Experimental Study of Fracture Initiation and Morphology in Low-Brittle Shales with Bedding Planes subjected to LN Fracturing

Liquid nitrogen (LN) can deteriorate rock structure and mechanical properties. It is an effective method for engineering applications in deep resource exploitation. For the first time, this work explored LN fracturing performances in low-brittle shales with bedding planes under true triaxial-confining stresses via a LN fracturing device. Microcracks and matrix structures were analyzed by scanning electron microscopy (SEM). Compared with hydraulic fracturing, LN fracturing can significantly lower fracture initiation pressure and promote fracture propagation. LN can thermally induce bedding planes and secondary pores around the borehole in low brittle shale, significantly deteriorating shale structure. Pre-existing fractures and bedding seams with graptolites and pyrite minerals are more likely to be activated by LN freezing. The increase in the differential stress ratio does not reduce fracture complexity. Curved cracks, branched cracks, and bedding planes can be thermally induced around the borehole, and the aperture is micro-level. The longer the bare hole, the more thermally induced microcracks around the borehole and the more complex macro fractures. Additionally, LN pretreatment can effectively lower fracture initiation pressure.