3:45 PM - 4:00 PM
[SCG49-02] Visualization of the spatiotemporal changes in the stress distribution accompanying with the fluid injection through photoelastic experiments
Keywords:photoelasticity, induced earthquake, laboratory experiment
For efficient energy resource recovery, the fluid injection has been an important procedure in recent years. When the fluid gets into a preexisting fault or weak plane, increment of the pore pressure induces earthquakes. Though indirect stress estimation is possible from borehole measurements and seismicity, direct observation of the spatio-temporal changes in the stress state is difficult in the deep underground. Photoelasticity is the unique effect that enables visualization of the differential stress strength by using transparent materials in the laboratory. In this study, we establish the experimental procedure to visualize the spatio-temporal changes in the stress distribution during crack propagation through the agar gel as a simulation of the fluid injection. We analyze the stress change after the injection in agar samples with or without a weak plane.
We solidified the agar gel with 1 % concentration in a thin cuboid container (150 mm × 150 mm × 30 mm). Put the gel sample between two polarization plates and quarter wave plates and shed light from behind. During the water injection by a syringe through an acrylic pipe of 4 mm in diameter, we took a video with 120 fps. A weak plane was made by coagulating the agar at two separate times.
After the water injection started, water cracked the gel sample, and we observed an intense stress concentration on the tip of the crack. The stress strongly concentrates on ahead the crack tip, which is similar to the theoretical pattern of the mode I crack. We tested the sensitivity of the agar gel in this study and found that the highest differential stress on the crack tip is around 1.34 × 105 Pa. When the crack tip reached the weak plane, the propagation direction changed along the plane. The crack propagation speed showed a clear difference in the two regions; on the weak plane, the crack propagated four times faster than other areas with similar injection rates. We observed a very high stress concentration when the water was initially injected into the sample, and a crack reached the fault plane. Previous studies in the resource mining fields observed the immediate increase of seismic activities after the fluid injection. The high stress concentration described above showed the similarities with observation evidence in the natural fields. Arrangement of this experimental method would deepen understanding of the stress state around the injection hole.
We solidified the agar gel with 1 % concentration in a thin cuboid container (150 mm × 150 mm × 30 mm). Put the gel sample between two polarization plates and quarter wave plates and shed light from behind. During the water injection by a syringe through an acrylic pipe of 4 mm in diameter, we took a video with 120 fps. A weak plane was made by coagulating the agar at two separate times.
After the water injection started, water cracked the gel sample, and we observed an intense stress concentration on the tip of the crack. The stress strongly concentrates on ahead the crack tip, which is similar to the theoretical pattern of the mode I crack. We tested the sensitivity of the agar gel in this study and found that the highest differential stress on the crack tip is around 1.34 × 105 Pa. When the crack tip reached the weak plane, the propagation direction changed along the plane. The crack propagation speed showed a clear difference in the two regions; on the weak plane, the crack propagated four times faster than other areas with similar injection rates. We observed a very high stress concentration when the water was initially injected into the sample, and a crack reached the fault plane. Previous studies in the resource mining fields observed the immediate increase of seismic activities after the fluid injection. The high stress concentration described above showed the similarities with observation evidence in the natural fields. Arrangement of this experimental method would deepen understanding of the stress state around the injection hole.