Creep failure of dry and fluid-saturated rock

Strength of the materials involved in the fluid injection is usually well assessed and injection pressure is maintained below the critical value to avoid induced seismicity. However, a microcracking activity could be triggered at stress levels well below the peak strength and could lead to rock failure in the long-term perspective, even if the effective stress state is not changing. Pore fluid could significantly affect the inelastic response and makes brittle creep in rock more pronounced. Triaxial creep tests are performed on dry and oil-saturated Berea sandstone specimens instrumented with strain gauges, LVDTs, and acoustic emission sensors. Recorded creep rate is highly correlated with the acoustic emission activity and sensitive to the experimental conditions (e.g., stress level). Dry and oil-saturated specimens exhibit similar behavior; however, some sensitivity to the type of the pore fluid and drainage condition is anticipated. Variation of the b-value might be one of the prognostic features for the transition between steady-rate and accelerating creep. Accurate representation of the in-situ condition in laboratory experiments is required for capturing the inelastic response of the material at constant subcritical stress level and addressing the long-term injection-induced seismicity.