Simulation of the hydraulic isolation efficiency during multistage hydrajet fracturing

Effective isolation between target zones is critical to multistage fracturing. Low pressure zone caused by the hydrajet is used to isolate stages in the multistage hydrajet fracturing technology. However, there are few researches available in the literature that investigates the hydraulic isolation efficiency considering the existence of multiple fractures. This paper builds a three-dimensional computational-fluid-dynamics model with FLUENT software to study the flow field along annular sections under different conditions. It is verified that the results obtained from the model are consistent with experimental data along the perforation tunnel. The mass flow rate ratio between two outlets is proposed to analyze the hydraulic isolation efficiency in hydrajet multistage fracturing. The results show that annular fluid converges into the perforation tunnel due to the high-speed hydrajet even though there is a pre-existing fracture behind. Effective hydraulic isolation can be generated by reducing the pressure differences between target zones and the annular injection velocity as well as by enlarging the jet velocity and the area ratio. It is found that there exists an optimum area ratio to get the maximum isolation efficiency. Sensitivity analysis indicates the pressure difference is the dominant factor affecting the hydraulic isolation efficiency. A fully quadratic model derived from the response surface method is presented to predict the hydraulic isolation efficiency for real fracturing. This study will be able to provide practical guidance for the optimization of hydrajet multistage fracturing.