Experimental Study on High Temperature on the Drillability and Microstructure of Basalt

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.