The THM Couple Model And Its Numerical Simulation On The Deformation And Fracture Of Reservoir Due To Dissociation Of Natural Gas Hydrate

With the advantages of wide distribution,abundant resources,clean,efficient and so on,Natural Gas Hydrate is regarded as one of the most development prospect and the most exploring potential new alternative energy sources in the 21 st century.The hydrate dissociation can give rise to heat transfer,pore fluid seepage,the mechanical property and the effective stress variation of hydrate bearing sediments.It is a Thermo-Hydro-Mechanical(THM)coupling effect precess with phase change.In the effect of Thermo-Hydro-Mechanical coupling,the phase change dissociation of hydrate may induce deformation and fracture of dissociation zone reservoir,thus may further cause the well wall collapse,the instable failure of the oceanic stratum,the aggravation of greenhouse effect,the pollution of the ocean environment.The research background of this paper is the deformation and fracture of hydrate bearing sediments due to the hydrate dissociation.The deformation and fracture law of hydrate bearing sediments is studied systematically by the combining method of laboratory experiment,theoretical analysis and numerical simulation in the condition of heat injection and depressurization dissociation.The mainly research achievements are as follows.(1)The quartz sand is selected as the sediment skeleton,and then the hydrate synthesis and heat injection dissociation experiment and depressurization dissociation experiment is carried out by the integrated experimental apparatus of Natural Gas Hydrate synthesis and dissociation,and then the variation law of temperature and pressure is obtained during the two dissociation modes.The result is shown that the pore pressure in the high pressure reactor appears positive exponent variation law with dissociation time during the inject heat dissociation of hydrate.Different measuring point temperature appears negative exponent law distribution in the hydrate bearing sediments along the heat transfer direction at the same time.With the inject heat time increase,the closer the heat source position,the bigger the change rate.The cumulative gas production appears natural logarithm law with the dissociation time in the different pressure condition during the depressurization dissociation of hydrate.The lower pressure,the higher dissociation efficiency,the larger the cumulative gas production in the condition of the same dissociation time.(2)The natural silty soil is selected as the sediment skeleton,and then the permeability experiment of containing different saturation methane hydrate sandy soil sediments is carried out by the integrated apparatus of hydrate bearing sediments synthesis and permeability test.The permeability variation law of hydrate bearing sediments with the hydrate saturation is obtained,and then the impact mechanism of hydrate saturation and its occurrence mode on the hydrate bearing sediments permeability is revealed.The result is shown that the hydrate bearing sediments permeability appears negative exponent variation law with the hydrate saturation.Occupying the pore center is the mainly forming mode of methane hydrate in the silty soil sediments pore,so the lower hydrate saturation can result in the significant decrease of the hydrate bearing sediments permeability.(3)The natural silty soil is selected as the sediment skeleton,and the triaxial loading permeability experiment of different saturation hydrate bearing sediments is carried out by the integrated experimental apparatus of hydrate bearing sediments synthesis and triaxial seepage test.The permeability variation law of different saturation hydrate bearing sediments with the action of different effective volume stress is obtained,and the impact mechanism of hydrate saturation and effective volume stress on the hydrate bearing sediments permeability is revealed.The result is shown that the hydrate bearing sediments permeability appears negative exponent variation law with the hydrate saturation and the effective volume stress respectively,and the impact of both on the hydrate bearing sediments permeability is non-independence.The impact mechanism of effective volume stress on the hydrate bearing sediments permeability is the compaction effect on the seepage channels,but the impact mechanism of hydrate saturation is the blocking effect on the seepage channels.(4)By analyzing the triaxial loading permeability experiment data,it is proved that the effective volume stress variation is the key factor that can cause the hydrate bearing sediments permeability variation in the condition of the general three-dimensional stress state.The impact of shear stress change on the permeability by causing the sample pore shape change can be neglected.(5)The natural silty soil is selected as the sediment skeleton,and the different saturation hydrate bearing sediments permeability experiment in the condition of loading-unloading is carried out by the integrated experimental apparatus of hydrate bearing sediments synthesis and triaxial seepage test.The permeability variation law of different saturation hydrate bearing sediments in the condition of the effective volume stress ascending and descending is obtained.The result is shown that the hydrate bearing sediments permeability appears negative exponent variation law with the effective volume stress during the effective volume stress ascending and descending,and the irreversibility is expressed,and the lower the effective volume stress,the more obvious the irreversibility.The higher the hydrate saturation,the greater the permeability damage rate,the worse the permeability recovery degree.(6)Based on the multi-field coupling theory,rock mechanics theory,seepage mechanics theory,elastic-plastic mechanics theory,heat transfer theory and other multi-disciplinary theories,the thermo-hydro-mechanical coupling model of the reservoir deformation and fracture due to the dissociation of Natural Gas Hydrate and the elastic-plastic constitutive model in the action of thermo-hydro-mechanical coupling which can reflect the deformation and fracture characteristic of reservoir stratum solid skeleton is established.And then the Galerkin method is used to disperse the model,the finite element equation of numerical simulation can be obtained.The model applicability is verified by the comparison of numerical simulation computation result and experiment result in terms of cumulative gas production during the inject heat dissociation of hydrate.(7)The USDFLD subroutine which consider the impact of "the effective volume stress" and"the hydrate saturation" on the mechanical property of hydrate bearing sediments is programmed by the Fortran language according to the dynamic variation relationship model of hydrate bearing sediments mechanical parameters.And then on the basis of thermo-hydro-mechanical coupling model,the numerical simulation method is used to analyze the mechanical parameters variation law,the stress state and strain state distribution law,the deformation and fracture law of dissociation zone reservoir and the sensitivity analysis of some factors that can influence the deformation and fracture of dissociation zone reservoir in the condition of injection heat and depressurization respectively.The result is shown that the reservoir borehole wall temperature and wellhead pressure are the two main factors that can influence the hydrate dissociation efficiency.If the dissociation mode is different,the distribution law of the effective volume stress and the volume strain will be different in the hydrate bearing sediments,and the effect degree of the effective volume stress and the hydrate saturation on the distribution law of the mechanical parameters will be also different.In the injection heat condition,the lower the permeability of hydrate bearing sediments,the greater the equivalent plastic strain of the reservoir.In the depressurization condition,controlling the depressurizing rate reasonably can effectively prevent the deformation and fracture of the reservoirs.(8)On the basis of thermo-hydro-mechanical coupling model and the USDFLD subroutine,the numerical simulation analysis of seedbed uplift and subsidence deformation due to the hydrate dissociation is carried out by the ABAQUS software.The result is shown that the higher the temperature,the more serious the uplift degree.The dissipation degree of pore pressure is different in the dissociation zone and undecomposed zone along the hydrate dissociation direction.The up covering layer permeability is the most significant factor that can impact the seabed uplift deformation.The lower the wellhead pressure,the faster the depressurization rate,the longer the depressurization time,the more serious degree the settlement deformation,and the settlement deformation quantity of the wellhead vicinity area is the maximum all the time.(9)On the platform of ABAQUS,the numerical simulation of the seabed slope instability and failure law is carried out by the finite element strength reduction program.The result is shown that the larger the hydrate dissociation amount,the greater impact on the slope stability,the lower the slope safety factor value.The larger the slope angle,the more easily landside,but the deeper the hydrate bearing sediments depth,the stronger the slope stability.