Coupled wellbore-reservoir heat and mass transfer model for horizontal drilling through hydrate reservoir and application in wellbore stability analysis

To address the wellbore instability risk during drilling in hydrate reservoirs, a fully coupled wellbore-reservoir model for horizontal drilling through a hydrate reservoir is presented, which considers the dynamic heat and mass transfer between the wellbore and reservoir and the coupling interactions between the fluid flow, heat transfer, and evolution of mechanical properties in hydrate-bearing sediments. A numerical method based on COMSOL Multiphysics + MATLAB was first proposed, and the accuracy of this model was verified by comparing it with experimental data and another mathematical model. The rules of heat and mass transfer between the wellbore and reservoir and the yield failure behaviors of the wellbore are thoroughly analyzed based on the data of hydrate-bearing sediments in the Shenhu area. The simulation results indicate that the temperature of the hydrate reservoir increases with the invasion of drilling fluid. The radii of the thermal disturbance zone at the toe and heel of the horizontal well are approximately 0.48 m and 0.64 m after 20 h, respectively. The formation shear strength near the wellbore gradually decreases from the initial 3.24 MPa-2.09 MPa due to hydrate decomposition; then, the formation begins to yield, and the yield ratio of the borehole reaches 92.56% after 20 h. Compared with the drilling fluid displacement and density, the inlet temperature of the drilling fluid plays an absolute leading role in the wellbore extension length. When the inlet temperature is less than 22.5 degrees C, the reservoir hydrate will not decompose. This work can provide a theoretical reference for the design of drilling fluids aimed at controlling borehole stability issues and for the design of the length of horizontal wells during drilling in hydrate reservoirs.