Hydraulic fracturing is a method routinely used in oil and gas extraction and in engineered geothermal systems. While applied frequently, there are many aspects of hydraulic fracturing that are not well understood, such as the effect of the vertical load and fracture geometry in the physical processes responsible for the development of hydraulically-created fractures. This paper presents an experimental study, in which prismatic granite specimens subjected to two different vertical loads (0 and 5 MPa) were hydraulically-fractured by applying a hydraulic pressure inside single and double pre-existing flaws. The fracturing processes were observed using High-Speed Video and High-Resolution cameras. In addition, the fracturing observed in the hydraulic fracturing tests was also compared to uniaxial compression tests performed on similar granite specimens with pre-fabricated flaws but no hydraulic pressure (“dry” tests). It was observed that cracking initiation was usually preceded by white patches, i.e. a process zone consisting of micro-cracks. There are two types of white patching, namely, “long-lasting” which last for several seconds before transforming into a physical crack and “short-lasting” which only exist for fractions of a millisecond, before transforming into a visible crack. The latter seems to be specific to hydraulic fracture tests since it was not observed in tests without hydraulic fracturing. While cracking was mostly tensile, there was also minor shearing along grain boundaries between segments of tensile cracks. Most importantly, a combined effect of loading conditions and existing flaw geometries on the fracturing processes was observed. At low bridging angles, there is a distinct difference of the coalescence pattern depending on the loading condition: direct coalescence when no vertical load was applied and no coalescence when a vertical load of 5 MPa was applied. At higher bridging angles, there is no effect of the loading conditions. This is consistent with the water pressure at failure (WPmax): for higher bridging angles, WPmax is the same regardless of the loading conditions, while for lower bridging angles WPmax is larger when a vertical load of 5 MPa is applied.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Mechanics of Materials
- Mechanical Engineering