Research On Porous Elastoplastic Mechanical Behavior Of Gas Hydrate Reservoir

Natural gas hydrate(NGH)is a kind of clean energy with great prospects,which has the advantages of wide distribution,rich in resources and high calorific value.However,in the process of exploitation,phase changes will occur,which leads to the lack of the reservoir structure,further reduce the strength of the reservoir,even cause the reservoir subsidence or collapse.It is necessary to study the deformation response of hydrate reservoirs during the exploitation process.At present,for the convenience of research,researchers regard the reservoir as an elastic medium in the deformation model of hydrate reservoirs.However,the hydrate reservoirs have poor cementation and shallow burial depth,so the reservoirs not only have single elastic deformation,but also have plastic deformation in the practical exploitation process.Hydrate reservoir is a porous medium,which will be affected by the coupling effects of temperature,stress,pore pressure and saturation during exploitation,resulting in complex deformation response process.Therefore,it is of great significance to establish porous elastoplastic mechanical model of hydrate reservoirs.In this paper,the elastic constitutive equation of porous media was derived based on the free energy balance of reservoir.Then,based on the plastic mechanics theory,the Cam-Clay model was modified to study the plastic part considering the coupling characteristics of multiple physical fields in porous media.The required parameters were calibrated by previous experiments to build the porous elastoplastic mechanics model of hydrate reservoirs.At the same time,the phase equilibrium curve of hydrate was modified to obtain the multiple phase change rule of hydrate in the reservoirs considering the interface effect,so as to describe the influence of phase change on the parameters of the model.Finally,combined with the actual data of Shenhu sea area in the South China Sea,the deformation response of three exploitation methods,namely depressurization,drained thermal stimulation and undrained thermal stimulation,was calculated and analyzed in detail.The results show that the interface effect has a great influence on the phase equilibrium conditions of hydrate and reservoir deformation.For a given pressure,the melting point of hydrate will lower with the decrease of pore size,which makes the phase equilibrium condition of hydrate appear as a region rather than a curve in the phase diagram.In addition,the reservoir deformation will be seriously underestimated if the interface effect is ignored.By comparing the results of the three exploitation methods,it can be found that the reservoir deformation caused by the drained thermal stimulation is the smallest,which is because the thermal expansion coefficient of the hydrate reservoir is too small.The deformation caused by the undrained thermal stimulation is the largest,because the gas generated under the undrained condition cannot be discharged in time,resulting in a sharp increase in pore pressure,causing an increase in deformation.According to the elastic deformation and plastic deformation under the three exploitation methods,it can be found that the plastic deformation takes a larger proportion in the hydrate exploitation process.Therefore,in the prediction of reservoir deformation and settlement,only by considering the influence of interface effect and plastic deformation,the error can be reduced to ensure the safety of later exploitation.