The wellbore instability mechanism in hot dry rock drilling

Abstract

Wellbore instability is one of the key factors limiting safe and efficient drilling in hot dry rock. A detailed understanding of the instability mechanism of hot dry rock wellbore is essential to achieve high efficiency in geothermal exploitation. Therefore, in this study, we established a numerical simulation model of wellbore stability that considers the heterogeneous properties of granite rock, and the coefficient of stability index for the wellbore of hot dry rock is proposed. The influence of heterogeneous characteristics of granite, in-situ stress, mud pressure, and temperature on the wellbore instability are analyzed. The results show that the granite heterogeneity leads to a significant disturbance of the near-wellbore stress. The degree of wellbore instability is more severe than that of the homogeneous rock formations. Radial tensile damage tends to occur at low mud pressures. High mud pressure and temperature difference between drilling mud and formation easily lead to tangential tensile failure. Shear damage occurs on the radial/tangential stress horizontal plane without temperature differences. However, a temperature difference dramatically increases the axial stress (the first principal stress), changing the direction of shear damage, which occurs mostly in the vertical plane or the wellbore's curved surface. Deeper wellbores are more severely unstable than shallow wellbores, and mud pressure can inhibit wellbore instability to a certain extent. However, increasing excess mud pressure can lead to tangential tensile failure. The temperature difference exacerbates the wellbore instability; especially after exceeding 260 °C, the wellbore shear thermal rupture is drastic. The study results are of great significance for the in-depth understanding of the mechanism of wellbore instability in deep hot dry rocks and for guiding the safe and efficient drilling of hot dry rocks.