Conventional hydraulic fracturing generally produces high breakdown pressure, results only in single major fracture morphology and increases the risk of seismic events during the stimulation of dry hot rock (HDR) reservoirs. Aiming at addressing the above bottlenecks during hydraulic fracturing, a new reservoir stimulation method, known as cyclic liquid nitrogen (LN2) fracturing, based on cyclic soft stimulation (CSS) and LN2 fracturing was explored in this paper. In cyclic LN2 fracturing, low-temperature LN2 was injected in a cyclic manner, i.e. alternating high-injection-rate and low-injection-rate (or stop injection). Hence, formation rocks would be subjected to fatigue damage under the combined action of alternating thermal stress and fluid pressure, which was expected to promote fracture initiation, propagation and bifurcation to form complex fracture networks and improve the stimulated reservoir volume. However, the research on LN2 fracturing was mainly focused on the mechanisms of cyclic or single cooling treatment on rock and the fracturing performance of LN2 fracturing. No works on the cyclic LN2 fracturing performance, especially subjected to in-situ stresses were published as far as we know. To verify the feasibility of developing HDR by cyclic LN2 fracturing, the fracture initiation and morphology of cyclic LN2 fracturing were revealed by using Polymethyl Methacrylate (PMMA) based on the self-developed true-triaxial experiment of cyclic LN2 fracturing. The effects of horizontal stress difference ratio and the number of cycles on cyclic LN2 fracturing performance were studied. Cyclic water fracturing experiments were also conducted as a comparison. The results show that cyclic LN2 fracturing can significantly reduce the breakdown pressure, with 47.1%~71.7% lower than cyclic water fracturing. Under the combined action of alternating thermal stress and fluid pressure, cyclic LN2 fracturing tends to form a complex fracture network characterized by “thermally-induced fractures + major fractures”. The fracture initiation and morphology of cyclic LN2 fracturing are not easily affected by the horizontal stress difference ratio. Complex fracture networks can still be produced by the cyclic LN2 fracturing under a larger horizontal stress difference ratio. Increasing the number of cycles can reduce more breakdown pressure and generate more pronounced complex fracture networks. When high-pressure LN2 was injected into the wellbore, the breakdown pressure was even higher than that of water fracturing, which indicated that cyclic LN2 cooling pretreatment was the key to enhancing the LN2 fracturing performance. In general, cyclic LN2 fracturing can achieve better fracturing performance with a relatively lower number of cycles and cyclic pretreat pressure compared with cyclic water fracturing. Cyclic LN2 fracturing was expected to provide a new way for the green, economic and efficient development of HDR. The results were expected to provide a theoretical and experimental basis for the development of HDR by using cyclic LN2 fracturing.
