Experimental study on hydraulic fracturing of granite under thermal shock

Abstract

As one of the key technologies for hot dry rock (HDR) geothermal exploitation, artificial reservoir is giant hydraulic fracturing at its core and takes the theory of hydraulic fracturing of rocks under thermo-mechanical coupling as its key scientific problem. With large samples of Shandong grey granite being the experimental subject, this paper conducted hydraulic fracturing experiment under triaxial stress at 20°C, 100°C, 200°C, 300°C and 400°C, analyzed the characteristic of water pressure loading curves in the experiment, and found that crack initiation pressure decreased remarkably after 300°C. In order to verify the factors controlling the influence of temperature on crack initiation, this paper established a strong transient thermal stress model, carried out numerical computation on this model and compared it with the experimental results. The results showed that the cooling effect of fracturing fluid for high temperature borehole can lead to thermal shock phenomenon and cause tensile stress near the borehole surface; the area near the borehole experienced two impact shocks, namely, elastic wave and thermal wave; the mechanism for the effect of temperature on the hydraulic fracturing of granite is not changes of rock mechanical parameters, but the thermal shock generated by the action of fracturing fluid of rock at high temperature.