2:30 PM - 2:45 PM
[SCG58-09] Slow stress-drop due to sintering of crushed particles of feldspar in granitoids during faulting under hydrothermal condition
Keywords:axial compression experiments, hydrothermal condition, mineral-phase-dependent-healing
To minimize a risk of induced earthquakes during the construction of a fracture reservoir in granitoids for the development of ultra-high enthalpy geothermal system, we conducted axial compression experiments on granite specimens (Oshima granite, Eehime Pref., Japan) under high-temperature conditions (400-750 deg. C) with a set of constant confining and pore water pressure of 104 MPa and 39 MPa, respectively. Those experimental conditions simulated quaternary granitoids buried at almost 4 km depth. We deformed the specimens with a constant strain rate of 2.5*10-6 /s.
For all temperature cases, we obtained the stress-strain curves that differential stress initially increased elastically and then decreased with failure after reaching the peak stress. At higher temperature, the peak stress became lower. The high temperature also brought a result of slow and gentle stress-drop. At 400 deg.C, to take c.a. 1.5 % axial strain required almost 15 minutes for the stress-drop of 303 MPa, while at 750 deg.C it required almost 65 minutes for that of 149 MPa. Microscopic observations on the experimental products elucidated that fracture distribution pattern was varied from localized to conjugated and pervasive ones with increasing the temperature. Additionally, the fractures through feldspar (both alkali-feldspar and plagioclase) grains were obscure and low porosity, while the pores due to fracturing in quartz grains were clear even at higher temperature. SEM observations of the samples deformed at temperature higher than 550 deg.C revealed that the size of crushed particles in feldspar was sub-micrometer in fault gouge zone within feldspar and they sintered each other. This sintering resulted in the healing of fractures. On the other hand, sintering of crushed particles was absent with the quartz at the temperatures below 750 deg.C.
The observed fracture healing depending on minerals resulted in the disconnection of fracture network in the granite rock. Additionally, that healing and crushing of feldspar grains possibly occurred simultaneously and resulted in the slow process of failure at high temperature. That process may reduce a potential of seismicity during the geothermal system development. On the other hand, fracturing related porosity in quartz could be sustainable below the temperature of 750 deg.C. Therefore, a combination of the fracture in the quartz and healing in the feldspars resulted in a framework of the porous media along the shear bands. Although we didn’t yet evaluate permeability of the deformed granite, the lifetime of the porous media under geothermal conditions, nor the stability against the deformation, our result implies the potential of the fracture reservoir forming in the ultra-high enthalpy geothermal system.
This research includes results obtained from a project, JPNP18008, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).
For all temperature cases, we obtained the stress-strain curves that differential stress initially increased elastically and then decreased with failure after reaching the peak stress. At higher temperature, the peak stress became lower. The high temperature also brought a result of slow and gentle stress-drop. At 400 deg.C, to take c.a. 1.5 % axial strain required almost 15 minutes for the stress-drop of 303 MPa, while at 750 deg.C it required almost 65 minutes for that of 149 MPa. Microscopic observations on the experimental products elucidated that fracture distribution pattern was varied from localized to conjugated and pervasive ones with increasing the temperature. Additionally, the fractures through feldspar (both alkali-feldspar and plagioclase) grains were obscure and low porosity, while the pores due to fracturing in quartz grains were clear even at higher temperature. SEM observations of the samples deformed at temperature higher than 550 deg.C revealed that the size of crushed particles in feldspar was sub-micrometer in fault gouge zone within feldspar and they sintered each other. This sintering resulted in the healing of fractures. On the other hand, sintering of crushed particles was absent with the quartz at the temperatures below 750 deg.C.
The observed fracture healing depending on minerals resulted in the disconnection of fracture network in the granite rock. Additionally, that healing and crushing of feldspar grains possibly occurred simultaneously and resulted in the slow process of failure at high temperature. That process may reduce a potential of seismicity during the geothermal system development. On the other hand, fracturing related porosity in quartz could be sustainable below the temperature of 750 deg.C. Therefore, a combination of the fracture in the quartz and healing in the feldspars resulted in a framework of the porous media along the shear bands. Although we didn’t yet evaluate permeability of the deformed granite, the lifetime of the porous media under geothermal conditions, nor the stability against the deformation, our result implies the potential of the fracture reservoir forming in the ultra-high enthalpy geothermal system.
This research includes results obtained from a project, JPNP18008, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).