In order to figure out the damage and failure characteristics of Longmaxi shale in deformation and failure under loading, three cylindrical specimens were prepared to conduct realtime uniaxial tests. Micro-focus X-ray computed tomography (μ-CT) at different stress levels were used to obtain reconstructed stereograms, and horizontal CT images were obtained for analysis. Based on the scanning results, the initial mineral and pore distribution were extracted and found that they have a combined influence on the compression strength of shale. The cracking evolution were first characterized quantitatively by the changing of pore ratio and crack volume. Moreover, the final crack morphologies were established and the failure degree was assessed by crack volume. The effect of internal mineral and pore evolution on the fracture mode was discussed and showed that the scattered mineral has little effect on the final crack and the increase in pore ratio related to the crack volume.
Increasing demand for natural gas and declining production from conventional high-permeability and high-porosity reservoirs have led to an increase in natural gas production from shale formations (Curtis & Montgomery 2002, Kerr 2010, Struchtemeyer et al. 2012). Owing to the geological characteristics of shale in gas reservoirs that generally show low penetrability and remarkable brittleness, large-scale hydraulic fracturing treatments are usually essential in the development of shale gas (Yang et al. 2012, Li et al. 2013, Roshan et al. 2016). The process of drilling, well completion and fracturing techniques all related to the failure process of shale. So the study of the damage process is thus of great significance for a better understanding of the disaster mechanisms of shale.
X-ray computed tomography (CT) is a powerful non-destructive technique and a direct method to observe the internal distribution of materials, which has unique advantage in imaging three-dimensional (3-D) structures and identifying the internal cracking of rocks under different loading conditions (Louis et al. 2007, Chen et al. 2015). Some researchers have used the CT method to observe the internal structural characteristics of shale (Islam & Skalle 2013, Wang et al. 2016, Ma et al. 2016, Mokhtari et al. 2016) and to learn the different fracturing results under unloading condition (Hou et al. 2014, Heng et al. 2014, Wu et al. 2016, Zhang et al. 2017). For the analysis of rock damage evolution, some previous studies (Yang et al. 1998, Ge et al. 1999, Zhang et al. 2003, Feng et al. 2004, Liang et al. 2010, Cao et al. 2011) have used the average CT value based on CT images to characterize. In recent years, Li et al. (2017), Wang et al. (2018) and Duan et al. (2018) used Micro-focus X-ray computed tomography (μ-CT) to study the progressive failure characteristics of shale. Some insights have been gained into the initial structure characteristics and the destruction results of shale. However, further research and interpretation are still required.
