Injection-Triggered Seismicity: an Investigation of Porothermoelastic Effects Using a Rate-and-State Earthquake Model

Abstract

Physical processes associated with injection-triggered seismicity were investigated through the use of a numerical model. We investigated the role of the following physical mechanisms on causing triggered earthquake events: fluid pressurization within the fault zone, poroelastically-induced stress due to fluid leakoff into the rock surrounding the fault, and thermoelastically-induced stress due to cooling of the reservoir rock. A model of a fault that had a direct hydraulic connection to an injection well was used to develop a numerical experiment. In the model, relatively cold fluid was injected into the fault for a period of one day, and then the well was shut-in. A rate-and-state friction framework was used to model the earthquake nucleation, rupture, and arrest processes. Four simulations were performed in order to isolate the effects of the different physical mechanisms. We observed that, depending on which physical mechanisms were active, the overall behavior in seismicity differed significantly between the four cases. For the reservoir and fault parameters used in this study, it was observed that the poroelastic and thermoelastic stresses were of the same order of magnitude as the change in fluid pressure within the fault zone. Of particular interest, the thermoelastic stresses introduced significant levels of heterogeneity in the distribution of effective stress along the fault, which ultimately led to a markedly distinct character in the individual earthquake events and overall seismic pattern. This study demonstrated that the physical mechanisms investigated herein have the potential to control behavior during injection-triggered seismicity. However, it should be clearly stated that the results presented in this paper cannot generally be extended to all scenarios related to injection-triggered seismicity, and further parametric studies must be performed in order to classify the range of geological and operational settings over which each physical mechanism may be important.