5:15 PM - 6:45 PM
[SCG46-P01] Behavior of the unfavorably oriented faults due to fluid injection: implication from laboratory experiments
Keywords:Pore poressure, Induced earthquake, Fault behavior
Fluid injection into the subsurface is essential for various subsurface applications including geothermal energy and carbon capture and storage. As a result, a fault with an unfavorably orientation to the maximum principal stress in the regional stress has sometimes been moved by fluid injection (e.g., Cochran et al., 2020; Lei et al., 2020). Sibson (1990) points out that unfavorably oriented faults can be moved not only as fluid-injection induced earthquakes , but also in natural earthquakes caused by increased pore pressure due to rising deep fluids. For example, the earthquake epicenters in the Noto Peninsula and Niigata Prefecture have located on the unfavorably oriented faults to the direction of maximum principal stress in the regional field (e.g., Sibson, 2007; Kato et al., 2008). Previous experimental studies have performed focusing on fluid injection parameters, such as water injection rate and/or injection volume (e.g., Passelegue et al., 2018), but there are few laboratory tests investigated the relationship between fluid injection and unfavorably oriented fault (Ji et al., 2022).
In this study, we conducted the fluid injection laboratory experiments on the samples with three different angles (30°, 40°, and 50°) of the simulated fault plane to the principal stress axis in order to investigate how the fault behavior changes with the angle. The results show that the shear stresses after the stress drops became almost the same values under the same effective pressure conditions regardless of the fault angle; therefore, friction coefficient showed the similar values. In addition, the time between the beginning of injection and the stress drop of the sample with large angle of the fault plane was shorter than that of the sample with small angle. When the friction coefficient is the same value, the normal stress on the fault plane increases with increasing the angle. It may result in limited pathway of the fluid, localized high pore pressure conditions, and finally the reduction of the time taken to the stress drop. This property corresponds to the observation: the foreshock occurred on unfavorably oriented fault though the optimal orientation fault, which is the epicenter of the mainshock, is located at the similar distance from the injection site (Cochran et al., 2020).
In this study, we conducted the fluid injection laboratory experiments on the samples with three different angles (30°, 40°, and 50°) of the simulated fault plane to the principal stress axis in order to investigate how the fault behavior changes with the angle. The results show that the shear stresses after the stress drops became almost the same values under the same effective pressure conditions regardless of the fault angle; therefore, friction coefficient showed the similar values. In addition, the time between the beginning of injection and the stress drop of the sample with large angle of the fault plane was shorter than that of the sample with small angle. When the friction coefficient is the same value, the normal stress on the fault plane increases with increasing the angle. It may result in limited pathway of the fluid, localized high pore pressure conditions, and finally the reduction of the time taken to the stress drop. This property corresponds to the observation: the foreshock occurred on unfavorably oriented fault though the optimal orientation fault, which is the epicenter of the mainshock, is located at the similar distance from the injection site (Cochran et al., 2020).