Experimental research on visible seepage of sandstone fissure using digital image-based method

The visual fracture seepage test devices independently developed apply the color tracer image digital processing technology to carry out the visual seepage tests on the sandstone fractures with vertical spatial angle under the coupling effects of loading and unloading with different osmotic pressure and normal stress gradations. By means of the millisecond frame division technology, the seepage state of the crack is captured and the color domain of the seepage area is partitioned. Based on the digital image technology, the binary processing is carried out. The parameters of the seepage area are identified by double overlay of layers, and the diffusion law of the seepage area under the action of osmotic pressure and normal stress is established. The characteristics of the change of the flow width of each section along the dominant seepage path of the fracture are studied, and the formula for calculating the visual seepage velocity is put forward. It is pointed out that the seepage velocity changes along the flow diameter into three stages: sudden increase, sudden decrease and uniform loss. A functional model for the peak velocity corresponding to the minimum section width is established under the coupling action of osmotic pressure and normal stress. Based on the measured data of the main seepage path, the power function for the stress normal, osmotic pressure and Reynolds number is established, the change point of seepage state in the inertia function area and viscous effect area is calibrated, and the prediction model for Reynolds number of visualized fissure flows is formulated. The new theories and research method for the key scientific issues such as identification of real seepage in fissure seepage path, real-time change of velocity vector and determination of fluid state are put forward.