Effect of injection flow rate on fracture toughness during hydraulic fracturing of hot dry rock (HDR)

Abstract

The fracture toughness of hot dry rock (HDR) is a test parameter affected by the injection flow rate and temperature in the hydraulic fracturing of enhanced geothermal systems (EGS), and it is crucial to the determination of fracture expansion criteria in artificial thermal reservoirs. To investigate the fracture toughness of hot dry rock during hydraulic fracturing as influenced by the injection flow rate and temperature, hydraulic fracturing tests were conducted using thick-walled hollow cubic granite specimens with pre-cracks at different specimen temperatures and injection flow rates on a self-developed real-time high-temperature true triaxial load hydraulic fracturing apparatus. The specimen's fracture toughness was calculated by plugging the breakdown pressure into the corresponding fracture mechanics equation. Additionally, the acoustic emission events of the specimens during fracturing were recorded. The test results show a 14.11%, 21.11%, 24.66%, 33.54%, 43.84%, and 57.89% decrease in fracture toughness for six temperature groups of specimens from 25 to 500 °C when the injection flow rate was varied from 10 ml/min to 1 ml/min, and the fracture toughness decreased by 45.45%, 56.99%, and 73.26% at the three injection flow rates of 10 ml/min, 5 ml/min, and 1 ml/min, respectively, when the specimen temperature was from 25 °C to 500 °C. This indicates that a reduction in the injection flow rate could lower the specimen's fracture toughness both independently and in a mutually reinforcing and controlled way together with the temperature factor. The independent effect of the injection flow rate was attributed to the mean fracture power, and the thermal shock excited by the rapid cooling of high-temperature specimens during hydraulic fracturing was believed to be the essential cause of its interaction with temperature to affect the fracture toughness. The test results were closer to the actual fracture toughness of high temperature rocks during hydraulic fracturing than the NSCB results, demonstrating the feasibility of this test method. To date, this work is the most reliable method for calculating the fracture toughness of hot dry rock during hydraulic fracturing.