11:00 AM - 1:00 PM
[SCG52-P14] 3D analysis of secondary shear band formation using sandbox shear experiments
Keywords:Shear zone, Riedel shear, Analogue modeling, Sandbox experiment
The behavior of the fluid in the crust is important for the crustal deformation style and the formation of oil and gas reservoirs. The distribution and density of the secondary shear band formed around the main shear band control the flow path, flow rate, flow velocity, and fluid pressure. Analogue sandbox experiments are one of the effective methods for investigating the characteristics of secondary shear bands. To examine their formation process three-dimensionally, we conducted sandbox shear experiments and digital image analyses.
The base of the experimental apparatus consists of two acrylic half boxes of 200 mm × 700 mm × 200 mm depth, width, and height. Each box can be moved past the other by geared motor drives. Some 50 acrylic sticks, 5 mm × 5 mm wide and height, and 380 mm long, are stacked, which are attached to the side walls of the half boxes. As the base half boxes are displaced, the confined acrylic sticks slip past one another and the initial rectangular configuration becomes a parallelogram, thus simulating distributed shear deformation. The wooden frame is mounted on top of the acrylic sticks and the washed No.8 quartz sand is filled with a thickness of 40–90 mm in the frame. The whole shear strain of the sand layer γ in one experiment is set to 0.13-0.25. The surface deformation in each experiment is recorded by sequential photographs and analyzed using Digital Image Correlation Method (DIC). Some experiments are scanned with X-ray Computed Tomography (XCT).
About 50 experiments were conducted while changing the shape of the frame, the method of sand filling, and the thickness of the layer. The main results are as follows. (1) The secondary shear bands are formed from a relatively low constrain area in the sand layer. (2) There are mainly two types of secondary shear band, the Riedel shear plane (R plane) and the anti-Riedel shear plane (R' plane). The relative growth of R and R’ planes varied with the constrained state of the layer. (3) The XCT images show that propagation of the secondary shear band occurs three-dimensionally. In other words, the depth at which formation begins differs for each secondary shear band. (4) Based on the DIC analysis, the surface deformation begins with the homogeneous strain and shifts to the localized strain due to multiple secondary shear bands. The shifts occur from γ = 0.10 to 0.24 depending on the frame constrains.
The base of the experimental apparatus consists of two acrylic half boxes of 200 mm × 700 mm × 200 mm depth, width, and height. Each box can be moved past the other by geared motor drives. Some 50 acrylic sticks, 5 mm × 5 mm wide and height, and 380 mm long, are stacked, which are attached to the side walls of the half boxes. As the base half boxes are displaced, the confined acrylic sticks slip past one another and the initial rectangular configuration becomes a parallelogram, thus simulating distributed shear deformation. The wooden frame is mounted on top of the acrylic sticks and the washed No.8 quartz sand is filled with a thickness of 40–90 mm in the frame. The whole shear strain of the sand layer γ in one experiment is set to 0.13-0.25. The surface deformation in each experiment is recorded by sequential photographs and analyzed using Digital Image Correlation Method (DIC). Some experiments are scanned with X-ray Computed Tomography (XCT).
About 50 experiments were conducted while changing the shape of the frame, the method of sand filling, and the thickness of the layer. The main results are as follows. (1) The secondary shear bands are formed from a relatively low constrain area in the sand layer. (2) There are mainly two types of secondary shear band, the Riedel shear plane (R plane) and the anti-Riedel shear plane (R' plane). The relative growth of R and R’ planes varied with the constrained state of the layer. (3) The XCT images show that propagation of the secondary shear band occurs three-dimensionally. In other words, the depth at which formation begins differs for each secondary shear band. (4) Based on the DIC analysis, the surface deformation begins with the homogeneous strain and shifts to the localized strain due to multiple secondary shear bands. The shifts occur from γ = 0.10 to 0.24 depending on the frame constrains.