore structure of sediments.Therefore,during gas hydrate exploitation,the seepage characteristics of reservoir present dynamic evolution with the phase transformation behavior of hydrate.Permeability and relative permeability are basic parameters to describe the seepage characteristics of sediments.Accurately describing the dynamic evolution mechanism of permeability and relative permeability during hydrate dissociation is the basis for evaluating and predicting the exploitation potential of hydrate reservoirs and designing hydrate exploitation schemes,which is also one of the key difficulties in hydrate exploitation research.The phase transition behavior of hydrate affects the seepage parameters by changing the pore structure of the porous medium,and the seepage parameters control the performance of gas and water production,so the study of the dynamic evolution characteristics of seepage parameters during hydrate dissociation involves complex problems from the microscopic pore scale to the macroscopic site scale.However,in previous studies,single research method from a single scale is usually used to explore the evolution of seepage parameters,resulting in the inability to effectively capture the complex pore assignment behavior of hydrates or the research results were often limited by scale.There is still a research gap in the quantitative correlation between the dynamic evolution of seepage parameters and the phase transition behavior of hydrates,which restricts the evaluation of field hydrate production potential.Therefore,it is necessary to combine the advantages of different research methods from multiple scales to jointly study the dynamic evolution characteristics of seepage parameters during hydrate dissociation.In this study,after clarifying the basic theory of multi-phase seepage in hydrate bearing sediments,the key factor influencing the evolution mechanism of seepage parameters with hydrate saturation is defined by exploring the seepage characteristics of hydrate bearing sediments and the evolution behavior of pore structure with hydrate dissociation at microscopic pore scale by LBM.Through the laboratory seepage experiment,the evolution law of the seepage parameters at the laboratory core scale is obtained.Based on the microscopic simulation and experimental results,a mathematical model is established to describe the evolution characteristics of seepage parameters in different types of sedimemts.The model is coupled into the numerical simulation program of TOUGH+Hydrate,and the gas and water production processes of different types of hydrate reservoirs are simulated,further verifying the feasiblity and reliability of the established model.Moreover,based on the results of numerical simulation,the hydrate reservoir structure and seepage parameters of differnet sites are defined,the key processes affecting the gas and water production rates in field trial tests are explored,and the characteristics of multi-phase seepage of reservoir during hydrate exploitation at macroscopic site scale are analyzed.This study mainly obtains the following conclusions and understandings:（1）Based on the microscopic pore-scale seepage simulation results,the dynamic evolution mechanism of permeability with hydrate dissociation is clarified.The key factors in the dynamic evolution of permeability induced by phase change of grain-coating and pore-filling hydrates are the evolution of pore throat size,and the decay rate of pore throat size with hydrate saturation largely determines the decrease rate of permeability.（2）The decay rate of the effective permeability of fine-grained argillaceous silt medium with hydrate saturation is significantly higher than that of coarse-grained medium;the effective permeability of porous medium has a greater decrease rate in the early stage of hydrate formation.The fundamental reason for the above phenomenon is that the hydrate occurrence forms in different media and different periods have different influences on the pore-throat size.（3）Based on the numerical simulation of the microscopic pore scale and the experimental results of the core scale,a new permeability evolution model was constructed:k_ir=(1-m（S_H）^1/2)（1-S_H）ⁿ,and the model parameters were determined.In the model,m is a parameter related to the characteristics of porous media such as particle size,distribution,porosity,etc.,n is the permeability attenuation index,and the larger the value of m and n,the faster the permeability decays with the saturation of the hydrate.Based on the experimental results,the permeability model parameters applicable to sand reservoirs are m=-1～1,and the argillaceous silt reservoirs are m=1.5～2.5,while the model parameters applicable to the Shenhu area in the South China Sea are m=1.734 and n=4.577.By adjusting the model parameters,the model can be applied to both coarse and fine particle deposits.This model is coupled into the TOUGH+Hydrate multiphase seepage numerical simulation program,which improves the solution and calculation function for the multiphase seepage process of different types of hydrate reservoirs.（4）The effect of the hydrate distribution on the relative permeability of gas and water is reflected in the fact that the presence of hydrate will significantly reduce the relative permeability of gas;the hydrate pore habit has no significant effect on the residual water saturation,while the increase of hydrate saturation will significantly increase the residual water saturation.In addition,based on the experimental data and numerical simulation identification results,it can be found that the residual water saturation of sandy reservoir is 0.2～0.3,and the residual water saturation of argillaceous silt is 0.5～0.7.Based on the research results of this paper,the dynamic evolution characteristics and key influencing factors of seepage parameters in hydrate bearing sediments at different scales can be obtained,which provides a theoretical basis for the long-term exploitation potential evaluation of hydrate reservoir and efficient exploitation of hydrate resource.