Experimental study of thermal-crack characteristics on hot dry rock impacted by liquid nitrogen jet

Liquid nitrogen jet fracturing is a novel stimulation technology, which is expected to be suitable for hot dry rock (HDR) reservoirs. Due to the large temperature difference between hot rock and cryogenic fluid, a great number of thermal cracks would be created during fracturing process, which is conductive to improve the penetration capacity of formation. In this study, a set of experiments were conducted to investigate the characteristics of thermal cracks. In these experiments, granite specimens with temperatures ranging from 200 ℃ to 300 ℃ were impacted by the low-pressure liquid nitrogen jet. The complexity and connectivity of cracks were quantitatively analyzed by a fractal method. The permeability and ultrasonic velocity of the granite specimens were tested in order to evaluate the damage conditions caused by thermal stress. Additionally, scanning electron microscope was adopted to analyze the microscopic characteristics of the thermal cracks. The results show that the heating process has a slight effect on thermal-crack generation compared with the liquid-nitrogen impact. The cracks mainly concentrate in the region near the impingement surface, due to the large temperature gradient there. The impacted rock breaks as the effects of tensile stress and shear stress. With an increase of initial rock temperature, the number of thermal cracks increases, and a more complex crack-network is formed in each specimen. Transient pulse evaluation and ultrasonic velocity measurements indicate that the impact of liquid nitrogen jet can improve the permeability and cause the damage of hot rock noticeably. This study demonstrates the important effect of thermal stress on crack generation during liquid nitrogen jet fracturing for HDR reservoirs, and the results shed light on the exploitation of HDR energy.