1:45 PM - 3:15 PM
[SSS06-P02] Effects of effective pressure on fault behavior triggered by fluid injection
Keywords:Induced seismicity, Effective pressure, Injection rate, Granite
Seismic activity induced by fluid injection, which associated with hydrocarbon reservoirs and geothermal energy, has been observed in recent years (e.g., Shapiroet al.,1997; Lei et al., 2008). In previous studies, the injection rate and the injection volume are thought to play a key parameter for the induced seismicity (e.g., McGarr, 2014; Passelegue et al., 2018 ). However, effective pressure is also considered to become one of the important factors of induced seismicity since elevating the fluid pressure during fluid injection decreases the effective pressure. We thus conducted laboratory experiments to investigated the contribution of the effective pressure to the mechanism and magnitude of fault slip caused by injection.
In the fluid injection tests, thermally cracked Inada granites were used. The samples were shaped into a cylinder with a precut surface at an angle of 30 degrees to the axial direction. Experiments were performed at a constant confining pressure of 33.5 MPa and pore fluid pressure of 2.2-23.9 MPa, by using a triaxial oil-medium compression test system at AIST. The pore fluid was injected at constant volumetric rates of 0.05 mL/min or 5 mL/min. Our preliminary results are summarized as follows. The behavior of shear stress drop varies by effective pressure condition. At injection rate of 0.05 mL/min, a sudden shear stress drop of ~6.6 MPa occurred under a high effective pressure of 24.6 MPa, on the other hand, a sudden stress drop of ~3.3 MPa and following slow stress drop of ~3.6 MPa were occurred during increasing fluid pressure to effective pressure of 9.6 MPa. At injection rate of 5 mL/min, the same behavior is confirmed, i.e. a rapid stress drop of ~5.3 MPa under high effective pressure of 20.4 MPa and a sudden stress drop of ~3.3 MPa and following slow stress drop of ~3.4 MPa under low effective pressure of 9.6 MPa. Compared with the results at a low effective pressure, the amount of the total stress drop shows almost the same value regardless of injection rate. These results suggest that the fault slip behavior and the amount of rapid stress drop may be affected by the effective pressure rather than the injection rate.
In the fluid injection tests, thermally cracked Inada granites were used. The samples were shaped into a cylinder with a precut surface at an angle of 30 degrees to the axial direction. Experiments were performed at a constant confining pressure of 33.5 MPa and pore fluid pressure of 2.2-23.9 MPa, by using a triaxial oil-medium compression test system at AIST. The pore fluid was injected at constant volumetric rates of 0.05 mL/min or 5 mL/min. Our preliminary results are summarized as follows. The behavior of shear stress drop varies by effective pressure condition. At injection rate of 0.05 mL/min, a sudden shear stress drop of ~6.6 MPa occurred under a high effective pressure of 24.6 MPa, on the other hand, a sudden stress drop of ~3.3 MPa and following slow stress drop of ~3.6 MPa were occurred during increasing fluid pressure to effective pressure of 9.6 MPa. At injection rate of 5 mL/min, the same behavior is confirmed, i.e. a rapid stress drop of ~5.3 MPa under high effective pressure of 20.4 MPa and a sudden stress drop of ~3.3 MPa and following slow stress drop of ~3.4 MPa under low effective pressure of 9.6 MPa. Compared with the results at a low effective pressure, the amount of the total stress drop shows almost the same value regardless of injection rate. These results suggest that the fault slip behavior and the amount of rapid stress drop may be affected by the effective pressure rather than the injection rate.