Linking Injection-Induced Seismicity to Permeability Changes

Abstract

The permeability along a fracture is not constant, but varies with geometrical complexities such as pre-existing cracks. The spatial permeability changes affecting fluid pressure distributions can generate a sophisticated source mechanism potential to interpret measured seismicity complexity in the field. In this study, the relation between injection-induced seismicity and changes in fracture permeability during hydraulic fracturing stimulation of naturally fractured reservoirs is investigated numerically. In the model, the infinite homogeneous rock is assumed to be impermeable and elastic and a plane-strain fracture is embedded in it with distributed cracks. The fluid flow in the fracture is realized through over pressure generated by constant-rate injection. When the over-pressurized fluid enters these cracks, the pressure varying trend is changed and the injection pressure tends to increase, rather than decrease monotonically. Also, the slipping is temporarily stopped along the whole fracture. When the barrier to fluid flow is overcome, a most prominent pulse-type slip at a limited slip speed occurs along the pressurized region. The slip pulse can induce an injection pressure drop reflecting the decrease of the stress level near the rupture tip. In the meantime, the slip pulse acts as the source mechanisms for these microseismic events during the fracturing stimulation operations. It is found that the stress drop and slip rate decrease with rupture growth. In addition, the stopping phase and the accelerating duration of the slip patterns are two interesting features to estimate the source sizes of the rupture complexities generated by the forced fluid flow along a fracture.