17:15 〜 18:45
[SCG40-P39] Temporary fracture sealing by silica precipitation in granite: insights from flow-through experiments under superhot conditions
キーワード：fracture, silica, seal, precipitation, flow-through, earthquake
Precipitation of silica is thought to influence the recurrence interval of fault slip events and the build-up of fluid pressure in seismogenic zones [1, 2]. Abundant quartz veins occur in seismogenic zones and in the region for slow earthquakes; however, our knowledge is still limited on the fracture sealing behavior with regards to the permeability and fluid pressure over time, and the resulting fluid pathway characteristics that follow a ruptured seal.
In order to investigate the temporal evolution of flow path permeability and changes during fracture sealing by silica precipitation, we utilized a flow-through apparatus that spans a temperature gradient (350°-430 °C) to simulate superhot natural conditions in which silica precipitation can occur. In this apparatus, 10-15 cm of granite core samples (9.8 mm x 50 mm) were housed in a stainless-steel (SUS) tube and have been cut lengthwise and polished to contain a rectangular fluid pathway along the approximate center of each core. A high silica solution (Si = 281 mg/kg, Al = 5.3 mg/kg) was flowed through the pathway for 48 hours along the temperature gradient at 0.2 mL/min, though future experiments may explore alternative durations and flow rates. Samples were analyzed using micro-XRF, X-ray CT, and SEM. Solution chemistry was analyzed by ICP-OES.
Preliminary results of a test experiment yielded a substantial amount of silica precipitation ~7 cm from the start of the granite, sealing a ~2 cm portion of two halves of one granite core together. This initial experiment did not yet have the pre-defined rectangular fluid pathway which is to be used for subsequent experiments. However, we can still see from X-ray CT analysis that crystal growth formed a sealing layer that spanned edge-to-edge across this portion of the slit (Figure 1). Towards the center, the seal spanned ~7 mm, growing in length closer to the edges with apertures or pockets up to 1 mm scattered throughout the seal. Further conclusions will be established following thin section analyses. Additionally, experimental data from upstream and downstream pressure gauges allow us to see that ~22 hours into the experiment, upstream pressure increased to a maximum just over 2 MPa differential pressure after 2.5 hours. During this timeframe, the differential pressure oscillated ~1 MPa every 10 min, while steadily rising overall (Figure 2). Following this maximum, the average differential pressure began decreasing, ultimately to ~0.5 MPa followed by a gradual increase back to 2 MPa ~46 hours into the experiment. This period witnessed oscillations as well that were less significant (~0.5 MPa), likely as a result of equipment instability. These preliminary results tell us that (1) we have successfully witnessed a quick formation of a silica seal in a narrow region, (2) the initial seal is sustained for brief, 10 minute intervals, then ruptures, and repeats followed by a slightly more stable, likely restricted flow path, and (3) we can use these timeframes to better understand how silica precipitation impacts earthquake slip recurrence intervals and to approximate slip magnitudes.
References: [1] Audet, P., & Bürgmann, R. (2014). Nature, 510(7505), 389-392. [2] Cox, S. F. (2016). Economic Geology, 111(3), 559-587.
In order to investigate the temporal evolution of flow path permeability and changes during fracture sealing by silica precipitation, we utilized a flow-through apparatus that spans a temperature gradient (350°-430 °C) to simulate superhot natural conditions in which silica precipitation can occur. In this apparatus, 10-15 cm of granite core samples (9.8 mm x 50 mm) were housed in a stainless-steel (SUS) tube and have been cut lengthwise and polished to contain a rectangular fluid pathway along the approximate center of each core. A high silica solution (Si = 281 mg/kg, Al = 5.3 mg/kg) was flowed through the pathway for 48 hours along the temperature gradient at 0.2 mL/min, though future experiments may explore alternative durations and flow rates. Samples were analyzed using micro-XRF, X-ray CT, and SEM. Solution chemistry was analyzed by ICP-OES.
Preliminary results of a test experiment yielded a substantial amount of silica precipitation ~7 cm from the start of the granite, sealing a ~2 cm portion of two halves of one granite core together. This initial experiment did not yet have the pre-defined rectangular fluid pathway which is to be used for subsequent experiments. However, we can still see from X-ray CT analysis that crystal growth formed a sealing layer that spanned edge-to-edge across this portion of the slit (Figure 1). Towards the center, the seal spanned ~7 mm, growing in length closer to the edges with apertures or pockets up to 1 mm scattered throughout the seal. Further conclusions will be established following thin section analyses. Additionally, experimental data from upstream and downstream pressure gauges allow us to see that ~22 hours into the experiment, upstream pressure increased to a maximum just over 2 MPa differential pressure after 2.5 hours. During this timeframe, the differential pressure oscillated ~1 MPa every 10 min, while steadily rising overall (Figure 2). Following this maximum, the average differential pressure began decreasing, ultimately to ~0.5 MPa followed by a gradual increase back to 2 MPa ~46 hours into the experiment. This period witnessed oscillations as well that were less significant (~0.5 MPa), likely as a result of equipment instability. These preliminary results tell us that (1) we have successfully witnessed a quick formation of a silica seal in a narrow region, (2) the initial seal is sustained for brief, 10 minute intervals, then ruptures, and repeats followed by a slightly more stable, likely restricted flow path, and (3) we can use these timeframes to better understand how silica precipitation impacts earthquake slip recurrence intervals and to approximate slip magnitudes.
References: [1] Audet, P., & Bürgmann, R. (2014). Nature, 510(7505), 389-392. [2] Cox, S. F. (2016). Economic Geology, 111(3), 559-587.