Experimental study on visualization simulation of coal powder dynamic migration and deposition in propped fractures

In view of the problem that the dynamic migration and deposition process of pulverized coal in propped fractures and the distribution characteristics of pulverized coal after sedimentation are blocked, the self-developed visualization simulation device for dynamic migration and deposition of pulverized coal was used to carry out simulation experiments of dynamic coal migration and deposition. Based on experimental feature image extraction and pixel gray level recognition, a grayscale matrix about the length and height of the visible crack plate is established, and the feature image is binarized. The ration of the number of pixels (the amount of coal powder) in each grid with a gray level of zero to the total pixels in the grid “Rc” is used to characterize the influence of coal powder deposition in the flow channel of propped fractures. The migration, deposition and plugging characteristics of coal powder at different positions of the propped fractures with different displacement and displacement time were compared and analyzed. The test results show that the migration and deposition of pulverized coal is mainly carried out in the flow channels formed by the interconnected accumulation pores. Due to the influence of gravity, more pulverized coal is deposited in the lower edge of the propped fracture. After the coal powder migrates into the proppant fractures, it will go through three stages of adsorption on the pore surface, bridging to block the pore roars, and filter cake to block the flow channel. Coal powder migrates from the accumulation area to the fracture outlet and deposits along the migration path. If the flow channel is not blocked, the migration and deposition equilibrium state will be reached. When coal powder does not block the flow channel, it presents “finger deposition characteristics”, otherwise it presents “massive deposition characteristics”. With the increase of displacement, the “finger deposition characteristics” tend to change to “massive deposition characteristics”. When the concentration of coal powder is constant and foreign coal powder continues to be replenished, the probability of flow channel clogging caused by coal powder increases with the increase of displacement and the blocking time will be advanced. In the early stage of drainage gas recovery, it is necessary to properly control the drainage to reduce the damage to the flow channel caused by the clogging of the pulverized coal deposition.