The interaction behaviors between the hydraulic fracture and natural fracture in hot dry rock

Abstract

Hydraulic fracturing is a prerequisite for efficiently extracting heat from hot dry rock (HDR), which is expected to create complex and inter-connected fracture networks between injection and production wells. Natural fractures are widely distributed within HDR and would prominently affect the propagation of hydraulic fractures. However, little work has been done utilizing granite specimens to investigate the interactive role between natural fractures and artificial fractures under the influence of thermal stress. We hereby performed high-temperature hydraulic fracturing tests on granite outcrops embedded with pre-existing natural fractures to reveal the fracture interaction behaviors. Consequently, the fracture interaction modes for guiding the generation of complex fracture networks were established. The findings indicate that a greater temperature differential between the injection fluid and the rock promotes the stimulation of natural fractures, attributable to the combined effects of thermal stress and fluid pressure exerted on the fracture interface. As the initial rock temperature rose to 180 °C, the activation of the natural fractures was likely to occur even under a relatively large injection rate and horizontal stress difference ratio, which unfavored the opening of the natural fracture. Additionally, with the increment of the rock temperature, the fracture conductivity of the generated fracture network rises accordingly. The average fracture conductivity of granite under 180 °C is around 5 times and 1.3 times higher than that of those under 25 °C and 100 °C, respectively. Furthermore, the reactivation of the natural fracture enlarges both the volume and surface area of the fracture networks in comparison to the crossing-only pattern, and this could be further enhanced as the hydraulic fracture propagated through the natural fractures afterward. The findings are expected to provide a comprehensive insight into the hydraulic fracturing of HDR with natural fractures.