Effects of secondary fractures on fault seismic rupture and aseismic slip during CO2 sequestration

Abstract

Fluid injection-induced fault activation and seismicity pose significant risks to the integrity of CO₂ geological sequestration projects. This study develops a computational model integrating two-phase fluid flow and geomechanics to investigate the influence of secondary fractures on fault activation and induced seismicity. The frictional strength variation of the fault and fractures is captured by a slip weakening model that incorporates healing mechanisms. A dynamic time-marching scheme is implemented to efficiently capture both slow aseismic slips and rapid seismic ruptures in a densely faulted/fractured reservoir, enabling detailed analysis of energy release during long-term CO₂ injection. Our results indicate that secondary fractures facilitate early-stage pressure dissipation, delaying fault slip and reducing seismic events. However, at later stages, secondary fractures contribute to increased seismicity, characterized by larger magnitudes and more extensive rupture zones. Critical pressure analysis reveals that seismicity propagates ahead of the fluid-pressurized zone, indicating that stress transfer plays a key role in triggering induced seismicity. Furthermore, we document a novel mechanism of seismic slip cascades developing in the fracture population, where fracture interactions boosted by stress transfer and aseismic deformation activate a series of fracture clusters to rupture in a bursting manner, promoting the spatial migration of seismic events beyond the fluid pressurization front. These findings provide new insights into the mechanisms of injection-induced seismicity, with far-reaching implications for CO₂ sequestration in fractured geological media.