Role of visco-elasto-plastic deformation in localization of injection-induced microseismic response

The risk assessment of injection-induced seismicity usually combines a poroelastic framework with a rate-and-state seismicity model. This allows for prediction of the stress changes caused by the injection and enables estimation of the frequency of (micro)seismic events in response to these changes. However, this constitutive framework neglects the time-dependent deformation, which is essential at subcritical stress regime. This work presents the laboratory brittle creep experiments on crystalline and sedimentary rock from the Illinois Basin, where microseismic activity was recorded during CO₂ storage operations. The specimens are instrumented with strain gauges and LVDT sensors to monitor their deformation over time, as well as acoustic emission sensors to capture the microcracking activity. The shear viscosity associated with the time-dependent response appears to be exponentially sensitive to the applied deviatoric stress and is measured in the range between 10¹⁷-10¹⁵ Pa·s for the secondary creep stages, and on the order of 10¹⁴ Pa·s during the initiation of the tertiary stage. Locally, the state of stress at the injection site is influenced by stratigraphy and heterogeneity of geologic formations, causing variations in acting deviatoric stress of about 1–2 MPa. Because of the exponential dependence of the shear viscosity on applied deviatoric stress, even small stress variations (on the order of a few MPa) can significantly affect the localization of the time-dependent deformation and shorten the time to failure in critically stressed zones, which cannot be accounted for within the poroelastic framework.