The discrete fracture process zone of shale on micrometer scale

In the shale formations stimulated by hydraulic fracturing, the development of micro-fractures invariably results in macro-fracture propagation and connects nano pore spaces, whose characteristics defined as a fracture process zone (FPZ) previously considered to be a crack zone ahead of a fracture, dominate fracturing process. However, there is still a lack of understanding of how micro-cracks nucleate to form FPZ and control fracture propagation. This paper reports on a discrete FPZ comprising the discontinuous FPZ (DFPZ) and the continuous FPZ (CFPZ) for shales, based on scanning electron microscopy (SEM) and digital image correlation techniques (DIC) from threepoint bending tests of shale, resulting mainly from shale heterogeneity in micro-scale due to the components difference, characterized with nanoindentation tests. DIC shows that DFPZ precedes CFPZ, occurring at the edge of hard components, and DFPZ closes completely in shales of weak heterogeneity or partly in those of strong heterogeneity, when CFPZ or fracture initiates. However, DFPZ reopens afterwards when CFPZ or a fracture propagates. SEM shows DFPZ controls fracture kinking and branching of a main fracture, influencing mode I fracture toughness, which is prominent in shales of strong heterogeneity where the CFPZ reduces in length and DFPZ closes, and in shales of weak heterogeneity the main fracture formed by CFPZ and DFPZ connecting.