Experimental study of hydraulic facture propagation behavior during temporary plugging re-fracturing in coal formation

Re-fracturing is an effective means of increasing production when the primary fracture declines in CBM exploitation. In order to study the crack propagation law of re-fracturing in coalbed methane reservoirs and optimize the parameters of temporary plugging, a simulation experiment of indoor temporary plugging and fracturing was carried out. In this study, a large-scale true triaxial fracturing simulation system was used, and the sample size was 30 cm × 30 cm × 30 cm. After collecting coal and rock in the Qinshui Basin, the standard experimental samples were obtained by wrapping concrete. The experiment first simulates the formation environment by loading triaxial stress, and then injects fracturing fluid into the wellbore with a constant displacement to simulate the initial fracturing. The plugging agent was injected into the wellbore and the re-fracturing experiment was carried out; at the end of the experiment, the fracture morphology of the re-fracturing was analyzed by CT scanning. The experimental results show that the temporary plugging effect is greatly affected by the particle size and dosage of the temporary plugging agent. The particle size of the temporary plugging agent is too large, which affects the sand suspension effect of the fracturing fluid. The temporary plugging agent is easy to accumulate in the wellbore, resulting in an abnormal increase in the re-fracturing construction pressure. The particle size of the temporary plugging agent is too small, and it is difficult to form effective plugging, during the re-fracturing, the fractures mainly propagate along the primary fractures. With the increase of the amount of temporary plugging agent, the plugging effect on the primary fracture is enhanced, and the rupture pressure of re-fracturing increases, which promotes the expansion of re-fracturing fractures in the direction perpendicular to the primary fracture; on the other hand, the temporary plugging agent enters the re-fracturing. After fracturing fractures, secondary plugging is formed, which promotes the formation of branched fractures and increases the complexity of re-fracturing. It is believed that temporary plugging and fracturing can promote the formation of branch fractures, improve the complexity of re-fracturing fractures, and increase the fracturing area to improve the effect of re-fracturing. The fracture morphology of the re-fracturing is mainly affected by factors such as the particle size of the temporary plugging agent and the matching degree of the primary fracturing fractures and the amount of the temporary plugging agent.