Borehole Rock Stability Under High Temperature Gradient of Geothermal Well Drilling

In high temperature geothermal well of drilling, when hot dry rock (HDR) meets the lower temperature drilling fluid, larger thermal stresses will be produced due to a large temperature gradient in borehole surrounding rock. Thermal stresses combined with non-uniform in-situ stress and wellbore pressure, which will endanger the stability of surrounding rock. A stress analytical model of borehole surrounding rock in HDR section was established, according to thermal and elastoplastic theory and finite element analysis of temperature distribution of surrounding rock. The stability factor is established to judge the stability of borehole surrounding rock, according to Mohr-Coulomb failure criterion. The stability of borehole surrounding rock in high temperature geothermal well of Yangbajing area in China is analyzed by solving the analytical model. When borehole with little internal pressure, the position of highest stability factor is the direction of the azimuth angle of 0° and 180°, and the position of lowest stability factor is the direction of the azimuth angle of 90° and 270°, while positions of highest stability factor and lowest stability factor are opposite when borehole with a very high internal pressure. The stability factor of the borehole increases first and then decreases with the increase of borehole internal pressure, and turning points in different direction are difference. With the increase of the distance from the axis of borehole, the stability factor of borehole surrounding rock is difference in different azimuth angle and different borehole inner pressure.