The influence of embedded discontinuous rock on hydraulic fractures propagation behaviors of hot dry rock in Gonghe Basin

The exploitation of hot dry rock (HDR) requires the enhanced geothermal system (EGS), whose core is to build a thermal reservoir by hydraulic fracturing. In addition to natural fractures, the embedded discontinuous rocks, such as quartz veins, are developed in granite (typical HDR). This characteristic demonstrates significant differences compared to sedimentary rocks, which makes it difficult to predict the propagation pattern of hydraulic fractures (HFs). Hydraulic fracturing experiments were conducted on the core specimens from the HDR reservoir in the Gonghe Basin. Moreover, the numerical simulation based on the extended finite element method (XFEM) was conducted to investigate the influence mechanism of embedded discontinuous rocks on the HF propagation behaviors of HDR. The results show that: (1) Due to the mechanical properties, the embedded discontinuous rock in granite, is prone to induce HFs to propagate along the interface between the rock matrix and discontinuous rock. (2) As the in-situ stresses gradually increase, the HFs encounter greater difficulty in penetrating the discontinuous rock. Furthermore, a larger tensile strength contrast between the embedded discontinuous rock and the rock matrix leads to the increased resistance to HF penetration through the discontinuity. (3) During the hydraulic fracturing of HDR in the field, HFs intersecting with large-scale geological discontinuities tend to propagate along the interfaces, inducing the slip of structural planes. This process is characterized by a gradual increase in wellhead pressure, a reduction in the number of microseismic events, and an elevated S/P-wave amplitude ratio in raw waveforms. Consequently, the likelihood of triggering seismic events exceeding ML1.5 is significantly enhanced. The research can provide theoretical and experimental support for the design of the fracturing technology for controlling induced-earthquakes during HDR exploitation.