Modeling of Injection Induced Seismic Events

Abstract

Abstract Using a BEM-based hydraulic fracture model and the Coulomb frictional law, the generation of slip and shear rupturing produced by fluid injection is investigated. A wide range of factors can contribute to injection-induced seismicity. In addition to injection rate, fluid viscosity and existing permeability, several other factors are considered in this paper to determine their effects on slip development during continuous fluid injection and during shut-in. These factors include dilatant strengthening, slip weakening of frictional coefficients, structural complexities along a rupturing path like jogs and branches and the application of far-field wave-form stresses. The numerical method is summarized and the theoretical formulation of the above physical processes associated with generation of seismic events is provided. The numerical results show a rich and complex local slip rate change during extension of the slip zone. Under fluid injection, the slip can first produce a seismic event either caused by rapidly increasing pressure or by abrupt slip initiation, and then slows. When the permeability is large enough, the slip pattern manifests itself as long-term slow slip period coupled with aseismic slip transients. The long-duration and low-magnitude seismic events are associated with the slipping state in the conditionally stable regime, in which fluid pressure is marginally equal to a value required to produce slip using the effective stress and the static frictional strength. A small perturbation in the fluid pressure caused by any of the above factors can stop and/or generate relatively faster slip rates. The slip continues varying with time after shut-in in the presence of the above influencing factors. The slow slip on a fault, which is difficult to detect by seismology, could be a significant process in dissipating elastic strain energy during hydraulic fracturing stimulations. In addition, the model can capture a tendency of the slip rate to become significantly large. The results demonstrate the importance of fracture and fault permeability enhancement on variations of fluid pressure and slip rates.