Impact of Injection Style on the Evolution of Fluid-Induced Seismicity and Permeability in Rock Mass at 410 m Depth in Äspö Hard Rock Laboratory, Sweden

An underground experiment in Aspo Hard Rock Laboratory is described to bridge the gap between findings from laboratory hydraulic fracturing tests and wellbore-size fluid stimulation in hard rock. Three different water injection schemes are tested quantifying seismic radiated energy, hydraulic energy pumped into the rock, and resulting permeability of the generated fractures. Six hydraulic fracturing tests are performed from a horizontal borehole 102 mm in diameter and 28 m long at 410 m depth and drilled from an existing tunnel in the direction of minimum horizontal stress. Fracture initiation and propagation are mapped by acoustic emission monitoring and impression packer in a rock mass volume 30 × 30 × 30 m in size. In tendency, the fracture breakdown pressure is lower and the number of fluid induced seismicity events is smaller if conventional monotonic hydraulic fracturing is replaced by cyclic, progressive injection and/or pulse pumping schemes. The related permeability of the generated fracture can be increased. The maximum permeability increase results from a combination of cyclic and pulse hydraulic fracturing. Laboratory testing of drill-cores from the long borehole show that dynamic pulsing in combination with progressive cyclic pressurization lower fracture breakdown pressure 10–20%. The interpretation for this result is that during fatigue hydraulic fracturing by cyclic/pulse pumping, a larger process zone develops which is accompanied by many smaller seismic events.