Research on Optimization of Shut-In and Backflow System After Fracture in Ganchaigou Shale Oil

Ganchaigou is a Paleogene Source rock in the Qaidam Basin, which was formed in the inland saline lake basin environment. It is a typical large-scale lithologic reservoir in the source. The production mode after fracturing has a significant impact on the oil production effect. Field statistics show that well plugging can make the shale oil reservoir have a stable production increase period, but there is no clear guidance for well plugging time and flowback system. This article focuses on the dynamic changes of the reservoir during the closed well period after the pressure of the Ganchaigou. Based on the theory of fracture dynamic changes and seepage, a closed well simulation model is established. Through the development of reservoir numerical simulation software, the closed well process of the fracturing well is simulated, and the variation of the fracturing fluid coverage range under different closed well times is studied to determine the optimal closed well time. The study shows that the optimal closed well time is inversely proportional to the permeability, and positively correlated with the injection displacement and injection volume, The optimal lockdown time for Ganchaigou is 18–30 days; Based on the analysis of the force acting on the proppant in hydraulic fractures after compression and the starting conditions, the critical flowback velocity for controlling backflow is determined. A flowback optimization model is established and the flowback working system is optimized. The factors affecting flowback are analyzed. Research shows that there is a maximum value for the diameter of the nozzle during the flowback process after compression, which may lead to backflow of the proppant, thereby affecting the flowback efficiency. During the closure period of the Ganchaigou crack, it should be used no more than 5mm nozzle, after the crack is closed, it should be used no more than 8mm nozzle ensures maximum flowback rate while reducing the risk of proppant backflow. The research results of this article are closely related to production practice, and study the dynamic changes and stimulation mechanisms of shale oil reservoirs after volume fracturing from the perspective of the entire life cycle. This has a good engineering guiding role in optimizing the actual well shut-in time after fracturing and determining the backflow system.