Runaway Versus Stable Fracturing During Hydraulic Stimulation: Insights from the Damage Rheology Modeling

Abstract

Injection-induced seismicity might have unexpectedly large earthquake magnitudes. The rate of occurrence of seismic events is controlled by natural factors such as stress field and rock properties, as well as by operational factors such as injection pressure. Two faulting modes induced by fluid injection are considered: stable pressure controlled, in which the rupture is constrained to the high pore pressure locations and seismicity propagates away from the injection well as a diffusional front; and runaway mode, in which the rupture propagates beyond the high pore pressure zone and produces high-magnitude earthquakes. In this study, we simulate the injection-induced seismicity using a poro-elastic damage rheology model and determine the setups that lead to stable and unstable conditions. The value of the damage–permeability coupling parameter, which controls the increase of permeability with damage, has a major effect on the stability of the stimulated process. The role of injection pressure is also shown here to control the rate and stability of seismicity growth. The model is unique in its ability to spontaneously branch fractures in conjugate directions. At low injection pressures and weak permeability–damage coupling, branching occurs, and segments gradually grow in the two conjugate directions. Under higher injection pressure and strong permeability–damage coupling, active branching causes unstable segments that rapidly grow and produce hazardous conditions. Activation of pre-existing faults is also shown to be unsafe when the fault is optimally oriented with respect to the stress field. If the pre-existing faults are in other orientations, they may decrease the seismic hazard and even stabilize the system.