Risk Assessment Of Sand Production For Natural Gas Hydrate Exploitation In The Northern South China Sea

In recent years,the development of natural gas hydrate(NGH)research has made gas hydrate recognized as an important follow-up energy with good prospect,which has the characteristics of wide distribution,large resource quantity,high energy density and no pollution.The commercial exploitation of NGH has not been realized in the world,only a few countries have carried out experimental exploitation,and most of these countries have encountered different degree of formation sand production in the experimental process.However,in order to realize the long-term commercial exploitation of NGH resources,the problems of formation sand production is the key point that must be overcome.Therefore,it is of great significance to study the theory and practical engineering of NGH exploitation by developing sand production risk assessment method for NGH exploitation.In this paper,the experimental production of NGH formation in the northern South China Sea is the engineering background.The strength criterion and stress-strain constitutive model of natural gas hydrate in the northern part of the South China Sea were established by simulation experiment.Based on the theory of rock mechanics,seepage mechanics,thermodynamics and physical chemistry,and the method of theoretical analysis and numerical calculation,a multifield coupling mathematical model is established for the risk assessment of sand production of NGH formation to take into account the phase transformation of hydrate decomposition,and the mathematical model is solved by the finite element software.This paper systematically studies the physical and mechanical responses in the formation of NGH,constructs a theoretical system of risk assessment method for sand production in NGH formation,and analyzes the influencing factors of sand production and the rule of sand producing in the process of NGH formation.This paper evaluates the harm degree of sand production in NGH formation in northern South China Sea,and provides theoretical support for future large-scale exploration and development of NGH resources in South China Sea.Firstly,combined with the experience of NGH experimental project at home and abroad,the characteristics of NGH formation,the characteristics of hydrate decomposition and the influence of hydrate decomposition on the formation,the formation mechanism of sand produciton risk and influence factors of sand production in NGH exploitation were studied.The sand production of NGH formation is a complicated process of multi phase and multiple physical field coupling,which is different from conventional oil and gas reservoir.Secondly,by simulating the physical and mechanical experiments of the NGH formation in the South China Sea,it can be found that the formation stress-strain curve of NGH has no obvious compaction stage,the elastic stage is short,the yield stage is long,the peak strength is not obvious,and the strain hardening characteristic is obvious.Assuming that the friction angle of NGH formation is invariable,the influence of effective confining pressure on the formation of NGH is non-linear,and the influence of hydrate saturation is introduced,the formation strength criterion of muddy hydrate siltstone is established.At the same time,a hyperbolic stress-strain constitutive model is established for the formation of NGH in the northern part of the South China Sea,which can reflect the strain hardening characteristics of the NGH formation under specific conditions,and can calculate the peak stress-strain relationship accurately.Thirdly,the temperature field model of NGH formation is established by the energy conservation principle in consideration of the factors such as heat conduction,heat convection,energy supply of upper and lower strata and wellbore in NGH formation,and hydrate decomposition endothermic.The model of hydrate decomposition phase change is constructed,including the kinetic model of hydrate decomposition,the phase equilibrium model of NGH and the decomposition heat model.Based on the generalized Darcy's law and the continuity equation of stratum skeleton and pore fluid,the fluid seepage field model of NGH formation is deduced and established,and the model takes into account the gas and water two-phase flow and strata anisotropy.Based on the theory of small deformation of solid mechanics and the principle of Terzaghi effective stress,the formation deformation field model of NGH formation is deduced.The conditions for determining the physical fields of the sand analysis of NGH formation are given,including boundary conditions and initial conditions.Based on the above models,the physical and mechanical parameter models for the risk assessment and analysis of the sand production in the formation of NGH are established,which comprehensively reflects the influence of the NGH decomposition and the change of strata stress state on the physical parameters in the process of NGH formation.It is also reflected in the formation of NGH mining sand evaluation problem is subject to a number of factors coupling effect.Finally,based on secondary development function of ABAQUS finite element software,the risk evaluation model of sand production in NGH formation was established,and the factors influencing the sand formation in NGH strata were analyzed,and the evolution process of formation plastic strain and sand yield of NGH was studied in the process of mining.Analyze and summarize the law of disaster.The results show that with the increase of mining time,the maximum equivalent plastic strain of NGH formation becomes larger,that is,the risk degree of formation destroying sand is increased.In the same NGH formation,the decomposition range of NGH increases with the increase of production pressure difference,At the same time,the sand-producing range of NGH formation and the severity of sand production will increase,and the increase of maximum plastic strain along with the increase of production pressure difference increases with the increase of the pressure difference;the influence of initial hydrate saturation change on sand formation is limited,The rock matrix of natural NGH formation has a great influence on the formation of sand production.The horizontal stress difference leads to the most concentrated compressive stress in the direction of the lowest horizontal geostress in the strata near the wellbore,which is the most easily sand-forming position,the greater the horizontal stress heterogeneity,the greater the maximum deformation region near the wellbore,and the greater the possibility of sand formation.At the same time,the NGH formation in the north of South China Sea was simulated,and the results showed that: in the northern part of the South China Sea,due to the low ground stress heterogeneity,the plastic yield range of the well surrounding rock was even.The maximum production pressure difference of step-down mining method should not exceed 5 MPa by using open hole completion method of straight well.When the horizontal well screen completion method is exploited,the liquid yield of the hydrate formation is greatly increased than that of the direct open hole completion method,and the sand content is reduced,and the maximum production pressure difference of the step-down mining method is 8.6 MPa.