Modelling of fault reactivation mechanisms and associated induced seismicity in rocks with different elastic materials

Abstract

The mechanical failure of fault plane during fluid injection can be conveniently approached by numerical methods, and the results can be applied in fault slip analysis to determine the corresponding magnitude of induced seismicity. During hydrofracturing, when faults are present and the fluid is injected into the fault, micro-seismic events are possible, although the magnitude is often somewhat larger than those associated with micro-seismic events produced from regular hydraulic fracturing because of the larger surface area available for fault rupture. This study considers the rate at which the changing elastic properties of materials influences the magnitude of seismic event during fault injection. The simulation is carried out under varying injection flow rates from 0.18 kg/s to 0.3 kg/s, and the thermo-hydro-mechanical (THM) model in FLAC3D is adopted. As the material elastic moduli increase significantly under isothermal injection, the resulted non-uniformity in the fault slip timing affects the magnitude of injection-induced seismicity. Rocks with lower moduli produced higher slip distance and seismicity during shear failure. However, in the coupled thermal case, the magnitudes of seismicity during injection are largely enhanced at lower elastic properties, which suggests that the energy of accumulated fluid pressure produces a larger rupture and longer slip displacement in cold injection than in the isothermal case. The resulting volumetric strain, both in the fault zone and in the matrix, is higher in lower moduli, meanwhile, it is much developed in non-isothermal injection as a result of the rock's response to the sum effect of thermal strain and the stress-induced strain.