We report experimental research on frictional strength and stability properties of shale fractures during slip. Longmaxi shale, Green River shale and Marcellus shale are selected for constant-velocity and velocity-stepping fracture shear experiments. Combing with theoretical analysis, micro controlling mechanism of mineralogy on frictional strength and stability properties of shale fractures is discussed. Results indicate that the fracture friction-stability relationship is largely affected by shale mineralogy. Frictional strength of shale fractures increases when tectosilicate content increases and phyllosilicate content decreases; during velocity-stepping experiments, three kinds of shale show velocity-strengthening behavior, which means an aseismic creep tends to occur; with the increase content of tectosilicate, shale fractures tend to have seismic slip while with the increase of phyllosilicate content, stable aseismic creep happens.
With the advances in horizontal drilling and multi-stage fracturing technologies, shale gas production has a substantial growth not only in North America, but also in other areas around the world (Jia et al., 2018; Wu et al., 2017). However, as the basic stimulation method to enhance shale gas recovery, hydraulic fracturing is implemented for almost every shale gas wells. After hydraulic fracturing operations, large-scale waste water re-injection has been linked to seismic activities (Bao and Eaton, 2016; Ellsworth, 2013; Elsworth et al., 2016), which has raised public concerns and makes hydraulic fracturing prohibited in many areas across the world.
The large-scale injection of waste water generates overpressures and decreases effective normal stresses which leads to reactivation of pre-existing faults and fractures in formations (Ellsworth, 2013), which is shown in Fig 1. In addition, the fracturing operations induced fractures may also be the source for seismic activities. Hence, the stability of pre-existing and induced fractures will decide whether seismic activities happen.
Fig. 1. Mechanism of induced seismicity by large-scale water re-injection. Natural fractures and hydraulic fracturing induced fractures may be re-activated by overpressure caused by fluid injection.