Experimental Investigations on the Effect of High Temperature and Pressure on the Mechanical Properties and Drillability of Granite

Abstract

ABSTRACT Hot Dry Rock (HDR) is an important clean energy resource. However, there are many problems in the exploration of HDR, which are closely related to the geological environment and properties of HDR. The representative lithology of HDR is granite. Granite has the characteristics of poor drillability, high hardness and strong abrasive property in underground high-temperature and high-pressure environment. Therefore, the effect of temperature and pressure on physical and mechanical properties and drillability of granite was explored with deep water pore pressure servo system and rock drillability tester. The result shows that Young's modulus and compressive strength increase with the increase of confining pressure. High temperature weakens the mechanical properties of granite under the same confining pressure. The specific manifestation is that density, mass, longitudinal wave velocity and drillability grade value decrease with the increase of temperature, while mass loss rate decreases by 2.2‰ and longitudinal wave velocity decreases by nearly 74%. The drillability grade value of granite increases with the increase of confining pressure, especially at room temperature. This study has certain guiding significance for revealing the mechanical properties of granite after heating under high pressure and efficient drilling in the exploration of HDR. INTRODUCTION At present, HDR is a new clean energy, which is a reliable energy resource (Goldstein et al., 2008). HDR is a dense high-temperature rock mass with a temperature between 150 °C and 650 °C, and buried at 3-10 km underground, with little or no liquid (Ma et al., 2022). The most typical reservoir rock of hot and dry rock is granite (H. Liu et al., 2020). However, there are many problems during drilling, such as high confining pressure, high rock hardness and poor drillability (Rossi et al., 2020). The physical and mechanical properties of granite have changed considerably under the influence of high temperature and high pressure (S. Liu & Xu, 2015). Therefore, it makes sense to study the effect of high temperature and pressure on the mechanical properties and drillability of granite. Lau et al. (1995) heated the granite from 75 °C to 125 °C, and found that the Young's modulus decreased slightly, and the decrease amplitude increased under low confining pressure. In addition, temperature seems to have no effect on Poisson's ratio. Dwivedi et al. (2008) explored various thermodynamic properties of Indian granite at temperatures ranging from 30 to 160 °C, mainly including Poisson's ratio, uniaxial compressive strength, Young's modulus and creep behavior, and scanned micro-cracks generated in the rock due to high temperature by scanning electron microscope (SEM).