Multi-fracture initiation sequence and breakdown pressure in horizontal wells during TDPF: A visualization experimental investigation based on PMMA

Abstract

Temporary plugging and diverting fracturing (TDPF) is an effective stimulation method to achieve multiple uniform fracture propagation in unconventional reservoirs. The plugging mechanism of diverters and the feasibility of diverted fractures have been investigated well in previous studies. However, few studies aim at the relationship between the multi-fracture morphology and injection pressure curve during TDPF because multiple fractures can not be directly observed during the fracturing process. This study established a visual true tri-axial fracturing physical experiment based on transparent polymethyl methacrylate (PMMA) to investigate the multi-fracture initiation sequence and breakdown pressure in the horizontal wellbore during TDPF. The critical fracture parameters, including completion method, injection flow rate, injection fluid type, fracture density, perforation depth and stress difference coefficient, were analyzed in detail based on twelve PMMA samples. The results showed that all experiments just produced one hydraulic fracture in the initial fracturing stage, but sequentially created multiple fractures after multiple diversion stages. Meanwhile, due to the effect of stress shadow, the break-down pressure of the first and second diverted fracture was 1.36 times and 1.67 times of the initial fracture respectively. In addition to the transverse fractures produced in the diversion stages, longitudinal fractures were also created along the horizontal wellbore, which decreased the multiple fracture initiation effectiveness. Optimized parameters, i.e. perforation completion method, high injection flowrate, moderate fracture density, uniform perforation depth and high-viscosity carrier fluid with diverters, were recommended to obtain more transverse fractures during TPDF. In addition, our results also revealed three fracture initiation sequences of multiple fractures in the horizontal well during TPDF. This study described the relationship between the visual multi-fracture geometries and breakdown pressure in horizontal wells during TPDF, which provides new insights into the propagation mechanism of multiple fractures and guidance for fracturing design in the oilfield.