Making complex fractures by re-fracturing with different plugging types in large stress difference reservoirs

In recent years, re-fracturing has attracted much attention in the development of old oil fields and unconventional reservoirs, but it wasn't always successful in the trials. It was still difficult to know whether the old fractures had been plugged successfully and how the plugging influenced the fracture behavior when the plugging agents were injected into wells. However, previous experiments and models were not convinced to simulate the real plugging-fracturing. This study designed a new method to simulate the plugging-fracturing by true tri-axial hydraulic fracturing test. Instead of injecting plugging agents to wellbores, the experiment made target plugging at old fractures. The plugging was simulated by glue the fracture boundary, which could simulate different types of plugging by gluing different region on the fracture. There were three plugging types studied in this paper, including tip plugging, partial plugging and complete plugging. Besides, the pressure feature, optimized plugging time, the effects of weak plane and layer interface were discussed. The results showed obvious vertical propagation of old fractures when meeting tip barriers, which was neglected in normal 2 d fracture-plugging models. Tip plugging was not so effective in real 3 d fracture, while partial plugging helped obtain a “S” shape fracture. To improve fracture complexity, weak planes and layer interface plays an important role in improving fracture complexity by diverting the fluids from vertical to horizontal direction and afford opportunities to find more potential points which could be easily fractured. This experiment focused on fracture behavior when it met different barriers, helping understand how to make successful plugging and improve fracture complexity in reservoirs with large horizontal stress difference and multilayers.