Thermoplastic fracture characteristics of granite suffering thermal shocks

Abstract

Thermal-shock-dependent thermoplastic fracturing is a prominent characteristic of hydraulic fracturing in high-temperature reservoirs, such as hot-dry-rock geothermal and deep oil–gas reservoirs. When cold water is injected, the rock surrounding the hydraulic fracture tip experiences sudden temperature changes, affecting the cohesive crack's tension-open in the fracture process zone (FPZ), i.e., the temperature-dependent plastic softening behavior. Based on our previously established thermoplastic cohesive fracture model, a description model of rock thermoplastic fracture properties related to thermal shock was proposed. Then, we performed mode-I fracture tests on high-temperature granite subjected to different thermal shock treatments and used digital image correlation (DIC) to characterize the fracture process (especially FPZ development). By comprehensively analyzing loading curves and DIC-based surface deformations, we quantificationally delineated seven thermoplastic fracture characteristics of thermal-shock-treated granite based on the proposed description model. We find that (1) cohesive tensile strength linearly decreases with increased thermal shock. (2) Critical COD and completely developed FPZ length linearly decrease only for temperature differences above 140 °C while they vary slightly for temperature differences from 0 °C to 140 °C (3) Plastic modulus, fracture energy, and accumulated dissipated energy change nonlinearly with the intensification of thermal shock. These three properties slightly vary for temperature differences from 0 °C to 140 °C but significantly reduce once the temperature difference exceeds 140 °C. (4) The temperature sensitivity modulus is mainly positive but follows different evolution laws with the temperature difference (≤140 °C: decreasing; >140 °C: increasing). For temperature differences of 0 °C and 140 °C, the temperature sensitivity modulus shifts from positive to negative only when COD is approximately at 0.5 times the critical value. The positive temperature sensitivity modulus implies that the yield surface of the cohesive crack contracts with enhanced thermal shocks. (5) We compared the linear elastic fracture parameter, energy release rate, with the thermoplastic fracture parameter, fracture energy. Our analysis reveals that the fracture energy is 3.34 ∼ 1.58 times larger than the energy release rate as the thermal shock intensifies, with a significantly different trend in the variability. This highlights the importance of considering thermoplastic fracture characteristics in hydraulic fracturing applications for high-temperature reservoirs. Furthermore, we put forward optimization recommendations based on our findings.