Effect of thermal shock on laboratory hydraulic fracturing in Laizhou granite: An experimental study

Abstract

Abstract This paper aims to investigate the effect of thermal shock on hydraulic fracturing in hot dry rock (HDR). To simulate the thermal shock effect caused during the injection of cold water, several natural granite specimens were slowly heated to various target temperatures and then rapidly cooled down by flowing water. Laboratory hydraulic fracturing experiments were performed on granite specimens under triaxial stress condition and room temperature. After that, computed tomography (CT) scanning technique, scanning electron microscope (SEM) observation, and surface profile scanning (SPS) tests were combined to analyze the influence of thermal shock on fracture morphology from micro-scale to macro-scale. Moreover, the characteristics of injection pressure curve were discussed. Experimental results show that single main hydraulic fracture with large aperture is created and approximately propagates along the direction perpendicular to the minimum principal stress before 300 ℃. When the thermal treatment temperature is higher than 300 ℃, multiple hydraulic fractures with branches tend to be created due to the existence of thermally-induced damage zone near the well hole. Experimental results of SPS tests indicate that the maximum height difference of the hydraulic fracture surface increases with increasing thermal treatment temperature. The tortuosity of hydraulic fracture significantly increases when the thermal treatment temperature is higher than 300 ℃. Additionally, the analysis of injection pressure response shows that apparent non-linear pressurization, and decrease in pressurization rate and breakdown pressure are observed after 300 ℃.