Study on the Fracturing Pattern and Damage Characteristics of Deep Shale by Liquid Nitrogen Fracturing

Abstract

ABSTRACT Liquid nitrogen (LN) can undermine the structure of the rock and its mechanical properties. It is also effective in engineering applications of deep resource exploitation. There are few lab experiments that use LN as a fracturing fluid directly. In this work, we have studied LN fracturing performances in deep shales under actual triaxial-confining stresses by a high-pressure LN fracturing device. Microcracks and matrix structures have been analyzed by scanning electron microscopy. The breakdown pressure and fracture morphologies have been compared with hydraulic fracturing. The result demonstrates that LN fracturing can obviously lower fracture initiation pressure and increase fracture complexity. The increase of differential stress ratio does not reduce fracture complexity. Microcracks and pore clusters can be induced on LN fracture surfaces, improving matrix permeability. Curved cracks, branched cracks, and horizontal bedding planes can be thermally induced around the borehole. Low fluid viscosity of LN can facilitate the fracture propagation and network generation during the fracturing process. The longer the bare-hole, the more thermally induced microcracks around the borehole, and the more complex macro fractures. Moreover, LN pretreatment can lower fracture initiation pressure actually. The productive findings obtained in this work are expected to provide an alternative for the sustainable development of deep shales resources. INTRODUCTION Deep shale gas resources below 3400 m are widely distributed and have the potential for sustainable natural gas development(Altammar et al., 2019).The deep shale formations have characteristics of low porosity and permeability and characterized by significant in-situ stresses, horizontal stress differences, and high temperature(Carpenter, 2017). Hydraulic fracturing is a traditional measure to stimulate these formations. However, the burden of hydraulic fracturing operations has been a topic of great importance to the energy industry and public alike(Han et al., 2018). Traditional hydraulic fracturing is challenging to generate complex fractures, and the production is usually less than expected (Cong et al., 2021). Cryogenic liquid nitrogen (LN, −196 °C at atmospheric pressure) fracturing is one possible method of solving these water-related issues(Yang et al., 2019). When LN is injected into the reservoir formation, the sudden heat transfer could cause shrinkage of the rock and exert a sharp thermal gradient that can induce numerous fractures at the surface of the formation rocks (Elwegaa et al., 2019). The significant thermal stress between LN and rock will lower fracture initiation pressure and create complex fractures(Wan et al., 2018). Therefore, LN fracturing will be an effective engineering method to develop high-temperature or deep formation resources(Wan et al., 2019).