New Leakage Dynamics Model Based on Thermal-Hydraulic-Mechanical Coupling Theory

At present, the basis of most leakage dynamics modeling is to describe the leak-off velocity by Carter model. But, most of the natural fractures are deformable and the rock on the fracture wall is thermoelasticity. When the lost drilling fluid invades, the flowing net pressure will deform the natural fractures and cause the leak-off. At the same time, the head loss caused by the leak-off drilling fluid and its thermo-hydro-mechanical coupling effect with the rock around the fracture wall will disturb the net pressure of flow in the fracture and affect the dynamic width of natural fracture, and influence leakage state. Based on the porothermoelastic of fractured rock, the thermo-hydro-mechanical coupling constitutive equation of fractured rock is established. Considering the thermo-hydro-mechanical coupling effect between the leak off and the rock around the fractured rock and the moving boundary conditions of the leak off and the original formation fluid in the two-phase contact area, the thermo-hydro-mechanical coupling leak off model of fractured rock is established. According to the control equation of natural fracture deformation, taking the flowing net pressure as the connecting hub, the thermo-hydro-mechanical coupling leakage dynamics model is established that simultaneously couples the fracture-deformation coupling system and the fracture wall thermo-hydro-mechanical coupling system, and then perform leakage simulation. The results show that a greater temperature difference causes a greater leakage rate and volume, whereas a smaller leakage front distance, and a greater the temperature difference causes the narrower the natural fracture width and the greater the flow pressure drop in the same leakage front distance, resulting in the smaller the leakage front-end distance. The new model may be utilized to simulate various problems related to the invasion of drilling fluids into the fractures, including predicting the dynamic width of natural fracture and borehole ballooning/breathing phenomena.