Experimental study on hydraulic fracturing of granite specimens under high temperature

Man-made geothermal reservoir is one of the key technologies in heat extraction in HDR.The fundamental issue is the massive hydraulic fracturing and the key scientific problem is hydraulic fracturing theory when considering the thermo-mechanical coupling effects on the rock.The self-developed “600℃ 20 MN servo-controlled rock tri-axial testing machine with high temperature and high pressure” was adopted to carry out the hydraulic fracturing experiments for large sized specimen under high temperature and tri-axial stress.The size of Luhui granite specimen is Φ200 mm×400 mm.Characteristics of the hydraulic loading curve and fracture morphology were analyzed after the experiment.The results show that fracture cracking model of granite specimens transformed from brittle cracking into continuous cracking with the increase of temperature.When the temperature is larger than 300 ℃，the main control criteria of fracture cracking becomes the temperature.When the temperature varies from the room temperature to 200℃，the control mechanism of the impact of temperature on the initiation pressure is the decrease of tensile strength of rock masses，while from 200℃ to 400℃，the control mechanism is the thermal stress caused by the decrease of temperature.The fracture propagation orientation changed from axial direction into radial direction with the increase of temperature.