Thermal-hydraulic-mechanical Coupling Numerical Simulation For Drilling Gas Hydrate Reservoirs

Natural gas hydrate is a new type of clean energy with great potential for development.However,it is difficult to achieve commercial exploitation of hydrate because of its easy decomposition.In this thesis,a heat transfer model of gas hydrate reservoir is established based on the law of energy conservation,and the dynamic simulation suitable for drilling process is realized by numerical method.The influence of formation conditions,drilling fluid and drilling process parameters on formation heat transfer is analyzed.A thermal-hydraulic coupling model was established considering the circulation of drilling fluid and drilling fluid invasion into reservoir,and the effect of drilling fluid invasion on heat transfer was analyzed.Based on solid mechanics theories such as rock mechanics and borehole stability mechanics,a hydraulic-thermalmechanical coupling model considering porous elasticity and thermal expansion of hydrate reservoir is established on the assumption that hydrate reservoir is elastic material.Using tsai-Wu criterion and Mohr-Coulomb criterion,failure index was established to judge the stability of shaft wall.A two-dimensional axisymmetric model was established with COMSOL Multiphysics,and a wellbore stability calculation app was developed for non-simulators.It is found that the influence of multiple parameters on reservoir temperature is nonlinear.As drilling fluid intrudes into the reservoir,the heat exchange between the fluid and the formation increases,and the temperature drops more dramatically along the radius near the wellbore.Under the coupling effects,the borehole wall strain changes periodically along the borehole depth,and the displacement in the middle of borehole wall is the largest.