Role of Fluid Diffusivity in the Spatiotemporal Migration of Induced Earthquakes during Hydraulic Fracturing in Unconventional Reservoirs

Hydraulic-fracturing-induced earthquakes exhibit an intricate pattern in spatiotemporal migration with respect to stage completions of fracturing horizontal wells. The underlying physical mechanisms remain uncertain. This paper investigates two field cases to quantify the effects of fluid diffusivity on the spatiotemporal migration of induced earthquakes during fracturing stimulation in shale reservoirs. First, the double-couple component approach is employed to determine the focal mechanisms of mainshock events. The relation plot of earthquake magnitudefault size as well as the spatial distribution of induced events are used together to characterize the fault distribution. Then, the tight rock analysis and triaxial compression experiments are conducted to determine the distinctive petrophysical and geomechanical properties of stimulated formations in both cases. The hydraulic diffusivity of fracturing fluids is then estimated from the spatiotemporal evolution of the seismicity fronts with respect to the fracturing sites. Finally, a poroelastic modeling and numerical simulation are performed to evaluate the time-dependent pore pressure diffusion and poroelastic stress perturbation during fracturing stimulations. Results suggest that the subvertical N-S-trending faults and NE45 degrees-trending hydraulic fractures generate the fracture and fault networks in the M(w)3.6 and M(w)3.4 cases, providing geological evidence to account for the spatial distribution of induced earthquakes within the networks. The shear stress gradient G is calculated to be 0.21 MPa/km for the M(w)3.6 case and -0.08 MPa/km for the M(w)3.4 case, indicating downward and slightly upward shear growth. The observed seismicity migration defines the hydraulic diffusivity of 0.15 m(2)/s for the M(w)3.6 event and 2 m(2)/s for the M(w)3.4 event. The poroelastic simulation characterizes the time-dependent pore pressure and poroelastic stress changes, matching well with the spatiotemporal migration of induced earthquakes during and after fracturing stimulations. The increasing pore pressure is the first-order factor in fault reactivation. Different time lags between stage completions and induced earthquakes indicate distinctive fluid diffusivity within the hydraulic fracture and seismogenic fault networks.