The Formation and Pressure-Bearing Characteristics of Plugging Layers in Hydraulic Fracture: A Visualization Experimental Study

Summary Temporary plugging and diversion fracturing (TPDF) is an important means for improving the artificial fracture complexity in shale gas reservoirs. At present, most scholars’ studies on TPDF mainly focus on the formation conditions of the plugging layer in a horizontal fracture and the fracture propagation behavior after the plugging layer is formed. However, there is a lack of thorough study on the formation and pressure-bearing characteristics of the plugging layer in vertical fracture. For this paper, we conducted a plugging experiment using temporary plugging particles in a hydraulic fracture by use of a visualization hydraulic fracture experimental device to analyze the formation and pressure-bearing characteristics of plugging layers. The research results show that (1) when the ratio of temporary plugging particle diameter to fracture width (d/w) becomes larger, the fluid viscosity and injection rate have less influence on the formation of the plugging layer, and the concentration of temporary plugging particles required to form the plugging layer decreases. When d/w is equal to 0.45, the plugging layer has difficulty forming if the fluid viscosity is greater than 3 mPa·s or the mass concentration of temporary plugging particles is less than 20 kg/m3. If d/w is equal to 0.60, the plugging layer has difficulty forming when the concentration is less than 10 kg/m3. When d/w is equal to 0.75, a plugging layer forms when the concentration is 2.5 kg/m3, and the formation is not affected by the fluid viscosity and injection rate. (2) A smaller d/w, higher carrier fluid viscosity and injection rate, or lower temporary plugging particle concentration all lead to more pronounced fluctuation of the fracture flow channels at the location where the plugging layer is formed. (3) If the plugging layer can form, it is denser and has stronger plugging ability when the temporary plugging particle diameter is smaller and fluid viscosity and injection rate are larger. (4) Due to different lengths and d/w, the plugging layer can be divided into three types according to its morphological change characteristics after pressure-bearing: failure-unstable, locally-damaged, and stable-unchanged plugging layer. To improve the probability of forming the plugging layer with higher stability, the fluid with a viscosity of 3 mPa·s, in which is a temporary plugging particle with a d/w of 0.75, is recommended to plug the hydraulic fractures under an injection rate of 0.65 m3/min.