This paper aims to investigate the effect of thermal shock on the hydraulic fracturing process in hot dry rock. Five granite samples with the dimensions of 80×80×100 mm3 were preheated (slow heating and rapid water cooling) for laboratory hydraulic fracturing experiments. P-velocity and microscopic observation was performed to observe the thermal damage. After the laboratory hydraulic fracturing experiments, the thermal shock effect on the injection pressure response, local hydraulic fracture geometry, and features of fracture surface was analyzed. Experimental results show that the P-velocity decreased with the increasing of thermal treatment temperature, while obvious thermal cracking occurred especially when the thermal treatment temperature was higher than 300 °C. Due to the obvious thermal shock effect, the breakdown pressure significantly decreased. Meanwhile, the nonlinear pressurization and initiation pressure were observed before breakdown. Additionally, due to the existence of thermally-induced micro cracks, the complexity of local hydraulic fracture and the roughness of hydraulic fracture surface significantly increased after 300 °C.

1. Introduction

Deep geothermal energy in hot dry rock (HDR) has been considered as a promising energy resource for power production (Brown 2009; Ghassemi 2012; Olasolo et al. 2016). Due to the ultra-low permeability of the tight crystalline rock, hydraulic fracturing is widely used to establish an engineered/enhanced geothermal system (EGS) for fluid circulation so that the heat stored in high-temperature rock mass can be extracted. The performance and lifespan of the EGS is closely related to the fracture geometry (Fu et al. 2016). Thus, it is significant for exploitation of deep geothermal energy to understand the initiation and propagation behavior of hydraulic fracture in HDR.

To date, many laboratory fracturing experiments has been performed on granite which is the typical lithology of HDR formation. The influence of grain size, cleavage anisotropy (rift, hardway, and grain planes), confining pressure, type of fracturing fluid (i.e., water, oil, and liquid and supercritical carbon dioxide), and injection rate on hydraulic fracturing process was thoroughly investigated (Ishida et al. 2000; Chen et al. 2015; Diaz et al. 2016;). The breakdown pressure and fracture geometry were analyzed (Ishida et al. 2012; Kizaki et al. 2012; Hampton et al. 2013; Zhuang et al. 2019; Hu et al. 2020).

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