Understanding the high temperature mechanical properties of rocks is one of the important basis for developing dry hot rock geothermal resources. The types of high temperature dry hot rocks are mainly igneous rocks. There are abundant geothermal resources in the active areas of modern volcanic belts, and they are mostly covered with basalt of unequal thickness. However, the effect of high temperature on the physical and mechanical properties of basalt is less studied. We use rock drillability, acoustic wave, XRD, and SEM testing methods to study the changes of basalt drillability, acoustic wave, mineral composition, and microstructure after high temperature heat treatment. Studies have shown that the drillability index of basalt decreases significantly when the temperature exceeds 300 ℃, and the sonic velocity loss rate of basalt decreases with a quadratic function with the increase in temperature. The threshold for a significant increase in the percentage of basalt weight loss is about 600 ℃. At the same time, significant mineral transformation and recrystallization occurred; mineral grain deformation occur in basalt at 400 ℃, and the crack width between mineral grains gradually increase at 600 ℃, and the number of new cracks increases significantly, and the crack width could reach about 1 μm. Many macroscopic cracks appear at 700 ℃ and 800 ℃, and intergranular pores appear at 800 ℃. From this point of view, the mineral transformation and fracture development caused by high temperature are important reasons for the changes in rock drillability.

In drilling engineering, the analysis of wellbore stability is directly related to the design of wellbore structure, the design of drilling fluid density window and the safety of drilling operations. Traditional strength criteria, due to mostly linear assumptions and the neglect of the intermediate principal stress effect, are difficult to accurately characterize the nonlinear failure characteristics of rocks under deep and complex geological conditions. Therefore, the selection of an appropriate strength criterion is the key to accurately evaluating wellbore stability. This paper uses an improved three-dimensional Hoek-Brown strength criterion and combines theoretical research with model application to establish a wellbore stability evaluation model that is more in line with actual working conditions. The research results show that this criterion not only retains the nonlinear characteristics of the Hoek-Brown strength criterion in the meridian plane, but also considers the influence of the intermediate principal stress on rock strength. The yield surface of the improved three-dimensional criterion in the principal stress space is a conical surface, and the yield curve satisfies the properties of being outwardly convex, smooth and closed. Through the comparative analysis of the true triaxial test data of six different rocks, it is proved that this criterion has good applicability to various rocks. When this criterion is applied to the wellbore stability analysis of the Jialingjiang Formation limestone strata in the Tongluoxia Gas Field in the eastern Sichuan Basin, the calculated collapse pressure equivalent density is 1.16 g/cm³. When drilling along the direction of the maximum horizontal principal stress, the collapse pressure equivalent density is the lowest, which is in line with the actual working conditions and is superior to the other four traditional strength criteria. The improved three-dimensional Hoek-Brown strength criterion provides a more accurate and practical method for wellbore stability analysis.