Influence of Increasing Mean Stress on Rock Fatigue Fracture Characteristics and Failure Mechanism

Pulsating hydraulic fracturing (PHF) is a highly potential fracturing technology. Because it can induce fatigue fracture of rocks and generate complex fracture networks by pumping fracturing fluid in pulse form. Understanding fatigue fracture characteristics and failure mechanism of rocks in PHF is crucial for optimizing fracturing construction parameters. In this paper, a series of fatigue fracture toughness experiments including monotonic loading, constant amplitude with constant mean stress(CACMS) cyclic loading and constant amplitude with increasing mean stress(CAIMS) cyclic loading are conducted, respectively. Moreover, fatigue fracture characteristics under these three cyclic loading forms are compared and analyzed. Furthermore, three-dimensional laser scanning is performed on the samples after fatigue fracture to investigate fatigue failure mechanism. Results show that the fracture toughness under CAIMS cyclic loading and CACMS cyclic loading are lower than that under monotonic loading. The samples subjected to cyclic loading failure dissipate more energy during failure, and the roughness of the rock failure surface is also greater than monotonic loading. The results of this paper help to understand the reasons for the higher efficiency of PHF, and provide guidance for the application of PHF technology. 1. INTRODUCTION Pulse hydraulic fracturing technology has been proven to be a promising technology (Wang et al., 2015; Ni et al., 2018; Lin et al., 2012). This technique induces fatigue damage to reservoir rock by regulating the pumping of fracturing fluid through pulse pressure or pulse flow rate (Ni et al., 2019). As a result, complex fracture network can be generated through the fatigue failure of reservoir rock (Zang et al., 2013; Xie et al., 2020). According to Hou et al. (2021), reservoir rock is subjected to cyclic load with a constant amplitude and an increasing mean stress (CAIMS) when fracturing fluid is pumped in the form of pulse flow rate. Understanding and optimizing the PHF mechanism requires knowledge of the fatigue failure characteristics of rock under this form of cyclic loading.