高温条件下花岗岩水压致裂的实验研究

Experimental study on hydraulic fracturing of granite specimens under high temperature

人工储留层建造是高温岩体地热开采关键技术之一,其核心是巨型水压致裂,关键科学问题是岩石热力耦合作用下的水压致裂理论。采用自主研制的"600℃20 MN伺服控制高温高压岩体三轴试验机",开展了Φ200mm×400mm的鲁灰花岗岩大试样高温三轴应力下的水压致裂实验,分析了实验的水压加载曲线特征和裂缝最终形态。研究结果表明:随温度的升高,花岗岩裂缝开裂模式从脆性开裂转变为连续开裂;温度升高至300℃后,裂缝起裂的主控条件向温度转化;从室温到200℃之间,温度对花岗岩水压致裂影响的控制机理是岩石抗拉强度的下降,而200℃到400℃起裂压力的控制机理是温度下降引起的热应力。随着温度的升高,裂缝的扩展方向从轴向改变为优先向径向深处发展。

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 isΦ200 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.