5:15 PM - 7:15 PM
[SSS10-P31] Effects of pore fluid pressurization rate and permeability due to friction surface roughness on friction behaviors
Keywords:Injection-induced earthquake, Friction behaviors, Pore fluid pressure
The increase in pore pressure caused by fluid injection into deep underground formations for geothermal development poses a risk of inducing seismic slip. An increase in pore pressure on a fault plane reduces the effective normal stress and shear strength, thereby making the fault plane more susceptible to slip. The distribution of pore pressure on the fault plane correlates with the distribution of shear strength, potentially influencing the occurrence of injection-induced earthquakes. The distribution of pore pressure on a fault plane is expected to depend on the rate of pore pressurization during fluid injection (vp) and permeability along the fault plane. However, the effects of vp and permeability along the fault plane on frictional behavior remain unclear.
This study aims to elucidate the influence of the vp and fault surface permeability, which is associated with surface roughness, on frictional behavior though fluid injection friction experiments.
The rock sample was a cylindrical Aji granite specimen with a diameter of 40 mm and a length of 80 mm. This sample was cut at an angle at 30 degrees relative to the axial direction. To introduce pore fluid to the friction surface, a 2.5 mm diameter hole was drilled perpendicular to the friction surface, extending through the center of the bottom surface. To create friction surfaces with different permeabilities, the surfaces were polished with waterproof abrasive paper with two different grain size, #80 and #3000.
Fluid injection experiments were conducted using a triaxial rock deformation apparatus. The main experimental conditions were a confining pressure of 60 MPa, an initial pore pressure of 0.1 MPa, and an axial loading rate of 0.1 mm/min. The experimental procedure involved maintaining a constant initial pore pressure and applying axial loading until a stick-slip and to keep the axial displacement was hold constant just before the next slip. Simultaneously, a syringe pump was used to control the pore pressure at constant vp. Two vp conditions were tested: vp = 0.1 MPa/min and 1.0 MPa/min. Downstream, the valve was closed, and the pore pressure was monitored using a downstream pore pressure gauge. Based on the experimental results, the effects of different vp and friction surface roughness on friction behavior were analyzed.
A comparison of the upstream pore pressure during slip in the vp = 0.1 MPa/min and vp = 1.0 MPa/min experiments on the #3000 specimens showed that, slip occurred at an upstream pore pressure of 6.9 MPa in the vp = 0.1 MPa/min experiment, and at 16.3 MPa in the vp = 1.0 MPa/min experiment. This indicates that the upstream pore pressure at the onset of slip was lower in the vp = 0.1 MPa/min experiment. This result is consistent with the findings of Passelegue et al. (2018) and is likely due to the more uniform distribution of pore pressure on the friction surface at lower vp. A comparison of the shear stress drop during slip revealed that, the vp = 0.1 MPa/min experiment resulted in a shear stress drop of 2.29 MPa, whereas the vp = 1.0 MPa/min experiment resulted in a shear stress drop of 3.30 MPa. The larger vp led to greater shear stress drop during slip, suggesting that the higher vp may result in larger seismic magnitude.
A comparison was made between the #80 and #3000 specimens in the vp = 1.0 MPa/min experiment. In the #80 sample, slip occurred at an upstream pore pressure of 24.6 MPa, whereas in the #3000 sample, slip occurred at 16.3 MPa. Thus, slip occurred at a higher upstream pore pressure in the #80 sample. It was initially expected that the #80 sample, which has higher permeability, would slip at a lower upstream pore pressure because the pore pressure transmission to the friction surface. However, the experimental results deviated from this prediction. One possible explanation is that the friction surface of the #80 specimen was not sufficiently polished.
References
F.X. Passelegue, et al.,2018, Geophysical Research Letters,45,12,837-12,846.
This study aims to elucidate the influence of the vp and fault surface permeability, which is associated with surface roughness, on frictional behavior though fluid injection friction experiments.
The rock sample was a cylindrical Aji granite specimen with a diameter of 40 mm and a length of 80 mm. This sample was cut at an angle at 30 degrees relative to the axial direction. To introduce pore fluid to the friction surface, a 2.5 mm diameter hole was drilled perpendicular to the friction surface, extending through the center of the bottom surface. To create friction surfaces with different permeabilities, the surfaces were polished with waterproof abrasive paper with two different grain size, #80 and #3000.
Fluid injection experiments were conducted using a triaxial rock deformation apparatus. The main experimental conditions were a confining pressure of 60 MPa, an initial pore pressure of 0.1 MPa, and an axial loading rate of 0.1 mm/min. The experimental procedure involved maintaining a constant initial pore pressure and applying axial loading until a stick-slip and to keep the axial displacement was hold constant just before the next slip. Simultaneously, a syringe pump was used to control the pore pressure at constant vp. Two vp conditions were tested: vp = 0.1 MPa/min and 1.0 MPa/min. Downstream, the valve was closed, and the pore pressure was monitored using a downstream pore pressure gauge. Based on the experimental results, the effects of different vp and friction surface roughness on friction behavior were analyzed.
A comparison of the upstream pore pressure during slip in the vp = 0.1 MPa/min and vp = 1.0 MPa/min experiments on the #3000 specimens showed that, slip occurred at an upstream pore pressure of 6.9 MPa in the vp = 0.1 MPa/min experiment, and at 16.3 MPa in the vp = 1.0 MPa/min experiment. This indicates that the upstream pore pressure at the onset of slip was lower in the vp = 0.1 MPa/min experiment. This result is consistent with the findings of Passelegue et al. (2018) and is likely due to the more uniform distribution of pore pressure on the friction surface at lower vp. A comparison of the shear stress drop during slip revealed that, the vp = 0.1 MPa/min experiment resulted in a shear stress drop of 2.29 MPa, whereas the vp = 1.0 MPa/min experiment resulted in a shear stress drop of 3.30 MPa. The larger vp led to greater shear stress drop during slip, suggesting that the higher vp may result in larger seismic magnitude.
A comparison was made between the #80 and #3000 specimens in the vp = 1.0 MPa/min experiment. In the #80 sample, slip occurred at an upstream pore pressure of 24.6 MPa, whereas in the #3000 sample, slip occurred at 16.3 MPa. Thus, slip occurred at a higher upstream pore pressure in the #80 sample. It was initially expected that the #80 sample, which has higher permeability, would slip at a lower upstream pore pressure because the pore pressure transmission to the friction surface. However, the experimental results deviated from this prediction. One possible explanation is that the friction surface of the #80 specimen was not sufficiently polished.
References
F.X. Passelegue, et al.,2018, Geophysical Research Letters,45,12,837-12,846.