Coupled Thermo-Hydro-Mechanical Behavior Of Hydrate-Bearing Sediments During Hydrate Dissociation

In the process of drilling and production of natural gas hydrate,if the hydrate bearing sediment is disturbed and the temperature and pressure conditions change,the hydrate in the sediment will dissociate and release natural gas into the sediment pores,which will increase the pore pressure,and then affect the sediment strength,even endanger wellbore stability.Therefore,it is necessary to study the pore pressure caused by hydrate dissociation and the change of mechanical behaviors of the hydrate bearing sediment.In this paper,the coupled thermo-hydro-mechanical behaviors during thermal dissociation process of hydrate are studied.The main research contents are as follows:Firstly,the mechanical behaviors of hydrate formation under triaxial undrained test were studied.A Mohr-Coulomb constitutive model considering the effect of initial hydrate saturation is established by using finite element analysis software ABAQUS.Combined with user-defined subroutine USDFLD,the changes of cohesion and elastic modulus with hydrate saturation was realized.The effects of hydrate saturation and initial effective confining pressure on the mechanical properties of hydrate samples were studied.The results show that the deviatoric stress,yield strength and failure strength of hydrate samples increase with the increase of hydrate saturation and initial effective confining pressure,and the correctness of the model is verified.Then,the thermo-hydro-mechanical coupling behaviors of hydrate formation are studied.Through the finite element analysis software ABAQUS combined with the user-defined subroutine USDFLD and DISP written in Fortran language,the thermo-hydro-mechanical behaviors are coupled and the pore pressure generated in the process of hydrate thermal dissociation is calculated,and the radial distribution law of the temperature,pore pressure and transient hydrate saturation of the hydrate bearing sediment in the process of hydrate dissociation as well as the variation law with time are obtained,and the influence of temperature and hydraulic conductivity on effective stress and settlement in the sediment is studied.The results show that the pore pressure generated by hydrate dissociation increases with the increase of temperature and the decrease of hydraulic conductivity,and the distance of pore pressure propagation decreases with the decrease of hydraulic conductivity.For the three cases with permeability coefficients of 8.4×10^-5 m·s^-1,8.4×10^-6 m·s^-1 and 8.4×10^-7 m·s^-1,the distance of pore pressure propagation is 23.54 m,15.13 m and 11.45 m after 8 days of drilling,respectively.The transient hydrate saturation decreases along the radial direction during the hydrate dissociation process,and increases with the decrease of the hydraulic conductivity.The effective stress of hydrate bearing sediment decreases with the increase of temperature,and increases with the decrease of hydraulic conductivity.The ground settlement caused by hydrate dissociation increases with the increase of the temperature rise and the decrease of the hydraulic conductivity,and decreases along the radial direction of the sediment.